Distribution pipe biofilms present a currently unquantified public health risk to consumers receiving water for domestic potable and non-potable use. The aim of this study was to quantify the numbers of legionellae, used here as model bacterial pathogens, that may accumulate, persist within and detach from distribution pipe biofilms. L. pneumophila recovered by standard culture from an 8 week-old biofilm formed within a novel pilot-scale water distribution system represented 1% of those present in the adjacent bulk water. A combined chlorine concentration exceeding 0.2 mg.L-1 eliminated culturable sessile legionellae altogether, though the reduction in FISH-positive cells represented just 75 ± 25% of the original amount, compared to a 5-log reduction in culturable cells during the same period. Where there was < 0.1 mg.L-1 combined chlorine, an exponential decay/loss of sessile L. pneumophila was observed (k = 0.37 - 0.41) over the course of a 38-day experimental period. The inoculation of the system with 1 μm fluorescent microspheres and legionellae demonstrated that removal of the latter was dominated by chemical disinfection, with erosion and biological grazing playing lesser roles. Under turbulent (Re ∼5000) conditions, larger clusters of biofilm become detached from substrata, with more than 90% of sessile legionellae mobilised into the bulk water phase. Interaction with both biofilms and a thermophilic Acanthamoeba isolate reduced the susceptibility of legionellae to thermal inactivation by between one and two orders of magnitude, though it increased their sensitivity to chemical (free and combined chlorine) disinfection.

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