AN INVESTIGATION OF THE EFFICACY OF A BROMINE-CONTAINING BIOCIDE ON AN AQUATIC CONSORTIUM OF PLANKTONIC AND BIOFILM MICROORGANISMS INCLUDING LEGIONELLA-PNEUMOPHILA

A biocide (1-bromo-3-chloro-5.5-dimethylhydantoin), was tested for eff icacy against planktonic and biofilm bacteria in a laboratory model to simulate a water system. BCDMH was continually added to the continuou s culture chemostat which was operated using an artificially softened water to simulate conditions within a cooling tower. Biofilm developed on stainless steel and mild steel tiles suspended in the culture. The consortium of bacteria growing in the chemostat model included legion ellae, pseudomonads, methylobacteria and actinomycetes. Flavobacterium , Alcaligenes and Achromobacter spp. which were grouped together as Gr am negative (GN) bacteria were also present. At biocide concentrations of 1 or 2 mg.1(-1) bacteria in the planktonic phase were dramatically reduced in the culture whereas only a 1 log drop in viable bacteria w as detected in the biofilm. When the concentration of BCDMH was increa sed to 4 or 6 mg.1(-1) a 3 log reduction was observed in the number of viable bacteria recovered from the biofilm. This indicates that bacte ria contained within a biofilm are more refractory to attack from anti -microbial agents such as BCDMH. Significantly, legionellae were more susceptible to the biocide than other members of the microbial consort ium: legionellae were not recovered from either biofilm of planktonic phases of the model system at BCDMH concentrations of 1 mg.1(-1).