Different persistence among strains of E. faecalis and E. faecium in sterile treated wastewater microcosms - effects of temperature and Ciprofloxacin

Intestinal enterococci are zoonotic bacteria found in various environments and they hold medical significance due to their inherent and acquired resistance. Significant quantities of enterococci are consistently discharged into aquatic environments through wastewater treatment plants (WWTPs). In this study, the culturability of several strains of Enterococcus faecium and Enterococcus faecalis in sterilized treated wastewater at two temperatures or with the addition of low and high concentrations of ciprofloxacin (CIP) was evaluated. All strains remained culturable throughout the whole studied period (108 days), but E. faecalis showed a slightly lower culturability. The culturability of E. faecalis and the majority of E. faecium strains remained unaffected by the presence of CIP, although certain E. faecium strains exhibited a negative impact. We conclude that E. faecalis and E. faecium have the potential to form persister cells that will survive in treated wastewater, and that the response to different temperatures as well as CIP concentrations was species- and strain-specific. This is important to take into consideration when designing experiments that aim at evaluating the risk of WWTPs as a source of human pathogens, resistant bacteria, and resistance genes.