Efficacy of Pseudomonas fluorescens for biocontrol of Escherichia coli O157:H7 on spinach

Control of foodborne pathogens on leafy green vegetables is crucial for consumer food safety. Biocontrol microbes may inhibit or suppress foodborne pathogen growth and enhance existing post-harvest intervention controls. The efficacy of Pseudomonas fluorescens for biocontrol of Escherichia coli O157:H7, a human pathogen, was determined on baby spinach. P. fluorescens strains 2-79, Q287, and Q8R-1 (non-pectolytic and non plant pathogenic) were applied on spinach using the dip inoculation method, prior to inoculation with E. coli O157: H7 strains 43894, 43895, and 35150 individually or as a cocktail mixture. The inoculated spinach was stored at 20 degrees C for 24 and 48 hours, and then evaluated for residual populations of E. coli O157:H7 using Restaino and Frampton E. coli O157:H7 chromogenic medium. In subsequent experiments, the effects of storage time (24 and 48 h) and temperatures (5, 10, 15, 20, 25, and 30 degrees C) on the efficacy of biocontrol were assessed. The reduction of E. coli O157:H7 by P. fluorescens was computed relative to the control. The mean reduction of E. coli O157:H7 by P. fluorescens ranged from 0.5-2.1 log CFU/g of spinach. The efficacy of biocontrol was significantly (P<0.05) affected by storage temperature as suppressive effects were greater at 15 degrees C (1.5-2.4 log CFU/g of spinach) than at other temperatures (<0.93 log CFU/g). As expected, the reduction of E. coli O157:H7 populations varied between storage times of 24 and 48 h. These results imply that when the ratios of biocontrol to pathogen is relatively equal, P. fluorescens could provide low to moderate reductions of E. coli O157:H7 populations on spinach.