Effect of growth conditions on inactivation of Escherichia coli with monochloramine.

Reduced susceptibility of bacteria to disinfection is a serious concern in drinking water distribution systems (DWDS), yet the mechanisms and conditions governing reduced susceptibility are not well characterized. The effects of growth temperature, growth rate, and growth mode (suspended growth versus growth in biofilms) on inactivation kinetics of Escherichia coli exposed to monochloramine were studied in order to understand growth conditions that may reduce susceptibility of bacteria to disinfectants in DWDS. Cells grown at a suboptimal temperature (20 degrees C) were significantly less sensitive to monochloramine inactivation (using 0.5 and 5.0 mg/L monochloramine (as Cl2)) than cells grown at an optimal temperature (37 degrees C). Cells grown in biofilms were also significantly less sensitive than cells grown in suspension. No difference in inactivation kinetics was observed for cells grown in monolayer versus multilayer biofilms and between cells grown at different growth rates in chemostat bioreactors. Biofilm cells were estimated to grow at specific growth rates (mu) averaging between mu = 0.08 and 0.13 h(-1), which were approximately within the range of tested suspended growth conditions (mu = 0.04-0.10 h(-1)) using fluorescence in situ hybridizations targeting 16S rRNA. This result indicates that the reduced susceptibility of biofilm cells to monochloramine inactivation is not related to their specific growth rate within the range tested in this study. This work suggests that growth at suboptimal temperatures and growth in biofilms are important factors contributing to reduced susceptibility of bacteria to inactivation with monochloramine.