Identification of tetracycline- and erythromycin-resistant Gram-positive cocci within the fermenting microflora of an Italian dairy food product

Aims: Microbiological and molecular analysis of antibiotic resistance in Gram-positive cocci derived from the Italian PDO (Protected Designation of Origin) dairy food product Mozzarella di Bufala Campana. Methods and Results: One hundred and seven coccal colonies were assigned to Enterococcus faecalis, Lactococcus lactis and Streptococcus bovis genera by ARDRA analysis (amplified ribosomal DNA restriction analysis). Among them, 16 Ent. faecalis, 26 L. lactis and 39 Strep. bovis displayed high minimum inhibitory concentration (MIC) values for tetracycline, while 17 L. lactis showed high MIC values for both tetracycline and erythromycin. Strain typing and molecular analysis of the phenotypically resistant isolates demonstrated the presence of the tet(M) gene in the tetracycline-resistant strains and of tet(S) and erm(B) in the double-resistant strains. Southern blot analysis revealed plasmid localization of L. lactis tet(M), as well as of the erm(B) and tet(S) genes. Genetic linkage of erm(B) and tet(S) was also demonstrated by PCR amplification. Conjugation experiments demonstrated horizontal transfer to Ent. faecalis strain JH2-2 only for the plasmid-borne L. lactis tet(M) gene. Conclusions: We characterized tetracycline-and erythromycin-resistance genes in coccal species, representing the fermenting microflora of a typical Italian dairy product. Significance and Impact of the Study: These results are of particular relevance from the food safety viewpoint, especially in the light of the potential risk of horizontal transfer of antibiotic-resistance genes among foodborne commensal bacteria.