Characterization of the bacteriocin production locus blp from Streptococcus thermophilus

The production of antimicrobial peptides or bacteriocins by LAB has received considerable attention, which is related to their potential use as natural preservatives in food products. Nevertheless, a relatively limited amount of information is available on antimicrobial peptide production in Streptococcus thermophilus. The aim of the present study was to characterize a Class II bacteriocin locus in S. thermophilus that resembles a part of the blp (for bacteriocin-like peptide) loci identified in S. pneumoniae and S. mutans, and that is therefore designated the S. thermophilus blp locus. The universally conserved region of these streptococcal blp loci encompasses genes encoding an ABC-transporter (blpA) and an accessory protein (blpB), and a typical quorum-sensing regulatory module composed of a two-component system (blpH and blpR, TCS09 in S. thermophilus) and the corresponding bacteriocin-like inducer peptide precursor encoding gene blpC. Interestingly, genes located downstream of blpRH in the three streptococci species are highly variable and encode bacteriocin-like peptides containing a typical double-Gly leader and potential immunity proteins. The sequenced S. thermophilus strains (LMG18311, CNRZ1066 and LMD9) also display a high variation in the number of bacteriocin encoding genes, with the LMD9 strain harbouring the most complex blp locus with the presence of 4 bacteriocin encoding genes (blpD, blpU, blpE and blpF). In order to investigate Blp-associated bacteriocin production in S. thermophilus, the mature part of the potential induction peptide BlpC was synthesized and increasing concentrations were added to the culture medium of S. thermophilus LMD9. At a concentration of 100ng/ml, 100% of LMD9 cells were found to produce bacteriocins. Northern blotting experiments showed that the amount of the blpD, blpE and blpF mRNAs increased linearly with BlpC concentration, while the level of the blpU mRNA remained constant. The antimicrobial compounds produced by LMD9 upon BlpC induction were mainly active against S. thermophilus strains. We showed by gene deletion that BlpD alone was sufficient to inhibit most of the indicator S. thermophilus strains, while inhibition of a few other strains and species required the presence of either BlpD and BlpU, or the association of the four bacteriocin peptides. In conclusion, we showed that the S. thermophilus blp locus is dedicated to bacteriocin production and that the inhibitory spectrum of S. thermophilus can be modulated through the combination of different blp-encoded bacteriocins