Staphylococcus aureus is responsible for numerous food poisonings due to the production of enterotoxins by strains contaminating foodstuffs, especially dairy products. Several parameters can modulate S. aureus proliferation including interaction with antagonistic flora such as Lactococcus lactis, a lactic acid bacterium widely used in dairy industry. Our goal is to better understand the mechanisms involved in this interaction in order to improve food safety. A S. aureus microarray was thus developed to investigate the effect of L. lactis on staphylococcal gene expression during mixed culture. The microarray, composed of 420 oligonucleotides targeting genes involved in virulence, stress response, central metabolism and cellular machinery, was designed to limit cross-hybridizations with lactococcal nucleic acids and its specificity with regard to S. aureus was experimentally validated. It was used to establish the transcriptomic profile of S. aureus during mixed cultures with L. lactis on chemically defined medium at constant pH (6.6). In these conditions, L. lactis hardly affected S. aureus growth. The expression of most genes involved in cellular machinery, carbohydrate and nitrogen metabolisms but also stress response was only slightly modulated: a short time-lag was observed in mixed compared to pure cultures. On the contrary, the induction of several virulence factors, including the agr locus and some (but not all) enterotoxins was strongly affected. This work clearly underlines the complexity of L. lactis antagonistic potential against S. aureus and opens promising ways for investigations into non antibiotic strategies against pathogenic bacteria.
This work was supported by « ANR- French National Research Agency », project GENOFERMENT