Transfer of erythromycin-resistance among strains and species of bacteria: plasmid conjugation method in enterococcal isolates

Abstract

To study if the resistance to macrolide in Enterococcus can be transferred between strains, and species of the same and different genera. Agar dilution was used to screen 30 enterococcal isolates that were resistant to erythromycin. Conjugation was performed by filter mating method. The 30 donor bacteria included 13 strains of Enterococcus faecalis, 16 strains of E. faecium, and 1 strain of E. hirae. The recipient bacteria included 1 strain of E. faecalis sensitive to erythromycin and resistant to tetracycline, and 1 strain of Staphylococcus aureus with the MIC against erythromycin of 0.25 approximately 1 microg/ml. Polymerase chain reaction was used to test the existence of ermB gene and the tranposons Tn1545 and Tn917 in the enterococcal isolates before and after filter mating. The transfer rate between different strains and species of the same genus were all 100%. The MIC(50) and MIC(90) against erythromycin of 13 conjugates were both 512 microg/ml, and Tn1545 and Tn917 were found in the ermB gene of 12 conjugates. 17 conjugates were obtained from 16 strains of donor E. faecium and 1 strain of E. hirae with the MIC(50) and MIC(90) both of 512 microg/ml. The ermB gene was found in 16 of the 17 conjugates, and 11 of the 16 conjugates showed the existence of Tn1545 and Tn917, Tn1545 existed in the ermB gene of 4 conjugates, and Tn917 existed in the ermB gene of 1 conjugate. 30 conjugates of Staphylococcus aureus were obtained by plasmid conjugation and transfer with a transfer rate of 100% and the MIC(50) and MIC(90) both of 512 microg/ml. The ermB gene was found in 28 of the 30 conjugates. Both Tn1545 and Tn917 were found in the ermB gene of 23 of the 28 conjugates, Tn1545 was found in the ermB gene of 4 conjugates, and Tn917 was found in the ermB gene of 1 conjugate. The resistance to macrolide of Enterococcus, related with the existence of ermB gene and transposons Tn1545 and Tn917, can be transferred between strains and species of same and different genera.