Modern, large scale Cheddar cheese manufacture is dependent on reliable acid production by Lactococcus lactis subspecies cremoris and subspecies lactis starter cultures. Any inhibition of acid production may affect cheese quality, disrupt production schedules and reduce profitability. The presence of antibiotic residues in manufacturing milk resulting from the treatment of mastitis in lactating cattle is a potential source of starter culture inhibition. Therefore, a range of antibiotic concentrations was assessed for measurable inhibitory effects on acid production and compared to the minimum detectable concentrations by approved screening test procedures. Antibiotics were selected from formulations approved for use on lactating cattle for the treatment of mastitis. Novobiocin, lincomycin, oleandomycin and oxytetracyline HCl, all non-b-lactam antibiotics, inhibited acid production of one or more L. lactis strains at antibiotic concentrations below the detectable limit of standard screening procedures. / Depending on the antibiotic, either or both the Bacillus stearothermophilus (var. calidolactis) disk assay and/or the Delvo SP assay were ineffective at detecting the antibiotics at concentrations required to inhibit the starter strains. Consequently, antibiotic residues below the detectable limits of these testing procedures could cause significant starter culture inhibition, disrupting cheese making schedules. Another potential source of starter culture inhibition is related to raw milk quality and the practice of refrigerated storage prior to processing. Previous studies differed as to whether the growth of psychrotrophic organisms would have a detrimental impact on subsequent acid production by starter bacteria employed in cheese manufacture. In this study, no inhibition of acid production by a commercial L. lactis subsp. cremoris strain was evident when grown in milk that had undergone short term temperature abuse. Antimicrobial systems native to bovine milk may also have an adverse impact on starter culture performance. The present study assessed the inhibitory effect of an activated lactoperoxidase system (LPS) on a range of L. lactis cultures. All of the strains were significantly inhibited when grown on reconstituted skim milk in the presence of an active LPS. Inhibition of acid production by strains grown on glucose was also observed, leading to further investigations to describe the inhibitory process. A non-phosphoenolpyruvate phosphotransferase (PEP/PTS) dependent glucose transport system, first observed in 1980 in one L. lactis subsp. lactis strain, was hypothesised as a link in strain variations in LPS sensitivity. However, the LPS sensitive L. lactis subsp. cremoris strains tested did not take up glucose in a PEP depleted state, most likely due to their inability to utilise arginine as an ATP generating energy source. The questions remain unanswered whether cremoris strains possess this glucose transport mechanism and whether it could contribute to strain variations in LPS sensitivity. / In a subsequent investigation, galactose phosphotransferase system (PTS) deficient L. lactis strain ATCC 7962 demonstrated log phase growth inhibition when grown on galactose in the presence of the model LPS. Previously reported LPS mediated effects on the glycolytic enzyme hexokinase do not appear to explain this result. The present study confirmed strain variability in sensitivity to the model LPS among both Lactococcus lactis subspecies lactis and subspecies cremoris strains. Further, the observation that dithiothreitol significantly alleviated the inhibition of a highly sensitive cremoris strain, implicated the involvement of sulphydryl groups as the target of the transient inhibitory factors. Data collected excluded the possibility that portions of the metabolic pathways involved in fructose and galactose metabolism are sensitive to the LPS in cells possessing PEP/PTS capability. This study also identified potential directions of further work to elucidate the mechanism(s) of LPS inhibition.

Lactococcus lactis est une bactérie lactique couramment utilisée dans l’industrie laitière. Elle assure en tant que ferment des fonctions technologiques multiples qui impactent la flaveur et la texture finale des produits. Cependant, la diversité fonctionnelle constatée au sein des levains de cette espèce impose de mettre en place un processus de sélections des souches. Ces travaux ont pour objectif d’identifier les déterminants de la spécificité organoleptique dite « crème » de la souche industrielle L. lactis ssp. lactis. Dans un premier temps, un diagnostic macro-cinétique de l’activité de ce ferment a été réalisé en lait pour évaluer l’impact sur la physiologie cellulaire (l’acidification, le stress oxydatif, et la thermisation différentielle du lait). Définir la singularité de notre souche d’intérêt nécessite d’évaluer la diversité fonctionnelle de levains laitiers de L. lactis ssp. lactis. Cette démarche s’est appuyée sur une approche de biologie intégrative du génotype au phénotype. Pour réduire le temps d’expérimentation, une sélection des variables discriminantes à été conduite. L’un de ses composés clef de cette typicité a fait l’objet d’une étude approfondie afin de tester les différents paramètres pouvant influencer sa synthèse. La dernière partie, plus applicative, s’est articulée sur la modélisation de la signature en fonction de quatre facteurs industriels (matière grasse, sèche, oxygénation et température) par utilisation de la méthodologie des surfaces de réponse. / The lactic acid bacterium Lactococcus lactis is widely used in dairy industry. Used as a starter, L. lactis subsp. lactis is in charge of many functional characteristics which determine the final texture and flavour of fermented products. However, the phenotypic diversity observed within the species requires strain selection development. This PhD aims at identifying the determinants of the creamy sensory specificity of the industrial strain L. lactis subsp. lactis. Firstly, a diagnosis of macro-kinetic activity of ND5F was carried out to assess the impact on cellular physiology of three environmental parameters (acidification, oxidative stress, and milk heat treatments). In order to assess the uniqueness of our strain of interest, a system biology approach from genotype to phenotype was implemented to study the functional diversity of L. lactis subsp. lactis starters. A group of nine strains representative of the genetic diversity of this subpopulation was made up. 82 variables selected as important dairy features; including physiological descriptors, production of metabolites and volatile organic compounds (VOCs); were monitored. This study demonstrated the phenotypic uniqueness of each of these genetically closely related strains, allowing strain discrimination in dairy products. To reduce the time-consuming experimentation, we developed a method of variable selection. Twenty VOCs were therefore identified as major phenotypic determinants of this diversity. They define four robust stain clusters depending on their metabolic orientations: lipolysis, proteolysis, glycolysis and unexpressed. Inclusion of genotypic diversity in addition to phenotypic characters led to adjust the functional classification by integrating strain unexpressed genotypic potentialities. Comparative proteomics analysis in milk identified protein markers of this specificity. After selecting a subset of forty-six proteins the three levels (genotype, phenotype and proteomics) involved in this diversity expression were integrated to provide a predictive classification of starter diversity.The Integration of our strain to this model enabled the identification of fourteen phenotypic determinants and seven proteins to explain ND5F specificity. The pentane-2,3-dione, a key aromatic compounds of this typicity was subjected to a comprehensive analysis to point out the different factors that may influence its synthesis. On a more applied aspect, the last part was focused on the signature modelling based on four industrial factors (milk fat and total solid level, oxygen and temperature) using the response surface methodology. It enabled to enhance the “creamy” organoleptic characteristics of the fermented products.