Reduced susceptibility of thin Pseudomonas aeruginosa biofilms to hydrogenperoxide and monochloramine

Pseudomonas aeruginosa attached to alginate gel beads in sparse, thin biofi lms exhibited reduced susceptibility to monochloramine and hydrogen peroxid e compared with planktonic cells of the same micro-organism. Disinfection r ate coefficients for planktonic bacteria averaged 0.55 l mg(-1) min(-1) for monochloramine and 3.1 x 10(-4) l mg(-1) min(-1) for hydrogen peroxide. Th e corresponding values for 24-h-old biofilm cells were 0.29 l mg min(-1) an d 9.2 x 10(-5) l mg(-1) min(-1) for monochloramine and hydrogen peroxide, r espectively. Several pieces of evidence support the interpretation that the reduced susceptibility of biofilm was not due simply to inadequate deliver y of the antimicrobial agent to the local environment of the attached cells . No correlation between biofilm susceptibility and biofilm initial areal c ell density was observed. Rapid delivery of hydrogen peroxide to the attach ment surface, and subsequently to the interior, of the alginate gel beads w as visualized by a direct experimental technique. Theoretical analysis of u nsteady diffusion and diffusion-reaction interactions also argued against a ny significant delay or barrier to antimicrobial or oxygen delivery. It was hypothesized that new genes are expressed when bacteria attach to a surfac e and begin to form a biofilm and that some of the resulting gene products reduce the susceptibility of the cell to antimicrobial agents including oxi dative biocides such as monochloramine and hydrogen peroxide.