Lytic bacteriophages reduce Escherichia coli O 157 : H 7 on fresh cut lettuce introduced through cross - contamination

Two outbreaks of Escherichia coli O157:H7 infections in 2006 associated with the consumption of bagged baby spinach and bagged shredded lettuce were collectively responsible for sickening 254 people and causing 3 deaths. Romaine lettuce contaminated with E. coli O157:H7, grown in California, was responsible for causing a multi-state outbreak of 60 infections in 2011. A previous multi-state outbreak associated with E. coli O145 and contaminated Romaine lettuce grown in Arizona resulted in 34 cases of foodborne illness. Bagged spinach and shredded lettuce are especially vulnerable to bacterial contamination due to the lack of a kill-step during their harvesting, processing and packaging. Concentrations of chemical disinfectants like sodium hypochlorite used during the processing of fresh cut leafy green commodities may not be sufficient to kill foodborne pathogens attached to intact or cut surfaces of leafy greens, and are primarily used to prevent cross contamination of leafy greens through water. Outbreaks of foodborne illness associated with the consumption of leafy greens emphasize the need for an effective natural The role of lytic bacteriophages in preventing cross contamination of produce has not been evaluated. A cocktail of three lytic phages specific for E. coli O157:H7 (EcoShieldTM) or a control (phosphate buffered saline, PBS) was applied to lettuce by either; (1) immersion of lettuce in 500 ml of EcoShieldTM 8.3 log PFU/ml or 9.8 log PFU/ml for up to 2 min before inoculation with E. coli O157:H7; (2) spray-application of EcoShieldTM (9.3 log PFU/ml) to lettuce after inoculation with E. coli O157:H7 (4.10 CFU/cm2) following exposure to 50 μg/ml chlorine for 30 sec. After immersion studies, lettuce was spot-inoculated with E. coli O157:H7 (2.38 CFU/cm2). Phage-treated, inoculated lettuce pieces were stored at 4°C for and analyzed for E. coli O157:H7 populations for up to 7 d. Immersion of lettuce in 9.8 log PFU/ml EcoShieldTM for 2 min significantly (p < 0.05) reduced E. coli O157:H7 populations after 24 h when stored at 4°C compared with controls. Immersion of lettuce in suspensions containing high concentrations of EcoShieldTM (9.8 log PFU/ml) resulted in the deposition of high concentrations (7.8 log log PFU/cm2) of bacteriophages on the surface of fresh cut lettuce, potentially contributing to the efficacy of the lytic phages on lettuce. Spraying phages on to inoculated fresh cut lettuce after being washed in hypochlorite solution was significantly more effective in reducing E. coli O157:H7 populations (2.22 log CFU/cm2) on day 0 compared with control treatments (4.10 log CFU/cm2). Both immersion and spray treatments provided protection from E. coli O157:H7 contamination on lettuce, but spray application of lytic bacteriophages to lettuce was more effective in immediately reducing E. coli O157:H7 populations fresh cut lettuce. Lytic bacteriophages reduce Escherichia coli O157:H7 on fresh cut lettuce introduced through cross-contamination