Transfer of Escherichia coli O157:H7 from ice to romaine lettuce and evaluation of E. coli ATCC 25922 as its surrogate
Kim, Jin Kyung
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Ice can be used to chill romaine lettuce and maintain relative humidity during transportation. E. coli O157:H7 may contaminate water used for ice. Potential contamination of lettuce is of concern since it is usually consumed raw or minimally processed. The potential for E. coli O157:H7 contamination of romaine lettuce with either ice contaminated with the pathogen or by transfer from lettuce surfaces via melting ice was determined using a simulated commercial operation. In order to evaluate pathogen transfer by these means in actual commercial facilities, the use of non-pathogenic surrogates is needed. Non-pathogenic E. coli strains were compared with E. coli O157:H7 based on cryotolerance, cell surface characteristics (hydrophobicity, zeta potential, and morphology) and attachment to lettuce. Survival of E. coli O157: H7 in water for extended periods can induce starvation. Starvation may enhance E. coli O157:H7 survival under subsequent stresses by changing surface morphology which may affect adherence to produce. After starvation (at 37°C for 4 h, 20°C for 24 h, or 4°C for 7 d), cryotolerance, cell surface properties and attachment of E. coli O157:H7 and a surrogate E. coli to lettuce were determined. The selected non-pathogenic surrogate E. coli was compared with E. coli O157:H7 to determine differences and similarities in attachment to and recovery from romaine lettuce in contact with contaminated ice under simulated commercial operation conditions. E. coli O157:H7 distributes onto other produce layers in shipping containers due to melted ice made of contaminated water and transfer from contaminated to uncontaminated surfaces. Based on cryotolerance and cell surface characteristics, E. coli ATCC 25922 is a useful surrogate for E. coli O157:H7 for studies involving attachment and recovery from chilled produce. E. coli ATCC 25922 grown at 37°C showed similar or better cryotolerance regardless of starvation conditions. However, starved cells of both strains attached to lettuce less than non-starved cells. E. coli ATCC 25922 showed the same attachment and recovery as E. coli O157:H7 from romaine lettuce under icing conditions using a simulated commercial operation condition. Overall E. coli ATCC 25922 is a useful surrogate for E. coli O157:H7 for produce attachment and recovery studies.