• Lactococcus lactis (Lc. lactis) like other lactic acid bacteria, is generally harmless for mammals. Lactococci are among the most extensively used bacteria in food transformations, including cheese-making, largely because of the high acidifying power of lactic acid produced during fermentation.
  

• Streptococcus agalactiae (S. agalactiae) is mainly found in the digestive tract and vagina, where colonization is usually asymptomatic. However, S. agalactiae is responsible for acute pathologies in newborns, provoking septicemia and meningitis (about 3 cases per 1000 births). Infections with S. agalactiae increasingly concern the elderly and immuno-depressed subjects.

These two species are closely related phylogenetically. As they share at least 50% of their gene pool, it is likely that they have numerous similar properties.

Our team recently found that both these bacteria shift from a fermentation to a respiratory metabolism if cofactors required to complete the respiratory chain (heme for both and heme and quinone for S. agalactiae) are present in an aerated environment. Both these species are thus able to assemble an active respiration chain, only if the environment supplies the missing factors: thus, L. lactis and S. agalactiae are indeed auxotrophs for heme, and heme and quinone, respectively.

Compared to fermentation, respiration allows a more efficient use of the carbon source and a very good conservation of cells in stationary phase in vitro.

We are now focused on the characterization of respiration metabolism at the molecular level (genetics and proteomics), and trying to better define its role in the colonization of natural niches of these two species. The role of respiration in S. agalactiae virulence is being studied in a murine model.