P10: Biocidal Efficacy Studies with Legionella Grown in Amoeba

Legionellae spp, implicated in Legionnaire’s disease, have been detected in many environments including domestic water, cooling water and freshwater lakes, ponds, etc... Legionella spp. are unique in their ability to survive predation by amoeba that normally graze on biofilms, and can rapidly amplify within their natural protozoan host.  Such passages through amoeba confer additional physiological features in Legionella, making them more virulent. Such mature infectious forms employ similar mechanisms to multiply within mammalian alveolar macrophages leading to Legionellosis.  However very little information is available on the efficacies of biocides against Legionella cells grown in protozoa.  Our study focuses on control of Legionella, in its amoeba-amplified forms, by various biocide regimes commonly used in cooling waters. A new method, for growing high concentrations of Legionella pneumophila using Acanthamoeba polyphaga as the sole source of nutrition, has been developed.  Our studies show that, in order to achieve 6 log reductions in amoeba-amplified Legionella, all biocides tested require two to four fold higher dose than those for axenic Legionella. Oxidizing biocides, at conventional dose levels, failed to control the more virulent form of the organism. This may provide an explanation for the survival of Legionella spp. in cooling towers that receive biocides at levels that are inadequate for controlling these hardier forms.  In addition, these studies demonstrate the need to revisit the current biocide use practices in order to achieve the new and stringent requirements for Legionella control in industrial water systems.