Aims: To determine the effectiveness of an alkaline cleaner used in food-processing plants and a lytic bacteriophage specific for Escherichia coli O157:H7 in killing wild type and r poS-deficient cells of the pathogen in a biofilm. Methods and Results: Wild type and rpoS-deficient cells were attached to stainless steel coupons (c. 7-8 log CFU per coupon) on which biofilms were developed during incubation at 22 degrees C for 96 h in M9 minimal salts media (MSM) with one transfer to fresh medium. Coupons were treated with 100 and 25% working concentrations of a commercial alkaline cleaner (pH 11-9, with 100 mu g ml(-1) free chlorine) used in the food industry, chlorine solutions (50 and 100 mu g ml(-1) free chlorine), or sterile delonized water (control) at VC for 1 and 3 min. Treatment with 100% alkaline cleaners reduced populations by 5-6 log CFU per coupon, a significant (P <= 0-05) reduction compared with treatment with water. Initial populations (2-6 log CFU per coupon) of attached cells of both strains were reduced by 1-2 log CFU per coupon when treated with bacteriophage KH1 (7-7 log PFU ml(-1)) for up to 4 days at 4 degrees C. Biofilms containing low populations (2-7-2-8 log CFU per coupon) of wild type and rpoS-deficient cells that had developed for 24 h at 22 degrees C were not decreased by more than 1 log CFU pe;coupon when treated with KH1 (7-5 log PFU ml-1) at VC. Conclusions: Higher numbers of cells of E. coliO157:H7 in biofilms are killed by treatment with an alkaline cleaner than with hypochlorite alone, possibly through a synergistic mechanism of alkaline pH and hypochlorite. Populations of cells attached on coupons were reduced by treating with bacteriophage but cells enmeshed in blofilms were protected. Significance and Impact of the Study: The alkaline pH, in combination with hypochlorite, in a commercial cleaner is responsible for killing E. coliO157:H7 in b1ofilms. Treatment with bacteriophage KH1 reduces populations of cells attached to coupon surfaces but not cells in blofilms.