Survival of Escherichia coli O157:H7 on strawberry fruit and reduction of the pathogen population by chemical agents

Survival of Escherichia coli O157:H7 was studied on strawberry, a fruit that is not usually washed during production, harvest, or postharvest handling. Two strains of the bacteria were tested separately on the fruit surface or injected into the fruit. Both strains of E. coli O157:H7 survived externally and internally at 23°C for 24 h and at 10, 5, and -20°C for 3 days. The largest reduction in bacterial population occurred at -20°C and on the fruit surface during refrigeration. In all experiments, the bacteria inside the fruit either survived as well as or better than bacteria on the surface, and ATCC 43895 frequently exhibited greater survival than did ATCC 35150. Two strains of E. coli also survived at 23°C on the surface and particularly inside strawberry fruit. Chemical agents in aqueous solution comprising NaOCl (100 and 200 ppm), Tween 80 (100 and 200 ppm), acetic acid (2 and 5%), Na₃PO₄ (2 and 5%), and H₂O₂ (1 and 3%) were studied for their effects on reduction of surface inoculated (10⁸ CFU/ml) E. coli O157:H7 populations on strawberry fruit. Dipping the inoculated fruit in water alone reduced the pathogen population about 0.8 log unit. None of the compounds with the exception of H₂O₂ exhibited more than a 2-log CFU/g reduction of the bacteria on the fruit surface. Three percent H₂O₂, the most effective chemical treatment, reduced the bacterial population on strawberries by about 2.2 log CFU/g.