Persistence of chlorine-sensitive Legionella pneumophila in hyperchlorinated installations.

Aims:  To study the persistence of Legionella over time in different disinfected facilities and analysing whether failures in bacterial eradication could be the result of a decrease in the susceptibility of the persistent strains to subsequent treatments. Methods and Results:  A long-term environmental surveillance was carried out in three different facilities associated with cases of Legionnaires’ disease (a hospital, a fishing boat and a hotel). Despite several hyperchlorination episodes, amplified fragment length polymorphism, pulsed field gel electrophoresis-SfiI and arbitrarily primed polymerase chain reaction methods demonstrated that a specific clone of L. pneumophila serogroup 1 was able to survive for 17, 5 and 10 years in the hospital, fishing boat and the hotel, respectively. Isolates from different years from the same facility showed similar minimal inhibitory concentration and minimal bactericidal concentration values against eight different disinfectants. Conclusions:  Hyperchlorination over long periods of time did not prevent the persistence of L. pneumophila. The lack of effectiveness did not appear to be the result of a decreased susceptibility of Legionella to chlorine. Hyperchlorination did not modify in vitro susceptibility of Legionella to other disinfectants to which the bacteria had not previously been exposed. Significance and Impact of the Study:  Persistent Legionella in treated installations remain sensitive to disinfectants; hence, new strategies of treatment, different from hyperchlorination, should be developed to achieve bacterial eradication.