Gamma irradiation and aqueous chlorine interventions to control pathogenic bacteria internalized in lettuce and on the surface of strawberries.

摘要

Although fruits and vegetables have been generally considered safe to eat, consumption of lettuce in raw salad has been implicated in outbreaks of food borne illnesses. However, the mechanism by which pathogenic microorganisms are mobilized through the plant tissue and effective methods for reducing and/or eliminating these pathogens are not certain. The objective of this work was to ascertain the uptake of E. coli O157:H7 from contaminated nutrient solution through the vascular system of lettuce and to use gamma radiation and aqueous chlorine as a means to reduce or eliminate the pathogenic bacteria.; Lettuce seedlings were transplanted in a hydroponic system seeded with E. coli O157:H7 at concentrations of 103, 10 5, and 107 CFU/ml. Viability of E. coli O157:H7 cells in leaves and stems after surface disinfection with 80% ethanol, followed by immersion in 0.1% (w/v) mercuric chloride was evaluated. Stem sections were placed outer or inner surface directly on Luria Agar plates supplemented with 100 μg/ml of ampicillin and incubated at 37°C for 24 hrs to detect E. coli O157:H7. Confocal laser scanning microscopy was used to detect E. coli O157:H7 internalized in lettuce. Lettuce leaves were harvested and treated with 200 ppm of aqueous chlorine, gamma radiation at 0.25, 0.50, and 0.75 kGy to evaluate the effectiveness for reducing E. coli O157:H7.; Fresh strawberries were immersed separately in the E. coli O157:H7 and Salmonella suspensions to give an initial population of about 5 log CFU/g. The strawberries were subjected to irradiation at 0, 0.14, 0.24, 0.375, 0.50, and 0.75 kGy.; All plants analyzed for surface contamination, tested negative for E. coli O157:H7. The internal part of the leaves from 14 out of 71 plants tested positive for E. coli O157:H7. About 13% of longitudinally sliced stems tested positive for E. coli O157:H7. At 3 days post challenge, change in E. coli O157:H7 concentration on roots from 3 to 5 and 5 to 7 log 10 CFU/g was associated with a 97% and 29% increase in bacterial count, respectively. Treatment of harvested lettuce leaves 200 ppm of aqueous chlorine, gamma radiation at 0.25 and 0.5 kGy failed to eliminate E. coli O157:H7 in the leaves. Gamma radiation of 0.75 kGy reduced E. coli O157:H7 to non-detectable levels.; The magnitude of E. coli O157:H7 and Salmonella were reduced to non-detectable levels at 0.5 kGy and 0.75 kGy, respectively.; These results demonstrate that E. coli O157:H7 can enter the lettuce plant through the root system and migrate to the edible portion of the lettuce plant tissue and therefore, renders surface cleaning ineffective. Therefore, there is a need to explore alternate practical and logical means such as irradiation to control and reduce the internalized pathogens in fruits and vegetables.