Safety aspects, genetic diversity and technological characterisation of wild-type Streptococcus thermophilus strains isolated from north Italian traditional cheeses

Antibiotic susceptibility, antimicrobial activity, genotypic and technological properties of 52 Streptococcus thermophilus isolates, collected from four north Italian traditional cheeses, was investigated. RAPD-PCR, was used to study genetic variability and distinguish closely related strains; the results showed a high degree of heterogeneity among isolates. All S. thermophilus studied were susceptible to ciprofloxacin, levofloxacin, penicillin G, ampicillin, mupirocin, nitrofurantoin, quinupristin/dalfopristin and rifampicin. Nine isolates were classified as resistant to tetracycline, 6 to streptomycin, 5 to oxacillin, 3 to erytromycin, 3 to vancomycin and only one to chloramphenicol. Through PCR-based detection, none of the common genetic determinants for vancomycin (vanA, vanB, vanC1, vanC2, vanC3, vanD, vanE, vanG) and erythromycin (ermB and ermC) were identified. The genetic basis of the tetracycline resistance phenotype in these strains was linked to tetS-tetL genes (8 isolates) or the tetM gene (1 isolate), and the integrase element int of the Tn916/Tn1545 family of transposons was negative. Four strains were able to produce antimicrobial compounds against Clostridium tyrobutyricum. The study highlights the importance of making an accurate safety assessment when using S. thermophilus as a starter culture.