Bacterial loss from biofilms exposed to free chlorine

Previous studies have demonstrated the increased resistance of biofilm bacteria to biocides when compared with freely suspended planktonic cells, but there have been few investigations of sloughing and losses from biofilms in response to biocide activity. A model biofilm system was used to demonstrate that the loss of sloughed bacteria (including Escherichia coli as a pathogen indicator organism) from actively growing biofilm reached levels of 10(6)-10(7) CFU ml-1 of eluted medium. The sloughing response was examined in response to a range of free chlorine concentrations from 0.6 to 5.0 mg l-1. While chlorine treatment greatly reduced bacterial sloughing, significant bacterial loss into the planktonic phase still occurred with free chlorine concentrations below 3.0 mg l-1, indicating active growth and sloughing of biofilm organisms. Exposure of the biofilm to higher levels of free chlorine resulted in inhibition of bacterial loss, though biocide removal was accompanied by a rapid, almost immediate, recovery of sloughing ability. This work demonstrates the endurance and speed of biofilm recovery upon quenching of chlorine residuals, highlighting a potentially significant public health risk from biofilm recovery and the sloughing of pathogenic organisms associated with the biofilm.