Survivability of vancomycin resistant enterococci and fitness cost of vancomycin resistance acquisition

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Abstract
Aims: To investigate the survivability of vancomycin resistant enterococci (VRE) under dry starvation conditions and the fitness cost of vancomycin resistance. Methods: VRE colonies on cotton swabs were incubated at room temperature in a sterile box and cultured weekly until cultures no longer showed growth. Negative swabs inoculated into brain heart infusion (BHI) broth were subcultured to blood agar after 24, 48, and 72 hours of incubation to resuscitate viable but non-culturable cells. Stability of the vancomycin resistance determinant and of the DNA fingerprint pattern was determined by multiplex polymerase chain reaction (PCR) and repetitive PCR, respectively. Tests for fitness cost were carried out on the same VRE isolates and 28 hospital vancomycin sensitive enterococci (VSE) isolates by incubation and measurement of optical density using a microplate reader and comparing maximum growth rate and lag phase duration between VRE and VSE, using independent samples t tests. Results: Mean maximum time of recovery by primary culture was 8.5 weeks for Enterococcus faecalis VRE and 21.8 weeks for E faecium VRE. Two of two E faecalis isolates were resuscitated after 24 hours in BHI broth, and two of five E faecium isolates after 72 hours. No fitness cost of vancomycin resistance was demonstrated. Conclusions: VRE can survive for prolonged periods in a dry starvation state, retaining their genetic complement, including vancomycin resistance determinants, and show little or no fitness cost of vancomycin resistance. Thus, the rate of entry required for VRE to become, and remain, endemic in the community is relatively small.

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