Evaluation of Plasmid Content and Tetracycline Resistance Conjugative Transfer in Enterococcus italicus Strains of Dairy Origin

Five Enterococcus italicus strains harbouring tet genes responsible for the tetracycline resistance were subjected to plasmid profile determination studies. For four strains tested the profiles showed between three and six plasmid bands, the size of which ranged between 1.6 and 18.5 kb. Southern hybridization experiments associated tetS and tetK genes with chromosomal DNA in all strains and tetM gene with plasmids of around the same size (18.5 kb) in two of the tested strains. The ability of the new species to transfer tetM gene was studied by transfer experiments with the tetracycline-susceptible recipient strains E. faecalis JH2-2 and OG1RF; mobilization experiments were performed with E. faecalis JH 2-2 harbouring the conjugative plasmid pIP501as helper plasmid. The results obtained show that the new enterococcal species was able to acquire antibiotic resistance by conjugation, but not to transfer its plasmids to other bacteria. Further PCR and hybridization experiments carried out to assess the presence of mobilization sequences also suggest that the tetM plasmid from E. italicus is a non-mobilizable plasmid.