title: 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 (10(9), 10(8), and 10(7) 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 degrees C compared with 7, 25, or 37 degrees C and were detected at an average of 73.5 +/- 16.0 microm 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.
authors.affiliation: Center for Food Safety and Quality Enhancement, Department of Food Science and Technology, University of Georgia, Athens 30602-2106, USA.
authors.forename: K
authors.initials: K
authors.lastname: Takeuchi
authors.forename: J F
authors.initials: JF
authors.lastname: Frank
mesh_terms: D001422:Bacterial Adhesion; D002470:Cell Survival / Q000187:drug effects; D002713:Chlorine / Q000494:pharmacology*; D019453:Escherichia coli O157 / Q000472:pathogenicity*; D005516:Food Microbiology; D018545:Lactuca / Q000187:drug effects / Q000382:microbiology*; D013696:Temperature
chemical_list: D002713:Chlorine