The possible development of resistance of bacteria found dominant in i
ndustrial water systems to the oxidising biocides hypochlorous acid an
d 3-bromo-1-chloro-5,5-dimethylhydantoin was investigated. Pseudomonas
aeruginosa, P. stutzeri and Bacillus cereus were cultured repeatedly
in the presence of sub-inhibitory concentrations of hypochlorous acid
and of 3-bromo-1-chloro-5,5-dimethylhydantoin. The minimum inhibitory
concentrations (MIC) of hypochlorous acid and of 3-bromo-1-chloro-5,5-
dimethylhydantoin decreased following initial exposure, but varied gre
atly during the period of the investigation. The MICs did stabilise to
wards the end of the study. Whereas the isolates used did not display
classical resistance, they did respond variably to successive exposure
, indicating that long-term treatment of water systems with hypochloro
us acid or 3-bromo-1-chloro-5,5-dimethylhydantoin would yield variable
degrees of control of microbial activity.