Phenotype and genotype alteration during adaptive evolution of Enterococcus faecalis to antimicrobials

The current worldwide emergence of resistance to antibiotics in bacteria constitutes an important growing public health threat. The mechanisms of the emergence and dissemination of resistance remain to be elucidated. Adaptation laboratory evolution provide an approach to investigate the acquisition of de novo mutations that confers drug resistance. In our study, 3 Enterococcus faecalis clinical isolates and E. faecalis ATCC29212 were evolved resistant to 8 kinds of antimicrobials spanning five chemical classes, including ciprofloxacin, levofloxacin, gatifloxacin, penicillin, imipenem, vancomycin, chloramphenicol and gentamicin. After 10 passages for 40 days, strains exhibited high level resistance to selected drugs except for imipenem. The greatest increase was observed in those evolved to quinolones, which caused >256-fold increase in MIC compared to the wild type. Cross-resistance and collateral-sensitivity were widely found after evolution. Through genotypic analysis of quinolones resistance strains, amino acid changes were observed in the QRDR region of GyrA and ParC. Substitutions occurred at GyrA were detected as Ser84Asn/Ser84Ile/Ser84Arg/Gly106Asp. However, Substitutions in ParC were found as Ser82Ile/Glu86Lys/Glu86Gly/His105Tyr. Compared with ancestral strains, the growth rates of evolved resistant strains slowed down and the logarithmic phase was delayed >7 h. While, the biofilm formation capacity of strains was not significantly changed by evolutionary adaptation. Our data verified that long-term exposure to sub-lethal concentrations of antimicrobial agents could selectively enrich drug resistant mutant of E. faecalis, conferring to cross-resistance and collateral-sensitivity towards other antimicrobials and fitness costs. Defining these effects can provide alternative antimicrobial strategies directed to mitigate the selection of antibiotic resistant microbes.