Surrogate selection for Escherichia coli O157:H7 based on cryotolerance and attachment to romaine lettuce

Using nonpathogenic surrogates in place of pathogens when evaluating commercial food processing operations offers safety advantages, but their usefulness may be limited if they do not behave in the same manner in challenge situations. Nonpathogenic Escherichia coli strains were compared with E. coli O157:H7 based on cryotolerance, cell surface characteristics (hydrophobicity, zeta potential, and morphology), and attachment to lettuce. Populations for all strains were reduced less than 1 log CFU/ml over 7 days of storage at -18 degrees C. After 1 day of storage, the survival rate for E. coli ATCC 25922 was 44.3%, similar to that of E. coli O157:H7 (49%). No capsule was produced by any of the strains. E. coli O157:H7 expressed curli at both 20 and 37 degrees C, whereas E. coli ATCC 25922 expressed curli only when grown at 20 degrees C. Hydrophobicity of E. coli ATCC 25922 was 53.5%, similar to that of E. coli O157:H7 (56.2%). The zeta potentials of nonpathogenic E. coli and E. coli O157:H7 cells were -4.95 to -10.92 mV. The zeta potential of E. coli ATCC 25922 was not significantly different (P > 0.05) from that of E. coli O157:H7 at 37 degrees C and was the closest value to that of E. coli O157:H7 at 20 degrees C. E. coli ATCC 25922 exhibited the greatest attachment to lettuce among the surrogates and was not significantly different from E. coli O157:H7 (P > 0.05). Based on cryotolerance and cell surface characteristics, E. coli ATCC 25922 is a useful surrogate for E. coli O157:H7 for studies involving attachment to fresh produce.