Listeria monocytogenes is the etiological agent of listeriosis in humans and ruminants causing bacteremia, central nervous system (CNS) infections, abortion, and gastroenteritis among other clinical outcomes. Recent studies have integrated whole genome sequence (WGS), epidemiology data, and host susceptibility to provide evidence for variation in virulence among strains, as a small number of hypervirulent clones have been found linked to a high proportion of human and ruminant invasive listeriosis cases, however, still little is known about variation in virulence across different L. monocytogenes subgroups. To assess and compare the genetic diversity of clinical listeriosis isolates from ruminants in the Upper Great Plains states, we used multilocus sequence typing (MLST) and found that the variation in virulence potential varies among clonal complex (CC), which is reflected in the epidemiology of L. monocytogenes. Based on these results, we evaluated the strains’ virulence potential in Galleria mellonella through larvae survival, LD50, and cytotoxicity, and monitored health index scores and bacterial concentrations post-infection as quantifiable indicators of virulence and immunogenicity. Our findings suggest that strains belonging to CC14, as well as isolates from MN infections are hypervirulent in G. mellonella, as they need a lower bacterial concentration to cause disease and produce a low-level infection that could help in evading the host immune response. We also identify genomic elements associated with strains causing three different clinical outcomes: bacteremia, central nervous system infections, and maternal-neonatal infections. By analyzing 232 whole genome sequences from invasive listeriosis cases, we identified orthologous genes of phage phiX174, transfer RNAs and type I restrictionmodification (RM) system genes along with SNPs in loci associated with environmental adaptation such as rpoB and the phosphotransferase system (PTS) associated with one or more clinical outcomes. Novel genetic variants may be associated with a particular virulence phenotype, as it is likely that strains causing the same clinical outcome share unique genetic elements. Variation in virulence among L. monocytogenes subgroups may confer an increased ability to cross host barriers or higher adaptability to food processing environments, thus the investigation of strainspecific genetic features can impact the design of prevention and management plans for listeriosis.

Salmonella (S.) enterica est reconnue comme le principal agent causal de la salmonellose chez l’homme et les animaux. La distribution épidémiologique de cette infection implique souvent des régions géographiques éloignées; il est ainsi nécessaire de posséder des méthodes fiables afin de pouvoir discriminer des souches responsables d’une épidémie. En raison des limites de la méthode de typage sérologique, de nombreuses méthodes de génotypage moléculaire ont été développées. En particulier, la méthode par PCR couplée à l’électrophorèse en champ pulsé, qui est utilisée pour la séparation et la caractérisation des molécules d’ADN, et le génotypage par analyse de répétition en tandem polymorphe ou MLVA (Multiple-Locus VNTR Analysis), sont des méthodes modernes qui permettent d’étudier le polymorphisme et la diversité génétique des souches de S. enterica liées à une épidémie. Dans notre étude, onze marqueurs contenant des régions de répétitions en tandem polymorphique (VNTR : Variable Number Tandem Repeats) sélectionnés à partir du génome de S. enterica Typhimurium LT2 ont été utilisés pour évaluer la diversité génétique de 206 souches de S. enterica sélectionnées entre 2001 et 2007. Ces salmonelles sont représentées par 31 sérotypes, ont été isolées à partir de trois sources: hommes, aliments cuits et crus. Chaque souche a été isolée à partir d’échantillon unique et n’était liée à aucun épisode d’intoxication alimentaire ou à une épidémie de salmonellose connue. La technique MLVA a permis de sous typer 107 génotypes regroupés dans un dendrogramme en deux branches distintes dont la première et constituée par Salmonella Typhi et la deuxième par les 30 autres sérotypes liés entre eux par un ancêtre commun. Parmi les sérotypes, quatre ont été répartis dans deux à cinq branches phylogénétiques. La représentation de la variation allélique des sérotypes de S. enterica a utilisé l’arbre minimum couvrant sans racine. Des variations alléliques pour des sérotypes de S. enterica précédemment ou nouvellement décrits ont été identifiées et des variants génétiques ont été répartis en types ou en variants MLVA à loci uniques, en variants différents par un locus (SLVs), en variants différant par deux loci (DLVs) et des variants différant par plus de deux loci. Quatre marqueurs (STTR3, STTR5, STTR8 et Sal20) ont présenté un indice de diversité élevé (DI> 0,80). En résumé, la technique MLVA peut être appliquée pour étudier le profil génétique de S. enterica avec une grande diversité de sérotypes. / Epidemiological distribution of this infection often involves large areas of geographically distant, and reliable methods to discriminate strains responsible for an epidemic are necessary. Due to limitations of serological typing method, many molecular genotyping methods have been developed. Some molecular methods and their applications are: PCR coupled to PFGE, which is used for the separation and characterization of molecular profiles, and MLVA (Multiple-Locus VNTR Analysis) genotyping, or so called analysis of polymorphic tandem repeats are modern methods that allow study of the polymorphic genetic diversity and discrimination of Salmonella strains related or unrelated to epidemics. In our study, 11 markers containing polymorphic tandem repeats (VNTR: Variable Number Tandem Repeats) selected from the genome of S. enterica Typhimurium LT2 were used to assess the genetic diversity of 206 strains of S. enterica selected in 2001-2007 period. These are represented by 31 Salmonella serotypes selected from three sources: human, food and animals. Each strain was isolated from a single sample and was not related to an episode of epidemic of salmonellosis. The technique MLVA has allowed subtyping of 107 genotypes grouped in a dendrogram into two distinct dispersion trees, the first for serotype Typhi and the second for the other 30 serotypes devided within two subgroups derived from a common ancestor. Four serotypes were dispersed in two to five phylogenetic branches. The representation of the allelic variation of serotypes of S. enterica used a minimum spanning tree. Allelic variations in the serotypes of S. enterica, previously or newly described, were identified and genetic variants were distributed in MLVA types in unique locus variants, in single locus variants or in variants different by a locus (SLVs), in variants different by two loci (DLVs) and in different variants by more than two loci. Four markers (STTR3, STTR5, STTR8, and Sal20) have shown a high Diversity Index (DI> 0.80). In summary, MLVA can be applied to study the genetic profile of S. enterica with a wide variety of serotypes.

Streptococcus agalactiae est la première cause d’infections invasives chez le nouveau né et, malgré la mise en place de stratégies de prévention, cette bactérie reste le principal agent étiologique des infections néonatales. Les souches de séquence type 17, dites hyper-virulentes, sont particulièrement associées avec les méningites, type d’infection ayant des conséquences lourdes en terme de mortalité et morbidité. Ce clone possède des caractéristiques uniques, telle que la fixation au fibrinogène, ainsi qu’un répertoire de protéines de surface qui lui sont spécifiques. Parmi ces protéines, Srr2 appartient à une famille de larges glycoprotéines streptococcales et staphylococcales impliquées dans la pathogénicité. Un domaine central de Srr2, le domaine BR, est responsable de la fixation spécifique du fibrinogène par le clone ST-17, ainsi qu’au plasminogène et à divers composants de la matrice extracellulaire. Cette protéine promeut ainsi l’adhésion et le franchissement des barrières cellulaires. L’interaction de Srr2 avec les systèmes fibrinolytique et de coagulation de l’hôte favorise la dissémination bactérienne par l’activation de la fibrinolyse, et la persistance de la bactérie dans l’organisme par la formation d’agrégats bactériens. La liaison de Srr2 avec le fibrinogène semble également promouvoir la persistance bactérienne en favorisant l’internalisation et la survie dans les macrophages. Ainsi, la protéine Srr2 confère un avantage pour le processus infectieux du clone ST-17 dans l’hôte, et constitue une cible vaccinale intéressante pour la prévention des infections à S. agalactiae. / Streptococcus agalactiae is the leading cause of invasive infections in neonates. Despite the implementation of prevention strategies, this bacterium remains the main etiological agent of neonatal infections. Hyper-virulent sequence-type 17 strains are particularly associated with meningitis, a type of infection with serious consequences in terms of mortality and morbidity. This clone has unique characteristics, such as fibrinogen binding, and a panel of specific surface proteins. Among these proteins, Srr2 belongs to a family of large streptococcal and staphylococcal glycoproteins involved in pathogenicity. A central domain of Srr2, BR domain, is responsible for the specific binding of fibrinogen by the ST -17 clone and also binds plasminogen and various components of the extracellular matrix. Thereby, it promotes adhesion and crossing of cellular barriers. The interaction of Srr2 with fibrinolytic and coagulation systems of the host could promote bacterial spread through the activation of fibrinolysis and the persistence of the bacteria in the host by the formation of bacterial aggregates. The interaction of Srr2 with fibrinogen also seems to promote bacterial persistence in promoting the internalization and survival of the bacteria in macrophages. Thus, Srr2 confers an advantage to the infectious process of the ST- 17 clone in the host and is an attractive vaccine candidate for the prevention of S. agalactiae infections.

Listeria monocytogenes is a zoonotic foodborne pathogen, transmissible from the natural agricultural environment to animals and humans. In recent years, the pig production industry has experienced a series of monetary losses as a result of the L. monocytogenes outbreak which threatened the economy of South Africa. This outbreak also had a detrimental effect on the health system of the country. In South Africa however, there is limited information regarding the genomic diversity of L. monocytogenes. Therefore, an overview of the genomic diversity of L. monocytogenes strains circulating at different levels of the pork production chain needs to be determined so as to be able to identify routes of contamination of the pathogen and thus improve meat safety. This study was aimed to determine the antimicrobial resistance patterns and population structure of L. monocytogenes isolated from pig farms in South Africa. Based on wholegenome sequence analysis, 77 isolates of L. monocytogenes were differentiated into four molecular serogroups with IIa (45.5%) being the most prevalent followed by IIc (26.0%), IVb (22.1%) and IIb (6.5%). Overall, 11 clonal complexes (CCs) were identified in this study, with the predominance being observed from; CC204 (23.4%), CC1 (19.5%) and CC2 (16.9%). Genetic elements associated with biocide, antimicrobial and heavy metal resistance were noted in 24.7 %, 48% and 11.7% of the isolates, respectively. Listeria pathogenicity island 1 and 3 that harbored clusters of virulence genes were present in 38.8% of the isolates. Five different plasmids were found in 68.9% of the isolates. This study has given baseline data on the genomic diversity of L. monocytogenes strains that are associated with biocides, heavy metal and antibiotics resistance genes. The data again demonstrated the genotypes of L. monocytogenes that are prone to contaminate the farm environment and possibly cause diseases in animals and humans. / Life and Consumer Sciences / M. Sc. (Life Sciences)