Effect of Growth Stage on Bacterial Resistance to Chlorine Disinfection

The mechanisms involved in microbial resistance to chlorine disinfection have not been fully elucidated. In this study, we investigate the impact of cell growth stage on the chlorine disinfection efficiency during water treatment. Specifically, we determine the role of growth stage on chlorination resistance and compare the bacterial inactivation behaviors at different growth stages for E coli strains K12 and O157:H7. The inactivation rate constants, based on Chick-Watson kinetic equations, were estimated and compared for stationary phase and those at lag and exponential phase. Our results show that growing both E. coli strains cells obtained from different growth stages exhibited lower inactivation efficiency at stationary phase compared to those from lag and exponential phase. This implies that microbes in wastewater treatment process with varying solids retention time may show different chlorine resistance. The coefficient n of dilution was the lowest at exponential phase, indicating that cell inactivation becomes more sensitive to disinfectant at stationary phase. Higher inactivation rates were observed for the pathogenic O157:H7 than for K12 at different stages of growth. The strain-to-strain variability in survivability to chlorine exposure at different growth stages needs to be considered when selecting indicator microorganisms for water quality monitoring.