Inactivation of environmental and reference strains of heterotrophic bacteria and Escherichia coli O157:H7 by free chlorine and monochloramine

Reference cultures from culture collections are often used in disinfection experiments instead of indigenous environmental bacteria, but they may not accurately represent the organisms found in drinking water distribution systems due to physiological differences. This may explain why disinfectant concentrations and contact times, determined under typical laboratory conditions, are not always sufficient to control microbial growth or survival in distribution systems. The objective of this study was to investigate the effect of chlorine or monochloramine disinfection on Escherichia coli O157:H7 and other heterotrophic bacteria obtained from a culture collection and strains isolated from various environments, including disinfected water. It was hypothesized that previous exposure of the environmental strains to sublethal concentrations of disinfectant may allow them to develop greater resistance. Accordingly, it was observed that environmental strains of E. coli O157:H7 were either equally or less susceptible to chlorine and monochloramine than the reference strain. In contrast however, environmental strains of Brevundimonas vesicularis, Pseudomonas fluorescens, and Sphingomonas paucimobilis were either equally or more susceptible to free chlorine or monochloramine than their corresponding reference strains. This was counterintuitive because the environmental strains were able to survive disinfection in the pipes from which they were isolated. It is hypothesized that upon culturing in the laboratory the environmental strains may have become susceptible to low levels of disinfectant. Other researchers have suggested that changes in culture conditions may impact disinfectant sensitivity by affecting cell permeability, cell composition, or growth rate. This emphasizes the importance of designing laboratory studies to mimic environmental conditions as closely as possible to accurately represent environmental inactivation kinetics, including proper handling of organisms (subculturing, media transfers, etc.) before their use in inactivation studies.