Stability, persistence, and evolution of plasmid-encoded VanA glycopeptide resistance in enterococci in the absence of antibiotic selection in vitro and in gnotobiotic mice

Long-term persistence of VanA glycopeptide-resistant enterococci (GRE) has been observed in the absence of antibiotic selection. In the present study, we examined fitness parameters of a glycopeptide-susceptible Enterococcus faecium parent strain and its plasmid-mediated, VanA-resistant derivative before and after 1,000 generations in serial transfer broth cultures with or without antibiotic selection. With the exception of the vanA-containing plasmid, the strains were otherwise isogenic. The stability of the plasmid-encoded vanA resistance determinant was also investigated in vitro and in gnotobiotic mice. Competition experiments revealed that GRE with newly acquired VanA resistance had a 4% reduction in fitness relative to their susceptible parental counterpart. The relative difference in competitive fitness between resistant and susceptible strains was not significantly changed after 1,000 generations. Environmental adaptation was observed in all strains and exceeded the biological cost of resistance. Thus, the evolved VanA-resistant E. faecium populations outnumbered their unevolved ancestral susceptible E. faecium strain in mixed cultures, but remained less competitive than the evolved parent. The glycopeptide resistance determinant was similarly stably maintained during long-term colonization in gnotobiotic mice without antibiotic selection. In vivo vanA plasmid transfer was observed. The results suggest that environmental adaptation, in vivo gene transfer, and plasmid maintenance system(s) favor long-term VanA GRE persistence without antibiotic selection and compensate for the biological costs of possessing the resistance genes.