Recovery of Chlorine-Exposed Escherichia coli in Estuarine Microcosms

Chlorination is one of the most widely used forms of disinfection to treat wastewater effluent. Studies have shown, however, that exposure to chlorine may result in cellular injury (defined as the inability to culture viable cells on commonly used selective growth media), rather than death, for fecal-borne bacteria. In addition, a small number of bacteria pass through the disinfection process relatively unharmed. Microbial contamination of aquatic habitats by wastewater effluent therefore may occur by the reacquisition of culturability by injured cells and/or the growth of the few cells that escaped the chlorination process unharmed. Laboratory microcosm experiments were performed to determine to what extent conditions found within estuarine waters are sufficient for chlorine-exposed Escherichia coli to recover (i.e., increase in numbers of culturable cells due to reacquisition of culturability and/or growth) and the water quality parameters responsible for this recovery. Suspensions of E. coli were exposed to 0.5 mg/L of chlorine for 5 minutes followed by dechlorination with sodium thiosulfate. The chlorine-exposed bacteria were introduced into nine 2-L microcosms each containing estuarine water collected from a different location in the Seacoast region of New Hampshire. The number of culturable cells in the microcosms was enumerated at 0, 10, 24, 48, and 74 hours. In all estuarine microcosms the number of culturable cells increased by factors ranging from 40 to 170 over the 74-hour incubation period. Linear regression analyses indicated that dissolved organic carbon followed by total dissolved nitrogen and ammonium were most significantly correlated with the amount of recovery of E. coli over the 74-hour incubation period. The extensive recovery observed in our study indicates that chlorine-exposed E. coli may increase significantly in numbers when discharged into estuarine waters posing a water quality and public health risk and that the regular monitoring of wastewater treatment effluent may underestimate its true impact on estuarine water quality.