wide range of other environmental sources. Raw celery, tomatoes and
lettuce were implicated on epidemiological grounds as a possible cause of
listeriosis which occurred in several hospitals in Boston, USA in 1979,
although direct microbiological evidence was missing. An outbreak of
listeriosis in Canada in 1981 was associated with coleslaw (see Section
7.9.5). Strains of L. monocytogenes can certainly grow on shredded
cabbage and salad vegetables such as lettuce at temperatures as low as
51 C and modified-atmospheres seem to have no effect on this organism.
In the UK, routine surveillance of foods by the Public Health Labora-
tories revealed that, out of 567 samples of processed vegetables and
salads examined, 87 (15%) were found to containListeriaspp. while 72
(13%) containedL. monocytogenesspecifically.
Two other psychrotrophic organisms which are readily isolated from
the environment areYersinia enterocoliticaandAeromonas hydrophila.
Both may be expected to be associated with vegetables and could grow to
levels capable of causing illness if care is not taken during the growth,
harvesting, storage and treatment of these commodities.
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