Long-term persistence of a Legionella pneumophila RAPD sub-type possessing the mip gene in a municipal shower despite repeated cycles of chlorination

The ability of Legionella pneumophila to colonise domestic water systems is a primary cause of outbreaks of Legionnaire’s Disease in humans. World Health Organisation guidelines recommend that drinking water is chlorinated to between 2-5 mg L-1, but L. pneumophila is repeatedly isolated from chlorinated water systems, indicating that this treatment is not effective at preventing colonisation of the system. Current UK guidelines recommend a one-off treatment of 20-50 mg L-1 of free chlorine to remove the bacterium from the system. In this study we report on the persistence of Legionella pneumophila serogroup one in a domestic shower system despite repeated cycles of chlorination at 50 mg L-1 for one hour exposure time, over the course of two and a half years. These isolates were subjected to in vitro phenotypic analyses as well as PCR analysis for the mip gene that codes for toxin production, and RAPD typing was performed in order to ascertain whether the strains recovered on different occasions were in fact the same strain. In this paper we report that seven isolates of Legionella pneumophila recovered over a two-and-a-half year period are in-fact the same strain, indicating that the bacterium can persist despite repeated cycles of chlorination after each successive isolation.