OBJECTIVE To study the emergence mechanism of enterococcal resistance to a variety of antibacterial agents, and to guide
the selection and use of antimicrobial agents in clinical and prophylactic practice. METHODS The experiments on induced resistance to gentamicin or
ampicillin were performed, in nine or seven Enterococcus faecalis strains or two or one E. faecium,
respectively, seven strains of E.faecalis and two of E.faecium were selected to by multiple-step method.The susceptibility of the induced strains resistant to ampicillin and gentamicin was measured by determining the MIC using agar dilution method. The gentamicin-resistant genes and pbp_5 gene in the induced strains were detected by PCR, then the nucleotide sequence of pbp_5 gene was analyzed. RESULTS The strains highly resistant to gentamicin were obtained from 3 tested strain of E. faecalis and 1 tested strains of E.faecium,the MICs of these laboratory resistant strains were with 2,4,64 and 64-fold increase,respectively, when compared with parent strains. The strains stably resistant to ampicillin were obtained from 4 tested strains of E.faecalis. The MICs of these laboratory resistant strains were with 16,16,64 and 8-fold increase,respectively, when compared with parent strains. The substitution of some amino acids was found between the positions of 453 and 560 in the PBP5 sequences in 2 of 3 induced strains resistant to ampicillin. CONCLUSIONS The results of in vitro experiments on enterococcal strains for induced resistance to gentamicin and ampicillin,which are performed by multiple-step method,show that acquired resistance could be occurred when exposing them to low level of some antibacterial agents for long term.