Este estudo objetivou investigar a distribuição e a relação entre os genes envolvidos com a virulência e formação de biofilme em 196 Enterococcus faecalis isolados de alimentos, clínicos e suabes cloacais de frangos de corte que receberam oocistos de Eimeria maxima e Eimeria acervulina e ração suplementada ou não com anticoccidiano. No primeiro experimento foi investigada a frequência dos genes agg, ace, tet(M), tet(L) e do operon bopABCD; e foi analisada a produção da enzima gelatinase em duas temperaturas de crescimento (36°C e 42°C). Assim como a capacidade de formar biofilme em meio de cultura suplementado com 10% de sangue, 10% de urina ou 0,75% de glicose por 70 Enterococcus faecalis isolados de frangos. Os Enterococcus faecalis isolados de frangos de corte que receberam oocistos de Eimeria maxima e Eimeria acervulina e ração suplementada ou não com anticoccidiano apresentaram capacidade de se adaptar a diferentes nichos biológicos, como por exemplo, sangue e urina. Foi observado um alto percentual dos genes dos fatores de virulência. No segundo experimento, foi avaliada a diversidade filogenética, com base no método de PCR, de 182 cepas isoladas de humanos, frangos de corte e alimentos. Estas cepas formaram quatro grupos filogenéticos (A, B, C e D) e quatro subgrupos (A1, A2, B1 e B2) com índice de similaridade entre 65 a 100%, demonstrando a adaptação de Enterococcus faecalis a diferentes ambientes. No terceiro experimento, foi determinada a capacidade de formação de biofilme de isolados clínicos e alimentares em diferentes meios de cultivo. Observou-se que o meio suplementado com 0,75% de glicose demonstrou ser o mais adequado para estabelecimento de forte aderência e, consequente formação do biofilme microbiano nos isolados de todas as origens. Em contrapartida, o meio suplementado com 10% de sangue foi o que registrou as maiores taxas de fraca formação de biofilme. Os estudos conduzidos demonstram características fenotípicas e genotípicas de Enterococcus faecalis que sugerem uma ampla adaptação ambiental entre os isolados pesquisados. / This study aimed to investigate the distribution and the relationship between genes involved in virulence and biofilm formation in 196 Enterococcus faecalis isolates from food, clinical and cloacal swabs of broilers that received oocysts of Eimeria maxima and Eimeria acervulina and diet supplemented or not with anticoccidial. The first experiment investigated the frequency of agg, ace, tet(M), tet(L) genes and bopABCD operon; production of gelatinase enzyme was analyzed at two growth temperatures (36°C and 42°C). Thus the ability to form biofilm in culture medium supplemented with 10% of blood, 10% of urine and glucose 0.75% for 70 Enterococcus faecalis isolates of chicken. The Enterococcus faecalis isolates from broilers that received oocysts of Eimeria maxima and Eimeria acervulina and supplemented or not with anticoccidial showed the ability to adapt to different biological niches, such as blood and urine. A high percentage of genes of virulence factors were observed. In the second experiment, we evaluated the phylogenetic diversity based on PCR method of 182 strains isolated from humans, broilers and food. These strains formed four phylogenetic groups (A, B, C and D) and four subgroups (A1, A2, B1 and B2) with similarity index between 65 and 100%, demonstrating the adaptation of Enterococcus faecalis to different environments. In the third experiment, the ability of biofilm formation of clinical and food isolates in different culture media was determined. It was observed that the medium supplemented with 0.75% glucose was shown to be more suitable for the establishment of strong adhesion and subsequent formation of the biofilm isolates from all sources. In other hand, medium supplemented with 10% blood was reported that the highest rates of weak biofilm formation. Studies conducted show phenotypic and genotypic characteristics of Enterococcus faecalis that suggest a broad environmental adaptation among the isolates studied.

Orientador: Ranulfo Monte Alegre / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia de Alimentos / Made available in DSpace on 2018-08-03T21:39:29Z (GMT). No. of bitstreams: 1 Santos_LucielenOliveirados_M.pdf: 943021 bytes, checksum: 7237d5c1a050d0bfae7083ba9026d36f (MD5) Previous issue date: 2004 / Resumo: As bacteriocinas são substâncias protéicas produzidas por bactérias, com atividade bactericida e bacteriostática contra bactérias sensíveis. Estes compostos são produzidos por microrganismos gram-positivos e gram-negativos, incluindo as bactérias láticas. Estas bactérias são extensamente usadas no processamento de alimentos devido a sua contribuição na vida de prateleira, textura e propriedades organolépticas. Algumas cepas de enterococos podem produzir bacteriocinas ativas contra bactérias patogênicas, tal como a Listeria monocytogenes. As bacteriocinas produzidas por Enterococcus faecium e Enterococcus faecalis são peptídeos pequenos, hidrofóbicos e termoestáveis. Dentro deste contexto este trabalho teve como objetivo estudar a produção de enterocina por E. faecium e E. faecalis. Inicialmente, as fermentações foram feitas em frascos Erlenmeyers, incubados a 37°C por 24h, para determinar as melhores condições de fonte de carbono (glicose, sacarose comercial ou açúcar mascavo), pHinicial (4,0; 6,0; 8,0 ou 10,0) e agitação (0 ou 200rpm). O meio basal modificado sem glicose foi usado como meio base sendo complementado com diferentes fontes de carbono. Em um segundo estágio, os experimentos foram conduzidos em fermentador de bancada por 24h. Durante as fermentações foram coletadas amostras em intervalos de 24h ou 1h, para a determinação da atividade de enterocina, massa celular seca, pH e conteúdo de açúcar. Os resultados obtidos a partir dos testes em frascos Erlenmeyers indicam que usando o E. faecium para produção de enterocina as melhores condições de cultivo foram pHinicial 10,0, sacarose e sem agitação. A máxima produção de enterocina foi 2,81UE. Os resultados indicaram que as melhores condições para produção de enterocina usando E. faecalis foram pHinicial 10,0, 200rpm e açúcar mascavo, com atividade de enterocina de 1,68UE. No fermentador, a maior atividade de enterocina foi 2,94UE após 21h de fermentação no experimento com pHinicial 10,0, 200rpm, sacarose e E. faecium. A atividade de enterocina foi de 1,30UE após 12h de incubação no experimento com pHinicial 10,0, 200rpm, açúcar mascavo e E. faecalis. A L. monocytogenes Scott A foi utilizada como microrganismo teste e foi inibida pelas enterocinas produzidas por E. faecium e por E. faecalis / Abstract: Bacteriocins are protein substances produced by bacteria that are bactericidal or bacteriostatic against sensitive bacterial species. Both gram-negative and gram-positive organisms, including the lactic acid bacteria, produce these compounds. These bacteria are extensively used in food processing, for their contribution to shelf life, texture and organoleptic properties. Some strains of entecococci can produce bacteriocins active against pathogenic bacteria such as Listeria monocytogenes. Bacteriocins produced by Enterococcus faecium and Enterococcus faecalis are small, hydrophobic and termostable peptides. The aim of this work was the study of enterocin production by E. faecium and E. faecalis. Initially, fermentations assays were carried out in Erlenmeyers flasks for determining the best conditions of carbon sources (glucose, sucrose and brown sugar), pHinitial (4,0; 6,0; 8,0 or 10,0) and agitation (0 or 200rpm). The modified basal medium without glucose was used as base medium and the different carbon employed. The flasks incubated at 37°C for 24h. In a second stage, the experiments were carried out in laboratory scale fermentor for 24h. During the fermentations samples were collected at intervals of 24 h or 1h, for determination of enterocin activity, biomass (cell dry mass), pH and sugar contents. The first phase in Erlenmeyers flasks indicated that using E. faecium for enterocin production the best culture conditions were pHinitial 10,0, sucrose and without agitation. The maximum enterocin production was 2,81UE. The results indicated that the best conditions for enterocin production using E. faecalis were pHinitial 10,0, 200rpm and brown sugar, with enterocin activity of 1,68UE. In fermentor the highest enterocin activity was 2,94UE after 21h of fermentation in experiment using pHinitial 10,0, 200rpm, sucrose and E. faecium. The enterocin activity was 1,30UE after 12h of incubation in experiment with pHinitial 10,0, 200rpm, brown sugar and E. faecalis. When it was used L. monocytogenes Scott A as microorganism test occurred its inhibition by enterocin of E. faecium and E. faecalis / Mestrado / Mestre em Engenharia de Alimentos

Master of Science / Department of Biology / Helmut Hirt / Enterococcus faecalis, a gram-positive member of the mammalian gastrointestinal flora, emerged as an important contributor to nosocomial infections and antibiotic resistance gene transfer. Lipoteichoic acid (LTA), a vital component of gram-positive cell walls, has been reported to function in numerous cellular processes, ranging from maintenance of cation homeostasis and virulence to modulating function and presentation of wall proteins such as adhesins and autolysins. Interestingly, LTA can be covalently modified by the addition of D-alanyl ester residues, which appear to help regulate its function by altering surface charge. In E. faecalis the process of esterification is catalyzed by four proteins encoded by the dlt operon. Mutants lacking a functional dlt operon display the inability to incorporate D-alanyl residues on LTA and are thus deficient in their ability to regulate the anionic charge of the outer envelope in response to extracellular cues. Recent evidence suggests that two-component systems are responsible for sensing environmental conditions and regulating dlt operon expression. Utilizing a reporter construct with the upstream promoter region of dlt fused to lacZ, we were able to determine how extracellular stimuli affect transcription of this operon by measuring [Beta]-galactosidase activity. Furthermore, we were able to identify specific response regulators important for bile salt, magnesium and polymyxin B signaling.

Les entérocoques sont des bactéries commensales de l'Homme majoritairement rencontrées dans le tractus digestif. En dépit du caractère bénéfique pour leur hôte, ces microorganismes sont retrouvés au deuxième rang des bactéries responsables d'infection nosocomiales en France ces dernières décennies. Diverses études tendent à montrer que le métabolisme énergétique constitue un facteur crucial pour le processus infectieux des microorganismes. Lors de ce travail, nous nous sommes intéressés à l'étude des métabolismes de différents polymères de glucoses chez Enterococcus faecalis : les maltodextrines et le gentiobiose. L'utilisation du maltose et des maltodextrines est, chez cette bactérie, directement coordonnée au niveau transcriptionnel par le répresseur MalR. L'activité de ce régulateur est rapidement modulée par le maltose qui représente l'inducteur du système et par un corépresseur protéique : la protéine P Ser HPr qui, à l'inverse, favorise la répression exercée par MalR. Le métabolisme des maltodextrines complexes, mais pas le métabolisme du maltose, est également réprimé par le régulateur pléiotrope CcpA en coordination avec son cofacteur P Ser HPr en présence de glucose. La répression catabolique de l'opéron genBA, impliqué dans le métabolisme du β glycoside gentiobiose, est aussi assurée par ce régulateur CcpA en présence de glucose. Cet opéron genBA est responsable de l'import du gentiobiose par un PTS ainsi que de son catabolisme grâce à une hydrolase. L'expression de cette structure opéronique nécessite la présence de l'activateur transcriptionnel GenR actif en présence de l'inducteur gentiobiose 6' P. / Enterococci are commensal bacteria of Humans predominantly encountered in the digestive tract. Despite their beneficial activity for their host, these microorganisms became the second leading bacterial cause of hospital acquired infections in France for last decades. Some studies showed that the central metabolism is a critical factor for microorganisms infection process. In this study, we worked on the characterisation of metabolisms of the different glucose polymers maltodextrins and gentiobiose in Enterococcus faecalis. The maltose and maltodextrins utilization is coordinated in this bacterium transcriptionally by the MalR repressor. The MalR activity is rapidly modulated by the inducer maltose and by the co repressor P Ser HPr which strengthens the MalR DNA binding. The metabolism of long maltodextrins is also repressed by the pleiotropic regulator CcpA in complex with its essential cofactor P Ser HPr in presence of glucose. The Catabolite repression of the operon genBA, involved in metabolism of the β glycoside gentiobiose, is assumed by CcpA in presence of glucose. This operon genBA allows the gentiobiose uptake with a PTS and its catabolism by a hydrolase. The expression of this latter operon requires both the GenR transcriptional activator and the inducer gentiobiose 6' P.

Orientador: Fabio Luiz Camargo Villela Berbert / Resumo: O sucesso do tratamento endodôntico depende da eliminação de microrganismos do sistema de canais radiculares. As soluções irrigadoras e métodos mecânicos são incapazes de eliminar completamente as bactérias, que penetram profundamente nos túbulos dentinários e sistema de canais radiculares. Novos métodos de descontaminação intracanal, como o laser, têm sido estudados. Este estudo ex vivo foi realizado para avaliar o efeito antibacteriano de um laser diodo de alta potência, a irrigação convencional e combinação destas técnicas em dentes contaminados com biofilme de Enterococcus faecalis. Sessenta e cinco dentes unirradiculados com canal único foram selecionados, suas coroas foram removidas e as raizes padronizadas em 15mm e foram preparados com limas manuais e Reciproc R50. Posteriormente foram preparados para o estudo microbiológico. Seus forames apicais foram selados com resina fotopolimerizável e a superfícies externa radicular foi impermeabilizada com adesivo epoxi e os espécimes foram fixados em placas de 24-poços e esterilizados com óxido de etileno. Cinquenta e cinco foram contaminados com E. faecalis. Após 21 dias de incubação, os espécimes foram divididos em três grupos experimentais (n = 15): NaOCl, Solução salina + laser (SS+laser), e NaOCl + laser; e dois grupos controle (n = 10): Controle positivo (C+) e controle negativo (C-). No grupo NaOCl os espécimes foram irrigados com 5 mL de hipoclorito de sódio-NaOCl a 2,5%; no grupo SS+laser, foram irrigados com 5 mL de ... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: The success of the endodontic treatment depends on the elimination of microorganisms from the root canal system. The irrigant solutions and mechanical methods are incapable of eliminating bacteria, which penetrate deeply into the dentinal tubules and root canal systems. New methods of intracanal decontamination, like lasers, have been studied. This ex vivo study was done to evaluate the antibacterial effect of a high-power diode laser, conventional irrigation and the combination of these techniques in teeth contaminated with Enterococcus faecalis biofilm. Sixty-five uniradicular teeth with single canal were selected, their crowns were removed, and roots were standardized with 15 mm and were prepared with manual files and Reciproc R50. Afterwards, they were prepared for microbiological study. Their apical foramen was sealed with light cured resin and their root external surfaces were sealed with Epoxi adhesive and the specimens were attached to 24-well microplates and sterilized with ethylene oxide. Fifty-five specimens were contaminates with E. faecalis. After 21 days of incubation, the specimens were divided into three experimental groups (n = 15): NaOCl, Saline + laser (SS+laser), and NaOCl + laser; and two control groups (n = 10): Positive control (C+) and negative control (C-). In the NaOCl group, the specimens were irrigated with 5 mL of 2,5% sodium hypochlorite - NaOCl; in the SS+laser group, they were irrigated with 5 mL of saline and then irradiated with highpower dio... (Complete abstract click electronic access below) / Mestre

L’argument selon lequel les antibiotiques bactéricides stimulent la formation d'espèces réactives de l’oxygène (ROS pour reactive oxygen species) chez certaines bactéries en augmentant leur activité respiratoire est sujet à discussions. Enterococcus faecalis a des propriétés intéressantes pour tester cette hypothèse. La respiration ne s’observe qu’en présence d'hème ou de fumarate. En l'absence d'hème, E. faecalis produit du superoxyde extracellulaire par l’autoxydation d’une demethylmenaquinone (DMK), un composant de sa chaîne respiratoire associé à sa membrane. En raison de cette propriété, E. faecalis est soupçonné de jouer un rôle dans la cancérogenèse colorectale. Nous montrons dans cette étude que les β–lactamines amplifient significativement la production de ROS. Cette augmentation, dépendante de DMK, n'est observée qu'en l'absence de respiration. Nos résultats pourraient également fournir une explication quant au risque accru de cancer colorectal observé chez des patients traités par des β–lactamines, comme le montrent de récentes études cliniques. / The proposal that bactericidal antibiotics stimulate the formation of reactive oxygen species (ROS) by increasing respiration is still a matter of debate. Enterococcus faecalis has interesting properties to test this hypothesis. Respiration occurs only in the presence of heme or fumarate. In the absence of heme, E. faecalis produces extracellular superoxide through autoxidation of demethylmenaquinone (DMK), a component of its respiratory chain. Due to this ability, E. faecalis is suspected to play a role in colorectal carcinogenesis. We show in this study that β–lactam antibiotics increase significantly the production of ROS. This boost of ROS formation is DMK–dependent and only observed in the absence of respiration. Our results could provide a mechanistic explanation for the observed increased risk of colorectal cancer by β–lactam antibiotics in several recent nested case–control studies.

Enterococcus faecalis is a well known nosocomial drug resistant pathogen that is responsible for urinary tract infections, bacteremia, wound infections and endocarditis through the formation of biofilms. It has been shown that 68 genes present within the core genome of E. faecalis are upregulated in biofilm formation. One of those 68 genes is a putative seleniumdependent molybdenum hydroxylase (SDMH). Adjacent to this gene are a series of open reading frames that have been postulated to play a role in the maturation of a labile selenium cofactor. The biosynthesis of this labile cofactor has yet to be studied at either the genetic or biochemical level. The addition of selenium to growth medium caused a significant increase in biofilm density and extracellular hydrogen peroxide by wild type E. faecalis. By site-directed mutagenesis gene products encoded in the SDMH operon were shown to be necessary for the selenium-dependent biofilm formation as well as extracellular hydrogen peroxide production. This biofilm and peroxide phenotype is inhibited both by tungsten or auranofin in wild type E. faecalis suggesting the SDMH is a necessary enzyme for selenium-dependent biofilm and peroxide formation. These results show that the gene products encoded within the SDMH operon are necessary for a selenium-dependent biofilm formation as well as extracellular hydrogen peroxide production. These mutants will provide the basis for defining the synthesis of the labile selenium cofactor and allow for an expanded understanding of the biological use of selenium.

Enterococcus faecalis is a Gram-positive bacterial species that is typically a member of the human gastrointestinal tract microbiota. However, E. faecalis is also a nosocomial pathogen, which is involved in urinary tract infections, soft tissue infections and endocarditis. In recent times, the occurrence of antibiotic resistance has complicated the treatment of these infections. One of the major differences between commensal and pathogenic strains of E. faecalis is that pathogens contain multiple mobile elements such as plasmids, transposons and integrative conjugative elements (ICE). These elements allow for the acquisition and transfer of virulence factors and resistance genes. Conjugative plasmids are a class of plasmids present in E. faecalis whose transfer to host cells is induced by a small pheromone peptide, cCF10 (LVTLVFV). This peptide is initially encoded as a 22-amino acid precursor (pre-cCF10) from the signal sequence of the chromosomal ccfA gene and is then proteolytically cleaved by signal peptidase II and Eep. Once pCF10 has been transferred a host E. faecalis cell, it is exceptionally stable. A replicon clone is maintained in greater than 85% of host cells over 100 generations in the absence of selection, suggesting the stability of pCF10 is intrinsic to the replicon. Three unique features of the replication initiation protein PrgW may contribute to this stability: (a) the interaction of PrgW with pre-cCF10, (b) disulfide bond formation at three conserved cysteines (C78, C275, and C307) in PrgW, and (c) processing of the nascent PrgW protein. Replication initiation proteins associated with theta replicons, such as pCF10, are often self-contained units. To initiate plasmid replication, the replication initiation protein (PrgW) binds to direct repeats (oriV) in its own coding sequence (prgW). In silico analysis of PrgW suggests the existence of three distinct domains within the protein. The first 122 amino acids are homologous to a conserved domain present in related replication initiation proteins, which includes a Helix-Turn-Helix (HTH) DNA binding domain. This suggests that this domain of PrgW has a DNA-binding function and binds to the oriV site in prgW. The following 61 amino acids are not similar to any known sequence, and are encoded by the DNA sequence containing the direct repeats in the oriV site. This domain may or may not have a distinct function. The remaining sequence forms a domain that contains cysteines C275 and C307, and is also similar to no known structure. It is hypothesized that this domain is related to the stability of pCF10. C307 appears to be critical, as previous experiments indicate that its mutation alone affects plasmid stability. Secondary structure analysis of this domain revealed the presence of multiple alpha-helices that contain distinct hydrophobic domains that possibly contribute to pre-cCF10 binding and PrgW tertiary structure. The positions of the conserved cysteines within these alpha-helices may stabilize a hydrophobic binding pocket that could potentially facilitate interaction with pre-cCF10. PrgW has a predicted molecular weight of 38.6 KDa and can be detected in Western blots as a band with an apparent approximate molecular weight (mw) of 36,000. Previous data from our lab indicates that, when overexpressed in E. faecalis, four bands of PrgW are present with observed molecular weights of 40,000, 36,000, 24,000 and 18,000. Time course experiments revealed that the 40,000 mw form is converted to a 36,000 mw form independent. The 40,000 mw form is unstable (with a complete turnover in 30 minutes) while the 36,000 mw form has a half-life of greater than 4 hours. The 24,000 mw band does not have a DNA binding motif and is likely a turnover product. When the three conserved cysteines (and only cysteines) in PrgW are replaced with alanine, the 40,000 mw form is still processed to the 36,000 mw form. However, the cysteine to alanine mutants accumulate the 36,000 mw form. / Microbiology and Immunology

Three studies were conducted to assess if the ability of Enterococcus spp. surviving the ensiling process and bovine gastrointestinal tract could impact risk of bovine mastitis. The first study determined ability of enterococci to survive 3 wk ensiling. Grass and corn crops were divided into 3 treatments: 2 commercial silage inoculants, 1 negative control. After wk 1, 2, and 3 of ensiling, dry matter and bacterial enumeration were performed. Addition of silage inoculant led to greater levels of enterococci in grass silage compared with negative control levels, but showed less difference in inoculated corn silage. The second study quantified enterococci shedding rates in lactating dairy cows. Using a 4 x 4 Latin Square design, lactating, ruminally fistulated Holsteins were inoculated with enterococcal isolates from silage inoculants, ensiled forages, or clinical mastitis cases. Over the 4-period study, each period consisted of rumen and fecal sampling (2 wk) followed by a wash period (10 d). There were no significant differences in rumen or fecal enterococci levels between the 4 treatments. Both rumen and fecal enterococci levels showed significant differences between baseline and treatment periods (period 3, 4). The third study analyzed similarity in enterococcal isolates of silage and bovine origin using pulsed-field gel electrophoresis patterns from SmaI restrictions. Dendogram analysis showed none of the isolates met or were greater than a 75% genetic similarity and produced a genetically diverse population. Results suggest Enterococcus spp. from silage inoculants are not likely to contribute to an increased risk of enterococcal bovine mastitis. / Master of Science

[ES] Los antibióticos han sido uno de los mayores logros de la medicina moderna y han contribuido al incremento de la esperanza de vida. Además, su uso en la producción ganadera primaria ha permitido incrementar significativamente su productividad. Sin embargo, su sobreuso ha llevado a la aparición de bacterias resistentes. Las bacterias resistentes a los antibióticos son causantes de miles de muertes anualmente, por lo que el control de las resistencias a los antibióticos y su monitorización es de suma importancia, tanto en bacterias aisladas de humanos, como en bacterias procedentes de animales de consumo y alimentos. La industria avícola representa uno de los sectores de producción primaria más importantes en España, tanto por producción como por volumen de ventas. En los productos avícolas, al igual que en otros productos alimentarios, se pueden encontrar bacterias resistentes a los antibióticos. Esto puede suponer un problema, especialmente por la posible transmisión de dichas bacterias resistentes al consumidor a través de la cadena alimentaria, ya que podrían reducir la eficacia de los tratamientos antibióticos en humanos. Entre los antibióticos más usados, tanto en clínica humana como en uso veterinario, se encuentran los β-lactámicos, familia a la que pertenecen las cefalosporinas, antibióticos considerados de importancia crítica. La resistencia a estos antibióticos viene determinada en la mayoría de los casos por la acción de β-lactamasas, destacando las de tipo ESBL (β-lactamasas de amplio espectro) y las de tipo AmpC. Otro grupo de antibióticos de importancia crítica son las quinolonas, entre las que se encuentra el ciprofloxacino. Aunque para estos antibióticos el mecanismo de acción suelen ser mutaciones en las dianas de acción, se han descrito genes codificados en plásmidos transmisibles que confieren resistencia a quinolonas. Además, las bacterias que presentan algunos de los genes codificados en estos plásmidos, suelen tener asociados también genes codificantes para ESBL. Las tetraciclinas, al igual que los β-lactámicos, son de los primeros antibióticos usados para tratar infecciones humanas. De los diferentes mecanismos de resistencia a las tetraciclinas, los más frecuentes son la protección del ribosoma y las bombas de eflujo, que se atribuyen a la adquisición de genes de resistencia presentes en plásmidos u otros elementos genéticos móviles. Existen otros genes que confieren resistencia a diferentes familias de antibióticos. Ese es el caso de los genes que otorgan resistencia a macrólidos, lincosamidas y estreptograminas tipo B (conocidos como genes MLS) y de los que los más comunes son los erm. En la presente tesis nos propusimos determinar las tasas de resistencia a antibióticos de uso clínico en Escherichia coli y Enterococcus spp. aislados de gallinas y huevos; conocer el efecto de un tratamiento antibiótico a las aves en la selección de resistencias a antibióticos; determinar la prevalencia de algunos de los principales genes de resistencia a β-lactámicos, quinolonas, tetraciclinas y genes de resistencia MLS, así como los cambios en las tasas de resistencia durante el crecimiento de las aves en una explotación comercial de gallinas reproductoras. En primer lugar, se determinó la prevalencia de resistencias a los antibióticos en aislados de E. coli procedentes de huevos de producción convencional, ecológica y doméstica. La mayoría de las resistencias se observaron en los aislados de huevos ecológicos (56,9% del total de resistencias) seguido de los aislados de huevos domésticos (24,8% del total) y convencionales (18,25% del total). En los 3 orígenes, la mayoría de las resistencias se observaron para la amoxicilina-clavulánico (AMC) (58,6% - 70,6%) y la tetraciclina (TE) (20% - 51,7%). Además, en el caso de los huevos ecológicos, se observaron valores importantes de resistencia a ácido nalidíxico (NA) y ciprofloxacino (CIP) (34,5%). Asimismo, fue en los aislados de provenientes de huevos ecológicos donde mayor diversidad de patrones de resistencias a antibióticos se observaron. En cuanto a genes de resistencia a antibióticos, TEM, tetA y tetB fueron los más frecuentemente observados en los 3 orígenes. Por último, la caracterización del riesgo indicó que el mayor riesgo viene dado por la resistencia a amoxicilina-clavulánico (para todos los orígenes) y que, atendiendo al origen de los huevos, es en los ecológicos donde más casos con una valoración de riesgo elevado existen. También se analizaron las resistencias a antibióticos en gallinas de 1 día de vida y el efecto que un tratamiento con amoxicilina tiene sobre la selección de resistencias entre los aislados de E. coli. Para ello, se realizó el estudio en una granja experimental con gallinas procedentes de un criadero comercial. Las gallinas no recibieron tratamiento en el día 1. Los animales se dividieron en grupos en función de la dosis del tratamiento (dosis completa, la mitad de la dosis o un tercio de la dosis). Se recolectaron los meconios de las gallinas y muestras fecales individuales. En los aislados de meconios (gallinas de 1 día de vida) se observaron resistencias en el 63,5% de los aislados y los antibióticos con las mayores tasas de resistencia fueron NA (80%), ampicilina (AMP) y AMC (70% ambos). La elevada presencia de resistencias en aves que de 1 día de vida y que no han sido tratadas, nos sugirió que la transmisión vertical desde las gallinas progenitoras puede tener un papel importante en la transmisión de resistencias. Durante el crecimiento de las gallinas se aplicaron los diferentes tratamientos con amoxicilina y se observó que, entre los grupos con tratamiento, independientemente de la dosis, no había diferencias significativas entre ellos, pero sí con el grupo control no tratado. En las aves tratadas se observaron elevadas tasas de resistencia, no solo a AMC, sino también a AMP, cefalotina, NA y TE, indicando que el tratamiento con amoxicilina no solo seleccionó cepas resistentes a este antibiótico o familia de antibióticos, sino a otros grupos de antibióticos no relacionados con los β-lactámicos. Tras los resultados obtenidos en la granja experimental, se estudió también la prevalencia de resistencias a antibióticos en explotaciones comerciales de gallinas reproductoras de la Comunidad Valenciana. Por un lado, en una de las granjas se estudió la presencia de resistencias en las gallinas de 1 día de vida y los cambios en la prevalencia de las resistencias a medida que las gallinas crecen. En los aislados de 1 día de vida, se observaron tasas de resistencia elevadas a AMP (100%), TE (98,53%), estreptomicina (S) (66,18%) y gentamicina (57,35%), además de resistencias a cefalosporinas, principalmente a la cefotaxima (CTX) (41,18%). A medida que las gallinas crecieron, se observó una disminución generalizada en la tasa de resistencias, y tanto en pollitas como en adultas, las resistencias más frecuentemente observadas fueron a AMP y a TE, mientras que las resistencias a cefalosporinas prácticamente desaparecieron. Esta reducción pudo deberse al bajo uso de antibióticos durante el crecimiento de las gallinas, que reduce la selección de cepas resistentes. En todos los aislados de las gallinas de 1 día de vida y de los aislados resistentes o con susceptibilidad intermedia a CIP, CTX y/o ceftazidima (CAZ), se detectaron por PCR las familias de genes CMY-2, SHV, TEM, qnrB y qnrS. En los aislados de las gallinas de 1 día de vida, solo se detectó el gen CMY-2. En el resto de aislados, CMY-2 fue de los menos frecuentes, mientras que los más detectados fueron SHV y TEM seguidos de qnrB y qnrS. Por otra parte, también se analizó la prevalencia de las resistencias a antibióticos en una granja de gallinas reproductoras que ya estaban en fase de producción. La mayoría de las resistencias se observaron para la AMP, la TE y el NA. Las resistencias a cefalosporinas, así como la presencia de aislados multirresistentes, fueron bajas. Entre las cepas con resistencia o susceptibilidad intermedia a CTX, CAZ y/o CIP, así como multirresistentes, se detectaron las mismas familias de genes que en el caso anterior. En esta explotación, los genes más frecuentes fueron TEM, qnrS y qnrB. Por último, de forma paralela, se analizaron las resistencias a antibióticos en aislados de Enterococcus faecalis y Enterococcus faecium en ambas explotaciones. Se observó también un cambio en la distribución de las especies de enterococos, pasando de aislar solo E. faecalis en las gallinas de 1 día de vida a ir predominando los aislados de E. faecium en las muestras de gallinas, tanto adultas como pollitas. En cuanto a las resistencias a antibióticos, entre los aislados de E. faecalis las mayores tasas de resistencias se observaron para TE y eritromicina (E), así como para CIP y C en el caso de los aislados procedentes de la explotación de gallinas adultas. En los aislados de E. faecium, la resistencias se observaron principalmente para TE y E, en ambas explotaciones. En ninguna de las explotaciones se aislaron cepas resistentes a vancomicina. A las cepas se les realizaron diferentes PCR para la detección de genes de resistencia en función del fenotipo. En los aislados resistentes a macrólidos y/o estreptograminas se detectaron los genes tipo MLS ermA y ermB. En el caso de los aislados resistentes a TE se detectaron los genes tetK, tetL, tetM, tetS y tetO. En las cepas de E. faecalis, los genes con la mayor tasa de presencia fueron los MLS y los tetM y tetL. En los aislados de E. faecium los que mayores tasas presentaron fueron los tetK, tetL, tetM y el ermB. Los resultados de la presente tesis muestran la presencia de bacterias resistentes a los antibióticos, tanto en gallinas como en productos avícolas (huevos en este caso) y que, en el caso de huevos, estas pueden ser más diversas en huevos ecológicos que en huevos de producción convencional. También se ha visto como el tratamiento con un solo antibiótico favorece la selección de resistencias a antibióticos de distintas familias y se ha demostrado también la elevada presencia resistencias a antibióticos en E. coli aislados de gallinas de 1 día de vida, incluyendo resistencias a cefalosporinas. Por último, también se ha comprobado como los genes de la familia SHV y TEM, así como los qnrB y qnrS se observan con frecuencia en E. coli no-susceptibles a cefalosporinas o ciprofloxacino, además de la elevada frecuencia de los genes ermB, tetK, tetL y tetM en aislados de enterococos procedentes de gallinas. / [CA] Els antibiòtics han sigut un dels majors assoliments de la medicina moderna i han contribuït a l'increment de l'esperança de vida. A més, el seu ús en la producció ramadera primària ha permés incrementar significativament la seva productivitat. No obstant això, la sobreutilització ha portat a l’aparició de bacteris resistents. Els bacteris resistents als antibiòtics són causants de milers de morts anualment, per la qual cosa el control de les resistències als antibiòtics i el seu monitoratge és de suma importància tant per a bacteris aïllats d'humans, com per a bacteris procedents d'animals de consum i aliments. La indústria avícola representa un dels sectors de producció primària més importants a Espanya, tant per producció com per volum de vendes. En els productes avícoles, igual que en altres productes alimentaris, es poden trobar bacteris resistents als antibiòtics. Açò pot supossar un problema, especialment per la possible transmissió d'aquests bacteris resistents al consumidor mitjançant la cadena alimentària, ja que podrien reduir l'eficàcia dels tractaments antibiòtics en humans. Entre els antibiòtics més usats, tant en clínica humana com en ús veterinari, es troben els βlactàmics, família a la qual pertanyen les cefalosporines, antibiòtics considerats d'importància crítica. La resistència per a aquests antibiòtics ve determinada en la majoria dels casos per l'acció de β-lactamases, destacant les de tipus ESBL (β-lactamases d'ampli espectre) i les tipus AmpC. Un altre grup d'antibiòtics d'importància crítica són les quinolones, entre les quals es troba el ciprofloxací. Encara que per a aquests antibiòtics el mecanisme d'acció solen ser mutacions en les dianes d'acció, s'han descrit gens codificats en plasmidis transmissibles i que confereixen resistència a quinolones. A més, els bacteris que presenten alguns dels gens codificats en aquests plasmidis, solen tindre associats també gens codificants per a ESBL. Les tetraciclines, igual que els β-lactàmics, són dels primers antibiòtics que van començar a usar-se per al tractament d’infeccions humanes. Dels diferents mecanismes de resistència a les tetraciclines, els més freqüents són la protecció del ribosoma i les bombes d’efluxe, que s'atribueixen a l'adquisició de gens de resistència presents en plasmidis o altres elements genètics mòbils. Existeixen també gens que confereixen resistència a diferents famílies d'antibiòtics. Aqueix és el cas dels gens que atorguen resistència a macròlids, lincosamides i estreptogramines tipus B (coneguts com a gens MLS) i dels quals els més comuns són els erm. En la present tesi ens vam proposar determinar les taxes de resistència a antibiòtics d'ús clínic en Escherichia coli i Enterococcus spp. aïllats de gallines i ous; conéixer l'efecte d'un tractament antibiòtic a les aus en la selecció de resistències antibiòtiques; determinar la prevalença d'alguns dels principals gens de resistència a β-lactàmics, quinolones, tetraciclines i gens de resistència MLS, així com els canvis en les taxes de resistència durant el creixement de les aus en una explotació comercial de gallines reproductores. En primer lloc, es va determinar la prevalença de resistències als antibiòtics en aïllats d'E. coli procedents d'ous de producció convencional, ecològica i domèstica. La majoria de les resistències es van observar en els aïllats d'ous ecològics (56,9% del total de resistències), seguit dels aïllats d'ous domèstics (24,8% del total) i convencionals (18,25% del total). En els 3 orígens, la majoria de les resistències es van observar per a l’amoxicilina-clavulànic (AMC) (58,6% - 70,6%) i la tetraciclina (TE) (20% - 51,7%). A més, en el cas dels ous ecològics, es van observar valors importants de resistència a l’àcid nalidíxic (NA) i ciprofloxací (CIP) (34,5%). Així mateix, va anar en els aïllats de provinents d'ous ecològics on major diversitat de patrons de resistències a antibiòtics es van observar. Quant a gens de resistència a antibiòtics, van ser el TEM, el tetA i el tetB els més observats en els 3 orígens. Finalment, la caracterització del risc va indicar que el major risc ve donat per la resistència a amoxicilina-clavulànic (per a tots els orígens) i que, atés l'origen dels ous, és en els ecològics on més casos amb una valoració de risc elevat existeixen. També es van analitzar les resistències a antibiòtics en gallines d'1 dia de vida i l'efecte que un tractament amb amoxicilina té sobre la selecció de resistències entre els aïllats d'E. coli. Per a això, es va realitzar l'estudi en una granja experimental amb gallines procedents d'una granja comercial. Les gallines no van rebre tractament en el dia 1. Els animals, es van dividir en grups en funció de la dosi del tractament (dosi completa, la meitat de la dosi o un terç de la dosi). Es van recol·lectar els meconis de les gallines i mostres fecals individuals. En els aïllats de meconis (gallines d'1 dia de vida), es van observar resistències en el 63,5% dels aïllats i els antibiòtics amb les majors taxes de resistència van ser NA (80%), ampicil·lina (AMP) i AMC (70% en tots dos). L'elevada presència de resistències en aus d'1 dia de vida i que no han sigut tractades, ens va sugerir que la transmissió vertical des de les gallines progenitores pot tindre un paper important en la transmissió de resistències. Durant el creixement de les gallines es van aplicar els diferents tractaments amb amoxicilina i es va observar que, entre els grups amb tractament, independentment de la dosi, no hi havia diferències significatives entre ells, però sí amb el grup control no tractat. En les aus tractades, es van observar elevades taxes de resistència no sols a AMC, sinó també a AMP, cefalotina, NA i TE, indicant que el tractament amb amoxicilina no sols va seleccionar soques resistents a aquest antibiòtic o família d'antibiòtics, sinó a altres grups d'antibiòtics no relacionats amb els β-lactàmics. Després dels resultats obtinguts en la granja experimental, es va estudiar també la prevalença de resistències a antibiòtics en explotacions comercials de gallines reproductores de la Comunitat Valenciana. D'una banda, en una de les granges es va estudiar la presència de resistències en les gallines d'1 dia de vida i els canvis en la prevalença de les resistències a mesura que les gallines creixen. En els aïllats d'1 dia de vida, es van observar taxes de resistència elevades a AMP (100%), TE (98,53%), estreptomicina (S) (66,18%) i gentamicina (57,35%), a més de resistències a cefalosporines, principalment a la cefotaxima (CTX) (41,18%). A mesura que les gallines van créixer, es va observar una disminució generalitzada en la taxa de resistències, i tant en polletes com en adultes, les resistències més sovint observades van ser a AMP i a TE, mentre que les resistències a cefalosporines pràcticament van desaparéixer. Aquesta reducció pot haver-se sigut degut al baix ús d'antibiòtics durant el creixement de les gallines, que redueix la selecció de soques resistents. En tots els aïllats de les gallines d'1 dia de vida i dels aïllats resistents o amb susceptibilitat intermèdia a CIP, CTX i/o ceftazidima (CAZ), es van detectar mitjançant PCR les famílies de gens CMY-2, SHV, TEM, qnrB i qnrS. En els aïllats de les gallines d'1 dia de vida, només es va detectar el CMY-2. En la resta d'aïllats CMY-2 va ser dels menys freqüents, mentre que els més detectats van ser SHV i TEM seguits del qnrB i qnrS. D'altra banda, també es va analitzar la prevalença de les resistències a antibiòtics a una granja de gallines reproductores que ja estaven en fase de producció. La majoria de les resistències es van observar per a la AMP, la TE i el NA. Les resistències a cefalosporines així com la presència d'aïllats multiresistents, van ser baixos. Entre les soques amb resistència o susceptibilitat intermèdia a CTX, CAZ i/o CIP, així com multiresistents, es van detectar les mateixes famílies de gens que en el cas anterior. En aquesta explotació, els gens més freqüents van ser TEM, qnrS i qnrB. Finalment, de manera paral·lela, es van analitzar les resistències a antibiòtics en aïllats de Enterococcus faecalis i Enterococcus faecium en totes dues explotacions. A més, es va observar també un canvi en la distribució de les espècies d’enterococs, passant d'aïllar només E. faecalis en les gallines d'1 dia de vida a anar predominant els aïllats de E. faecium en les mostres de gallines, tant adultes com en polletes. Quant a les resistències a antibiòtics, entre els aïllats de E. faecalis les majors taxes de resistències es van observar per a TE i eritromicina (E), així com per a CIP i C en el cas dels aïllats procedents de l'explotació de gallines adultes. En els aïllats de E. faecium, la resistències es van observar principalment per a TE i E en totes dues explotacions. En cap de les explotacions es van aïllar ceps resistents a vancomicina. A les soques se'ls van realitzar diferents PCR per a la detecció de gens de resistència en funció del fenotip. En els aïllats resistents a macròlids i/o estreptogramines, es van detectar els gens tipus MLS ermA i ermB. En el cas dels aïllats resistents a TE es van detectar els gens tetK, tetL, tetM, tetS i tetO. En els ceps de E. faecalis, els gens amb la major taxa de presència van ser els MLS i els tetM i tetL. En els aïllats de E. faecium els que majors taxes van presentar van ser els tetK, tetL, tetM i el ermB. Els resultats de la present tesi mostren la presència de bacteris resistents als antibiòtics tant en gallines com en productes avícoles (ous en aquest cas) i que, en el cas d'ous, aquestes poden ser més diverses en ous ecològics que en ous de producció convencional. També s'ha vist com el tractament amb un sol antibiòtic afavoreix la selecció de resistències a antibiòtics de diferents famílies i s'ha demostrat també l'elevada presencia resistències a antibiòtics en E. coli aïllats de gallines d'1 dia de vida, incloent resistències a cefalosporines. Per últim, també s'ha comprovat com els gens de les famílies SHV i TEM, així com els qnrB i qnrS s'observen amb freqüència en E. coli no-susceptibles a cefalosporines o ciprofloxací, a més de l'elevada freqüència dels gens ermB, tetK, tetL i tetM en aïllats d’enterococs procedents de gallines. / [EN] Antibiotics have been one of the most important achievements in modern medicine and have contributed to life expectancy increasing. Furthermore, its use in livestock production has led to a significant raise in productivity. However, its overuse has caused the apparition of resistant bacteria. Antibiotic resistant bacteria cause thousands of deaths annually, so the control of antibiotic resistance and their monitoring is of utmost importance for bacteria isolated from humans as well as bacteria isolated from food and food-animals. Poultry farming is one of the most important sectors of primary production in Spain for both production volume and sales. Poultry products, as for any other food product, antibiotic resistant bacteria can be found. These may pose a risk, especially for the possible transmission of the resistant bacteria to consumers via the food chain, since these resistant bacteria could reduce the efficacy of antibiotic treatments in humans. Among the antibiotics most used, both in human clinical practice and veterinary medicine, are the β-lactams, family to which the cephalosporins belong, antibiotics considered as of critical importance. Resistance to this family is determined in many cases by β-lactamases, highlighting ESBL (Extended Spectrum β-lactamases) and AmpC types. Another family of critically important antibiotics are the quinolones, among which is ciprofloxacin. Even though for these antibiotics the main resistance mechanism are mutations in the action targets, genes conferring resistance to quinolones coded in transmissible plasmids have been described. Also, bacteria carrying some of these plasmids encoded genes usually present ESBL genes. Tetracyclines, like β-lactams, are among the first used antibiotics to treat human infections. Of the different tetracycline resistance mechanisms, the most frequent are the ribosome protection and efflux pumps and are usually attributed to the acquisition of resistance genes carried in plasmids or other mobile genetic elements. Genes conferring resistance to different antibiotics families at once also exist. This is the case of genes that confer resistance to macrolides, lincosamides and streptogramins B (known as MLS genes) and of which the most common are the erm genes. In this thesis we set out to determine the rates of antibiotic resistance against clinically used antibiotics in Escherichia coli and Enterococcus spp. isolates from hens and eggs; to know the effect of an antibiotic treatment on the birds in the selection of antibiotic resistance; to determine the prevalence of some of the main resistance genes for β-lactams, quinolones, tetracyclines and MLS resistance genes, as well as to determine the changes in the antibiotic resistance rates during the rearing of the chicks in a commercial farm of breeding hens. Firstly, the antibiotic resistance prevalence was determined in E. coli isolated from eggs coming from conventional, organic and backyard production. Most of the resistances were observed in the isolates from organic eggs (59.9% of total resistance), followed by backyard eggs isolates (24.8% of the total) and conventional eggs isolates (18.25% of the total). In all 3 origins, resistance was observed mainly for amoxicillin-clavulanic acid (AMC) (58.6%-70.6%) and tetracycline (TE) (20%-51.7%). What is more, in organic eggs, important values of resistance against nalidixic acid (NA) and ciprofloxacin (CIP) were observed (34.5%). Likewise, more patterns of antibiotic resistance were observed in isolates from organic eggs. Regarding the antibiotic resistance genes, TEM, tetA and tetB were the most observed in all 3 origins. Finally, the risk characterization indicated that the greater risk comes from resistance to amoxicillin-clavulanic acid (for all the origins) and that, in relation to the egg’s origin, it is in organic eggs were more cases with a high risk value existed. Antibiotic resistance in one-day old chicks and the effect that an amoxicillin treatment has in the selection of resistances in E. coli isolates were also analyzed. For this, the experiment was carried out on an experimental farm with hens purchased from a commercial breeding farm. The chicks did not receive any treatment on the first day. The birds were divided into groups depending on the antibiotic dose they would receive (complete dose, half of the dose and one third of the dose). Meconium and individual fecal samples were taken. In the meconium isolates (one-day old chicks), antibiotic resistances were observed in 63.5% of the isolates and the antibiotic with the highest resistance rates were NA (80%), ampicillin (AMP) and AMC (70% both). The high prevalence of resistance among untreated one-day old chicks, suggested to us that vertical transmission from hens may have an important role in resistance transmission. During the growth of the hens, different amoxicillin treatments were applied, and it was observed that, among the treated groups, with independence from dose, no significant differences were present, but there were with the untreated control group. In the treated birds, high resistance rates were observed, not only for AMC but also for AMP, cephalothin, NA and TE, indicating that the amoxicillin treatment not only selected resistant strains to this antibiotic or this antibiotic family, but also other antibiotic groups unrelated to β-lactams. After the results obtained in the experimental farm, the prevalence of antibiotic resistances in commercial farms of the Valencian Community were also studied. On the one hand, in one of the farms it was studied the presence of antibiotic resistance in one-day old chicks and the changes in the prevalence of the resistance as hens growth. In isolates from one-day old chicks, high resistance rates were observed for AMP (100%), TE (98.53%), streptomycin (S) (66.18%) and gentamycin (57.35%), as well as resistance to cephalosporins, mainly cefotaxime (CTX) (41.18%). As hens grew, a generalized decrease in antibiotic resistance was observed, and in pullets and adult hens the most frequently observed resistance was to AMP and TE, whilst cephalosporin resistances practically disappeared. This reduction may be due to the low usage of antibiotics during the rearing of hens, that could have reduced the selection of resistant strains. In all one-day old chicks isolates and all isolates with resistance or intermediate susceptibility to CIP, CTX and/or ceftazidime (CAZ), family genes CMY-2, SHV, TEM, qnrS and qnrB were detected by PCR. In one-day old chick isolates, only CMY-2 was detected. For the rest of the isolates CMY-2 was among the least frequently detected genes, while SHV and TEM were the most detected genes followed by qnrB and qnrS. On the other hand, the prevalence of antibiotic resistance in a hen farm during production stage was also analyzed. Most observed resistances were for AMP, TE and NA. Cephalosporin resistance, as well as multi-resistant isolates, were low. For the isolates with resistance or intermediate susceptibility to CIP, CTX and/or CAZ and for all the multi-resistant isolates, the same family of genes as before were detected. In this farm, the most observed genes were TEM, qnrS and qnrB. Finally, in parallel, antibiotic resistances in Enterococcus faecalis and Enterococcus faecium were analyzed in both farms. Moreover, a change in the distribution of the enterococcus species was observed, coming from isolating only E. faecalis in one-day old chicks to the predominance of E. faecium in pullets and adult hens. For the antibiotic resistances, among the E. faecalis isolates the highest rates of resistance were observed for TE and erythromycin (E), and for CIP and C in isolates from the farm of adult hens. In E. faecium, resistances were mainly observed for TE and E in both farms. Resistance to Vancomycin was not observed in any of the farms. Different PCRs for the detection of antibiotic resistance genes were made depending on the resistance phenotype. In macrolide and/or streptogramins resistant isolates, the MLS genes ermA and ermB were detected. For the TE resistant isolates, the genes tetK, tetL, tetM, tetO and tetS were detected. In E. faecalis isolates, the most frequently detected genes were the MLS and tetM and tetL. In E. faecium isolates, the ones with the highest rates were tetK, tetL, tetM and ermB. The results of the present thesis show the presence of antibiotic resistance bacteria both in poultry and in poultry products (eggs in this case) and, that in eggs, the resistances can be more diverse in organic eggs than in conventional. It has been seen too how the treatment with one antibiotic can favour the selection of antibiotic resistances to different families and, also, the presence of antibiotic resistant E. coli in one-day old chicks has been proven too, including resistance to cephalosporins. Likewise, it has been proven how the genes of the SHV and TEM families, as well as qnrB and qnrS genes are frequently observed in cephalosporins or ciprofloxacin non-susceptible E. coli, apart from the high prevalence of ermB, tetK, tetL and tetM genes in enterococcus isolates from hens. / Fenollar Penadés, A. (2020). Estudio de la transmisión de resistencias a antibióticos mediante métodos moleculares en el sector avícola y su implicación para la salud pública [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/149399