Escherichia coli and Enterococcus spp. Indigenous to Wastewater Have Slower Free Chlorine Disinfection Rates than Their Laboratory-Cultured Counterparts

Most published data on chlorine disinfection of bacteria are from experiments conducted using reference-strain bacteria cultured in a laboratory. However, indigenous environmental bacteria, such as those in wastewater, can be more resistant to disinfection than their laboratory-cultured counterparts. To investigate this phenomenon, we conducted controlled experiments to systematically quantify and compare free chlorine inactivation kinetics of laboratory-cultured Escherichia coli and Enterococcus faecalis to corresponding E. coli and enterococci sourced from wastewater, without confounding factors related to the sample matrix. To allow direct comparison between bacterial populations, dissolved and particulate constituents of the sample matrices that could influence disinfection kinetics were removed using sequential centrifugation steps prior to disinfection experiments. The first-order chlorine inactivation rate constants of laboratory-cultured E. coli (k = 18.6 L mg-1min-1) and E. faecalis (k = 12.7 L mg-1min-1) were over an order of magnitude greater than those of wastewater-sourced E. coli (k = 0.65 L mg-1min-1) and enterococci (k = 0.18 L mg-1min-1) in PBS. These results indicate that wastewater bacteria were less susceptible to free chlorine inactivation than corresponding laboratory-cultured bacteria. Results from control experiments suggest that the observed differences in disinfection rates were due to cell-related differences between the bacterial populations and not caused by matrix effects, aggregation, or purification procedures utilized.