Response of S. thermophilus LMD-9 to bacitracin: involvement of a BceRS/AB-like module and of the rhamnose-glucose polysaccharide synthesis pathway.

Streptococcus thermophilus is a lactic acid bacterium of major importance to the dairy industry as it is found in numerous cheeses and is one of the two bacterial species involved in the fermentation of yogurt. Bacterial two-component signal transduction systems (TCSs) play important roles in the process of bacterial environmental adaptation. S. thermophilus LMD-9 possesses eight such TCS systems; however, their functions have thus far been only poorly investigated. Here, we focused on two of the TCSs in LMD-9, TCS06 and TCS07, whose encoding genes are located close to each other on the chromosome, and are associated with those of ABC transporters. TCS06 homologs are frequently found in Lactobacillales, but their function has not yet been determined, while TCS07 and its upstream potential ABC transporter are homologous to the BceRS/AB system, which is involved in bacitracin resistance in Bacillus and Streptococcus species. To investigate the function(s) of TCS06 and TCS07, we constructed and characterized deletion mutants and performed transcriptional analysis in the presence and absence of bacitracin. We show here that both TCS06 and TCS07 regulate the genes in their close vicinity, in particular those encoding ABC transporters. We propose that the response of S. thermophilus to bacitracin includes i) a bacitracin export system, regulated by TCS07 and constituting a BceRS/AB-like detoxification module, and ii) the modification of cell-envelope properties via modulation of rhamnose-glucose polysaccharide synthesis, at least partially regulated by TCS06.