The extent of coliform injury after chlorination and the relationship between regrowth of injured coliforms and assimilable organic carbon concentrations (AOC) were studied. Seeded (Escherichia coli E25), sterilized tap water (AOC concentration adjusted to 400mg C/l) was incubated for a period of 3 weeks (25-28°C) after chlorination (0.5 and 1mg/l). Bacteria were enumerated using m-Endo agar LES and m-T7 agar. Chlorination resulted in a 3 log reduction in coliform counts within one minute. No viable colonies were found for 27% of the water samples enumerated with m-Endo agar LES. Injured coliforms were enumerated from all the water samples except one and were present in higher numbers than non-injured coliforms in the majority of the water samples. During the second and third weeks of incubation, only injured bacteria were observed in chlorinated samples. Additional AOC contributed little to the survival and regrowth of chlorinated bacteria.

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