Bacterial loss from biofilms exposed to free chlorine

Previous studies have demonstrated the increased resistance of biofilm bact eria 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 demonstra te that the loss of sloughed bacteria (including Escherichia coli as a path ogen 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, signifi cant bacterial loss into the planktonic phase still occurred with free chlo rine concentrations below 3.0 mg l(-1), indicating active growth and slough ing of biofilm organisms. Exposure of the biofilm to higher levels of free chlorine resulted in inhibition of bacterial loss, though biocide removal w as accompanied by a rapid, almost immediate, recovery of sloughing ability. This work demonstrates the endurance and speed of biofilm recovery upon qu enching of chlorine residuals, highlighting a potentially significant publi c health risk from biofilm recovery and the sloughing of pathogenic organis ms associated with the biofilm.