Enterococci from Appenzeller and Schabziger raw milk cheese: Antibiotic resistance, virulence factors, and persistence of particular strains in the products

Enterococci are natural residents of human and animal intestinal tracts, and grow to high numbers in a variety of cheeses. The aim of this study was to determine the diversity of enterococci in two types of artisanal raw milk cheese (Schabziger and Appenzeller) and to investigate whether particular strains with triple resistance against chloramphenicol (Chl), tetracycline (Tet), and erythromycin (Ery) persist in the production system. Of 46 cheese samples, a total of 312 Enterococcus strains were isolated over a 5-month period on selective agar plates containing Chl, Tet, or, Ery. Enterococcus faecalis was the predominant species (80.7%), followed by Enterococcus faecium (5.1%), and Enterococcus durans (11.7%). According to the phenotypic resistance patterns, a selection of 150 strains was analyzed with PCR for the presence of genes encoding resistance to Ery (ereA, ereB, mphA, ennA, ermB, ermC, rnrsA/mrsB, mefA/mefE), and Tet (tetM, tetL). Because virulence factors have been linked to the pathogenicity of enterococci, the strain selection was also tested for the presence of the following virulence factors: Agg, GelE, Cyl, Esp, EfaAfs, EfaAfm, Cpd, Cob, and Ccf. All tested strains contained at least two of the nine virulence genes taken into analysis. Pulsed-field gel electrophoresis patterns of the isolates showed a limited persistence of several strains over a period of 1 to 2 months in Schabziger, and more than 2 months in Appenzeller. Finally, the enterococcal flora in the two types of cheeses seems to be rather unrelated. Within 150 strains from 25 different cheese samples (11 Appenzeller and 14 Schabziger), 41 pulsed-field gel electrophoresis patterns could be identified, and only I of these was found in enterococci from both types of cheese.