Susceptibility of Enterococcus spp. to chlorhexidine and Virkon® S: Effect of environmental factors, adaptation and implications for antimicrobial resistance

Enterococcus spp. are potential hospital pathogens known for their intrinsic resistance to many antibiotics and resilience to many environmental factors. Biocides are frequently used to eliminate these bacteria from surfaces within the hospital environment, yet reports of decreased susceptibility to these compounds and possible antibiotic cross-resistance have become sources of concern. In this study, the susceptibility of 64 enterococci isolates to two biocides frequently used in clinical settings, Virkon® S and chlorhexidine, was tested in the absence and presence of two levels of organic matter. This collection was composed of veterinary isolates from a hospital environment (n = 34), veterinary clinical isolates (n = 10), veterinary commensal isolates (n = 10) and human clinical isolates (n = 10). Furthermore, bacterial adaptability to these biocides and possible cross-resistance with antibiotics was also explored, with a special emphasis towards efflux activity. A higher direct loss of efficiency due to the presence of organic matter was observed for Virkon® S, while chlorhexidine's efficacy showed a more isolate-dependant variation. Verapamil proved to be an efficient inhibitor for chlorhexidine efflux in some of the tested isolates. Adaptability to this biocide was also associated with a potential development of resistance to doxycycline, gentamycin and streptomycin, although the associated mechanisms responsible for this phenomenon still need to be elucidated.