Antimicrobial activities of Streptococcus thermophilus natural strains and occurrence of thermophilin genes and gene clusters.

"Streptococcus thermophilus is an important component of many natural and commercial starter cultures used in cheeses and fermented milks. Till now, the possibility of use of bacteriocin-producers belonging to this species as biopreserving agents in dairy products has been little investigated.. The aim of this study was evaluating the natural occurrence of bacteriocin-producers in the species . S. thermophilus by combining phenotipic and genetic approaches.. The study was conducted on 75 strains of S. thermophilus isolated from whey and milk cultures for cheesemaking, different cheeses made from raw milk and plants. . Most strains moderately inhibited Staphylococcus aureus and none inhibited Listeria innocua and Clostridium tyrobutyricum. Fourteen were active against at least one of the other bacterial indicators tested, i.e. Enterococcus faecium, Escherichia coli, and common starter lactic acid bacteria belonging to the species Lactobacillus delbrueckii subsp. lactis, L. helveticus (Figure 1), S. thermophilus and S. gallolyticus subsp. macedonicus. . Three strains were active against E. coli.. Strains with antimicrobial activity were only a part of those possessing known bacteriocin genes. In particular, just 14 of the strains possessing bacteriocin genes were active against one or more indicators. . In the species S. thermophilus the ability to inhibit other starter bacteria seems more common than inhibition of spoiling or pathogenic microorganisms, and the selection of bacteriocinogenic S. thermophilus strains for biopreservation must be based also on the effects exerted on the natural lactic acid bacteria microbiota as evaluated in production trials. . "