Survival and methods for detection and control of Escherichia coli O157:H7 on strawberry fruit

The survival and methods for detection and control of a foodborne human pathogen, Escherichia coli O157:H7, were studied on strawberry fruit. The bacterial populations increased during storage of fruit inoculated by dipping in E. coli inoculum (5 log CFU/mL) at 23°C. Placing a 25-μL droplet (4.3 log CFU/g) of the pathogen on the surface or injecting the same amount into the fruit resulted in survival at 23, 10, 5, and −20°C. The survival was generally greater when storage temperature was higher and the bacterium was injected into the strawberries.; E. coli O157:H7 and E. coli strains were studied for the production of volatile compounds using Super Q porous polymer (Alltech., Deerfield, IL) collections, and analysis by gas chromatography and mass spectrometry. The predominant compound produced on plate count agar (PCA) was indole with lesser amounts of other components including methyl ketones. The bacteria also produced aliphatic alcohols when grown in tryptic soy broth (TSB). There were quantitative differences in the volatile compounds produced among the bacterial strains tested. Headspace trapping of E. coli O157:H7-inoculated strawberries yielded no detectable bacterial-produced volatile compounds. However, a TSB wash of inoculated strawberry fruit produced all the volatile compounds emitted by control bacterial broth cultures.; The effects of natural volatile compounds and chemical agents on the recovery of E. coli O157:H7 from inoculated strawberry fruit were also studied. The results demonstrated that (E)-2-hexenal, the major wound volatile compound from plants, was effective in killing E. coli O157:H7 on PCA, but it was not effective when the pathogen was on strawberry fruit. Five chemical agents, namely sodium hypochlorite, hydrogen peroxide, sodium phosphate, acetic acid, and Tween 80, caused limited reduction (up to 1.4 log CFU/g more than a water wash alone) of the pathogen on strawberry fruit (4.6 log CFU/g). The most effective treatment was 3% hydrogen peroxide. Collectively, the data indicate that due to its capacity to survive on fruit, the relative ineffectiveness of decontaminating agents, and the known low cell number required for infection, E. coli O157:H7 contaminated strawberry fruit would present a health hazard.