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Interactions
of Escherichia coli O157:H7, Salmonella Typhimurium and Listeria Monocytogenes
Plants Cultivated in a Gnotobiotic System.
Int J Food Microbiol. 2005 Mar 1;99(1):7-18.
Jablasone J, Warriner K,
Griffiths M.
Department of Food Science, University of Guelph, Guelph, ON, Canada N1G 2W1.
The growth and persistence of Escherichia coli O157:H7, Salmonella typhimurium
and Listeria monocytogenes on a diverse range of plant types over extended
cultivation periods was studied. When introduced on the seed of carrot, cress,
lettuce, radish, spinach and tomato all the pathogens became rapidly established
shortly after germination, attaining cell densities of the order of 5.5-6.5 log
cfu/g. In general, Es. coli O157:H7 and L. monocytogenes became established and
persisted at significantly higher levels on seedlings (9 days post-germination)
than Salmonella. Es. coli O157:H7 became internalized in cress, lettuce, radish
and spinach seedlings but was not recovered within the tissues of mature plants.
Internalization of Salmonella was also observed in lettuce and radish but not
cress or spinach seedlings. In contrast, L. monocytogenes did not internalize
within seedlings but did persist on the surface of plants throughout the
cultivation period. Co-inoculation of isolates recovered from the rhizosphere of
plants did not significantly affect the numbers or persistence of human
pathogens. The only exception was with Enterobacter cloacae, which reduced Es.
coli O157:H7 Ph1 and L. monocytogenes levels by ca. 1 log cfu/g on lettuce. With
the bioluminescent phenotype of Es. coli O157:H7 Ph1, it was demonstrated that
the human pathogen became established on the roots of growing plants. Scanning
electron micrographs of root seedlings suggested that Es. coli O157:H7 Ph1
preferentially colonized the root junctions of seedlings. It is proposed that
such colonization sites enhanced the persistence of Es. coli O157:H7 on plants
and facilitated internalization within developing seedlings. The results suggest
that the risk associated with internalized human pathogens in salad vegetables
at harvest is low. Nevertheless, the introduction of human pathogens at an early
stage of plant development could enhance their persistence in the rhizosphere.
The implications of the study with regards to on-farm food safety initiatives
are discussed.
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