Penetration of Escherichia coli O157:H7 into Lettuce Tissues as Affected by Inoculum Size and Temperature and the Effect of Chlorine Treatment on Cell Viability

Abstract

Penetration of Escherichia coli O157:H7 into iceberg lettuce tissues and the effect of chlorine treatment on cell viability were evaluated. Attachment of different inoculum levels (109, 108, and 107 CFU/ml) was examined by determining the number of cells at the surface and the cut edge of lettuce leaves (2 by 2 cm). E. coli O157:H7 attached preferentially to cut edges at all inoculum levels, with greater attachment per cm2 of lettuce at higher inoculum levels. A longer attachment time allowed more cells to attach at both sites. Immunostaining with a fluorescein isothiocyanate–labeled antibody revealed that cells penetrated into lettuce leaves from cut edges. Cells showed greater penetration when lettuce was held at 4°C compared with 7, 25, or 37°C and were detected at an average of 73.5 ± 16.0μm below the surfaces of cut tissues. Penetrating cells were mostly found at the junction of lettuce cells. The viability of attached cells after treatment with 200 mg/liter (200 ppm) of free chlorine for 5 min was examined by plating on tryptic soy agar and by a nalidixic acid elongation method. Although chlorine treatment caused significant reduction in attachment (0.7- and 1.0-log reduction at surfaces and cut edges, respectively), cells remained attached at high numbers (7.9 and 8.1 log CFU/cm2 at surfaces and cut edges, respectively). Elongated cells were observed in stomata and within the tissues of the lettuce, indicating they were protected from contact with chlorine.