ANTIMICROBIAL ACTIVITY, ANTIBIOTIC RESISTANCE AND THE SAFETY OF LACTIC ACID BACTERIA IN RAW MILK VALTELLINA CASERA CHEESE

The study involved 75 autochthonous lactic acid bacteria (LAB) obtained from Valtellina Casera Protected Designation of Origin cheese and provides data on their antibiotic resistance (AR), antimicrobial activity and safety aspects. High biodiversity was revealed, assigning the isolates to 16 species. In the curd Streptococcus thermophilus was predominant, while Enterococcus and the Lactobacillus casei group prevailed in ripened cheese. One Lactobacillus rhamnosus was resistant to penicillin G. The presence of 19 tetracycline-resistant strains indicates that tetracycline resistance is emerging in dairy sources. Seven Enterococcus faecalis, one Lactococcus garvieae and one Lactoc.lactis subsp. lactis showed the presence of the int gene that is responsible for the transfer of tetracycline resistance. Six En.faecalis inhibited Listeria monocytogenes and closely related LAB, and four of these harbored the entAS48 gene. Genes associated with virulence factors were detected in En.faecalis but not in En.faecium. A low spread of AR in wild LAB and a low occurrence of safety hazards in enterococci strains were highlighted. Practical ApplicationIn recent years, raw milk cheese have received increasing attention for their microbial biodiversity and as a source of food bio-preservatives, but they could also provide a vehicle for antibiotic resistance. The results of this study showed that wild microbiota of Valtellina Casera cheese is characterized by an important activity inhibiting growth of Listeria monocytogenes. These probably actively contribute to ensure the safety and quality of this cheese. The presence of tetracycline resistance among En.faecalis and Str.thermophilus may indicate that this resistance is emergent in dairy sources, a point that needs further investigation. En.faecium from artisanal Valtellina Casera cheese did not show antibiotic resistance and did not harbor any of the virulence genes associated to the hospital-associated clade according to European Food Safety Authority.