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

Streptococcus thermophilus is an important component of the starter cultures used in many dairy products. Strains of this species are mainly selected on the basis of acidifying capacity, while the possibility of use of bacteriocins producers belonging to this species as protective agents in dairy products has been little considered. In recent years, several reports have described S. thermophilus strains as bacteriocin producers and some of the antimicrobial peptides have been purified and sequenced. These proteinaceous compounds, called thermophilins, have been found to inhibit pathogenic and spoilage microbes such as clostridia and Listeria. Bacteriocin producing S. thermophilus strains able to dominate in natural ecosystems might constitute a mean to increase the safety of products obtained with natural cultures. Therefore, in this study the natural occurrence of bacteriocin producers in the species S. thermophilus was evaluated by examining 75 strains isolated from whey and milk cultures for cheesemaking, different cheeses made from raw milk and the plants Plantago lanceolata and Rosa canina. Though PCR tests showed the presence of genes encoding known thermophilins in most strains, only sixteen were active against at least one of the bacterial indicators tested. The involvement of a proteinaceous compound was ascertained in all the strains showing antimicrobial activity. These included representative strains of Clostridium tyrobutyricum, Enterococcus faecium, Escherichia coli, Listeria innocua, as a non pathogenic counterpart of L. monocytogenes, Staphylococcus aureus and common starter lactic acid bacteria belonging to the species Lactobacillus delbrueckii subsp. bulgaricus and lactis, L. helveticus, S. thermophilus and S. macedonicus. Only the two isolates from plants were active against E. coli. All the active strains inhibited Staphylococcus aureus, most inhibited some starter microorganisms and none inhibited Listeria innocua and Clostridium tyrobutyricum. Therefore a limited potential of natural thermophilin producers as biopreserving agents was put in evidence. For the two strains with a wider inhibition spectrum, i.e. S. thermophilus PRI36 and PRI45, the arrangement of the genes encoding the most active components of the thermophilin 9 complex was elucidated. Results highlighted a high level of variability in sequence and gene content.