“The maintenance of a chlorine or chloramine residual to suppress waterborne pathogens in drinking water distribution systems is common practice in the United States, but less common in Europe. In this study, we investigated the occurrence of Bacteria and Legionella in water-main biofilms and tap water. We tested samples from a chloraminated distribution system in the United States and a system in Norway with no residual. We used a real-time quantitative polymerase chain reaction (qPCR). Despite generally higher temperatures and assimilable organic carbon levels in the chloraminated system, we found that total Bacteria and Legionella were significantly lower in water-main biofilms and tap water of that system (p < 0.05). Legionella were not detected in the biofilms of the chloraminated system (0 of 35 samples). They were frequently detected in biofilms from the no-residual system (10 of 23 samples; maximum concentration = 7.8×104 gene copies cm−2). This investigation suggests water-main biofilms may serve as a source of legionellae for tap water and premise plumbing systems. Residual chloramine may aid in reducing their abundance. ” From: Occurrence of Legionella spp. in Water-Main Biofilms from Two Drinking Water Distribution Systems. Michael Waak, et al. Environ. Sci. Technol. June 14, 2018
Growth of Legionella in biofilms can be reduced if the amount of assimilable organic carbon (AOC) in the water is controlled. Legionella bacteria in ground water enter municipal water treatment facilities and exit to distributed potable water systems in low, often not detectable, numbers. When conditions are suitable at point of use, Legionella multiply in biofilms. Legionella sloughing off biofilms can spread to people by aerosol from warm water-containing mechanical equipment such as cooling towers, showers, misters, and fountains.
Controlling Legionella growth in biofilms can reduce risk of legionellosis. Legionella biofilms are enhanced if assimilable organic carbon (AOC) is present in drinking water distribution systems. AOC is defined as the “…fraction of labile dissolved organic carbon that is more easily assimilated by microorganisms than other types of organic carbon. AOC consists of a broad range of low-molecular-weight organic carbon molecules, such as sugars, organic acids, and amino acids.” From: “Effects of Assimilable Organic Carbon and Free Chlorine on Bacterial Growth in Drinking Water”. X Liu et al. June 2015. PLoS One 10(6): e0128825.
Reducing assimilable organic carbon (AOC) in aerobic potable water-supplied systems slows growth of Legionella bacteria in biofilms. “The biofilm on glass (average steady-state concentration, 23 +/- 9 pg ATP cm-2) exposed to treated aerobic groundwater (0.3 mg C liter-1; 1 ug assimilable organic carbon [AOC] liter-1) did not support growth of the organism, which also disappeared from the biofilm on chlorinated polyvinyl chloride (CPVC) (49 +/- 9 pg ATP cm-2) after initial growth.” In contrast, at 10-fold higher AOC levels, “L. pneumophila attained a level of 4.3 log CFU cm-2 in the biofilms on glass (1,055 +/- 225 pg ATP cm-2) and chlorinated polyvinyl chloride (CPVC) (2,755 +/- 460 pg ATP cm-2) exposed to treated anaerobic groundwater (7.9 mg C liter-1; 10 ug AOC liter-1)”. From: “Biofilm composition and threshold concentration for growth of Legionella pneumophila on surfaces exposed to flowing warm tap water without disinfectant”. van der Kooij et al. March 2017. Appl Environ Microbiol 83:e02737-16.