Survival of hospital- and community-associated Enterococcus faecium following exposure to in-use concentrations of the biocide sodium dichloroisocyanurate (NaDCC).

Objectives: Hospital-associated infections with vancomycin-resistant Enterococcus faecium (VREfm) have increased dramatically in Denmark. A cornerstone in infection control is effective cleaning and disinfection. This study investigated the survival and resuscitation/growth of clinical isolates of E. faecium exposed to the chlorine-releasing disinfectant, sodium dichloroisocyanurate plus detergent (NaDCC Plus).

Methods: To assess biocide efficacy, we modified a method developed to characterise the dose-time-response of bacteria to antibiotics. E. faecium isolates (n = 59) were screened in 96-well plates containing 50-1400 ppm free available chlorine. Bacteria were exposed for 10 min, after which the biocide was inactivated with a neutralizer. Cells were collected by centrifugation, new broth added, and after 20-22 h, viability was recorded as growth/no growth. For a subset of strains the impact of shorter biocide exposure times were examined, as was the influence of longer incubation times.

Results: E. faecium survived exposure to relatively high concentrations of NaDCC Plus, average 415 ppm of free available chlorine (SD ± 78 ppm), compared to recommended in-use concentration (1000 ppm). "Outbreak" clones did not prove more tolerant to NaDCC Plus compared to other VREfm clones, hospital-associated vancomycin-susceptible E. faecium (VSEfm) or community-associated VSEfm. Shorter exposure time and extended incubation time in broth both significantly increased the concentration needed to eradicate E. faecium, with some isolates surviving higher concentrations than the recommended in-use concentration.

Conclusion: Our results indicate that if an exposure time of 10 min is not achieved, the efficacy of the disinfectant will not be sufficient.