The role of biofilm and slime formation and the effect of chlorine, monochloramine, hydrogen peroxide and silver on Pseudomonas aeruginosa

Pseudomonas aeruginosa represents a model for a hygienically relevant organism which can be isolated from technical water systems. It could be demonstrated that the cells, having formed a biofilm, were significantly less susceptible to chlorine as a disinfectant compared to monochloramine. The elevated chlorine tolerance was attributed to the formation of protective extracellular polymeric substances including the polysaccharide alginate which was shown to chemically react with chlorine. In order to investigate further disinfectants, the effect of hydrogen peroxide and silver ions was compared on slime-forming (mucoid) and isogenic nonmucoid strains of P. aeruginosa. The mucoid bacteria were slightly more susceptible to hydrogen peroxide and/or silver ions than the nonmucoid bacteria. When applied in combination, hydrogen peroxide and silver ions displayed a weak synergistic effect on both mucoid and nonmucoid cells. The decrease of viable count was accompanied by a decrease of bacterial respiratory activity determined with the redox dye 5-cyano-2,3-di- tolyl-tetrazolium chloride (CTC), by an increase in the proportion of bacteria staining with the membrane-impermeable DNA-binding fluorochrome propidium iodide and by a decrease of the proportion of highly fluorescent cells after staining with the DNA-binding fluorochrome 4',6-diamidino-2-phe- nylindole (DAPI). These results indicated that the bactericidal agents interfered with cellular electron transport processes, enhanced cell membrane permeability and affected DNA structure. Slime-formation had no influence on bacterial response to hydrogen peroxide, since (i) slime removal by washing of mucoid cells did not alter their susceptibility- , and (ii) no chemical interaction was observed between hydrogen peroxide and purified alginate as the major slime component. Catalase activity seemed to be involved in the response of P. aeruginosa to hydrogen peroxide, since (i) the more resistant nonmucoid revertant strains revealed up to 3-fold higher specific catalase activities than the more sensitive mucoid parent strains, and (ii) catalase-deficient mutants derived from the mucoid strains revealed drastically enhanced sensitivity to hydrogen peroxide.