Amoebae in domestic water systems: resistance to disinfection
treatments and implication in Legionella persistence.
Department of Public Health-Environment, School of
Pharmacy, University of Paris, Chatenay-Malabry Cedex, France.
AIMS: Monitoring of microbial changes during and after
application of various disinfection treatments in a model domestic water
system. METHODS AND RESULTS: A pilot-scale domestic water system
consisting of seven galvanized steel re-circulation loops and copper
dead legs was constructed. Culture techniques, confocal laser scanning
microscopy after fluorescent in situ hybridization and viability
staining with the BacLight LIVE/DEAD kit were used for planktonic and
biofilm flora monitoring. Before starting the treatments, the system was
highly contaminated with Legionella pneumophila and biofilm populations
mainly consisted of beta-proteobacteria. In the water and the biofilm of
the loops, continuous application of chlorine dioxide (0.5 mg l(-1)), or
chlorine (2.5 mg l(-1)) were very effective in reducing the microbial
flora, including L. pneumophila. Heterotrophic bacteria, although
strongly reduced, were still detectable after ozone application (0.5 mg
l(-1)), whereas with monochloramine (0.5 mg l(-1)) and copper-silver
ionization (0.8/0.02 mg l(-1)), the contamination remained significantly
higher. Monochloramine and copper-silver did not remove the biofilm.
During copper-silver application, Legionella re-growth was observed.
Only chlorine dioxide led to detectable effects in the dead leg. Amoebae
could not be eliminated, and after interrupting the treatments, L.
pneumophila quickly recovered their initial levels, in all cases.
CONCLUSIONS: Chlorine dioxide, applied as a continuous treatment, was
identified in this study as the most efficient for controlling L.
pneumophila in a domestic water system. Chlorine dioxide showed a longer
residual activity, leading to improved performance in the dead leg.
Amoebae resisted to all the treatments applied and probably acted as
reservoirs for L. pneumophila, allowing a quick re-colonization of the
system once the treatments were interrupted. SIGNIFICANCE AND IMPACT OF
THE STUDY: Control of microbial contamination requires maintenance of a
constant disinfectant residual throughout the water system. Treatment
strategies targeting free-living amoebae should lead to improved control
of L. pneumophila. Such treatment strategies still have to be
PMID: 15479410 [PubMed - indexed for MEDLINE]