listeria monocytogenes, a food-borne pathogen associated with severe disease, forms resistant biofilms which enable it to persist in food processing environments. recently, surface active compounds and other microbially secreted substances have been reported to antagonize biofilm formation (irie et al. 2005. fems microbiol lett 250, 237-243; valle et al. 2006. proc. natl. acad. sci. u.s.a. 103, 12558-12563). these findings led us to investigate the use of secreted substances as a method for listeria biofilm reduction. we screened isolates obtained from paper mill slimes for novel surfactant producing organisms, but were unable to find an isolate of interest. we then turned our investigation to the interactions between pseudomonads and listeria. conflicting reports regarding the influence of pseudomonads on the growth of listeria biofilms led us to investigate the effect of substances secreted by pseudomonas putida and pseudomonas aeruginosa pao1 on l. monocytogenes egde biofilms. listeria biofilms were grown on stainless steel and polystyrene at 22, 30, and 37°c and exposed to conditioned medium prepared from either p. putida or p. aeruginosa. p. putida was found to secrete substances that significantly enhance listeria attachment at 37°c. in contrast, p. aeruginosa was found to inhibit biofilm formation and disperse mature biofilms at temperatures < 30°c. treatment with p. aeruginosa conditioned medium reduced biofilms on stainless steel by 1.65 log cfu/cm2 and virtually eliminated all biofilm on polystyrene. the influence of temperature suggests that this reduction in biofilm formation may involve the flagella of listeria.