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

Abstract

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. With regard to technological properties, after 6 h of incubation in milk 25% of the streptococcal strains could be defined as fast acid producers, while after 24 h the majority of isolates (79%) displayed only weak acidification activity. Reduction activity was generally low; in fact, none of these S. thermophilus strains showed a Eh < −102 mV). All the studied S. thermophilus 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. PCR-based detection did not identify any of the common genetic determinants for vancomycin (vanA, vanB, vanC1, vanC2, vanC3, vanD, vanE, vanG) or erythromycin (ermB and ermC). 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 provides new evidence of the resistance of S. thermophilus to antimicrobial agents, confirming the importance of including an accurate safety assessment of phenotypic/biotechnological data when carrying out strain selection for dairy applications.