Natural waters are treated by various chemical and physical processes to improve their quality and to prevent disease spread. Among the last stages of water treatment is disinfection (the only treatment in Israel). This process is carried out by application of chlorine, chloramine or chlorine dioxide in order to inactivate bacteria, viruses and other single cell organisms found in water systems. Survival of some of these bacteria or microorganisms following disinfection is still a serious problem. Bacteria were isolated from disinfected water with residual free chlorine of nearly 1mg/L. The present study analysed the composition of cellular fatty acids of coliform bacteria such as Klebsiella oxytoca, Escherichia coli, Citrobacter freundii and Enterobacter taylorae exposed to different environmental conditions, including disinfection, and the effect of biochemical changes on their survival potential. Nutrition (high and low), temperature (6°C and 35°C) and aggregation (0.45–8μm sized aggregates) were tested with selected coliforms for their impact on chlorine resistance and cellular fatty acids alteration. Nutrition effect on the fatty acid composition of these bacteria showed a variable pattern with each bacterial strain; nevertheless bacteria grown in diluted medium displayed higher resistance to chlorine. Whilst temperature effect results showed that bacteria grown at lower temperatures are more resistant to chlorination, fatty acids analysis failed to reveal a constant pattern and variability was found among the bacterial strains tested. Bacterial species that formed aggregates in the water environment were more resistant to chlorine. The results of this study indicate that several processes are involved in bacterial resistance/adaptation to chlorine stress and together they form a barrier against external damages caused by chemical disinfection. The implication of these results on future disinfection practices is discussed.

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