Agriculture Reference
In-Depth Information
in some combination be conducive to pathogen survival, growth, and, in some
instances, increased virulence in a leafy vegetable-associated outbreak (Table 1.1).
These results refl ect a “snapshot” of the pathogen under the selected test or environ-
mental conditions, in addition to a spectrum of fi tness characteristics of the pathogen
assessed.
Two studies relevant to concepts of persistence of specifi c pathogen strains in a
preharvest environment and direct links to human illness are worth noting. A survey
of a family garden subsequent to the O157:H7 illness of a child playing in the raw
manure-amended garden revealed that strains indistinguishable from the child's strain
were detectable in soil samples from the garden for
69 days, and that incidence was
much higher in soil sampled during ambient temperatures compared to 4 °C (Mukherjee
and others 2006). Similarly, strains of
S.
Enteritidis Phage Type 30 associated with
at least one outbreak linked to raw almonds, and possibly a second (Table 1.1), were
isolated over at least a 5-year period from soil drag swab samples obtained in an
orchard linked to the outbreak (Uesugi and others 2007). The
Salmonella
strain,
indistinguishable from outbreak strains, was isolated from soil more frequently during
and after harvests (average 20-42% of samples, Aug-Dec), and in
>
50% of soil
samples following a heavy rain event. Although the virulence and infectiousness of
an environmental pathogen strain cannot be compared to related human clinical
strains, the sets of
E. coli
O157:H7 and
S.
Enteritidis PT 30 environmental strains
noted above are closely related epidemiologically to the corresponding clinical strains.
It can be speculated that persistence of these pathogen strains in the garden and orchard
environments may relate directly to the evolution of fi tness characteristics that
correlate also with virulence (Manning and others 2008).
Manure-amended soil, plants and plant debris appear to be benefi cial to the survival
of
E. coli
O157:H7 and
Salmonella
(Table 1.5). Ruminant-digested grasses and feeds
and crop debris have nutrients supporting survival and possibly growth of enteric
pathogens under the appropriate environmental conditions, including temperature,
moisture, and atmosphere (Brandl 2006). For example,
E. coli
cells present naturally
in cow feces placed in shaded and nonshaded fi elds increased 1.5 log after 6 to 8 days,
declining fast in nonshaded fecal samples and then rebounding
>
1 log in nonshaded
samples after rain events (Van Kessel and others 2007). In contrast,
E. coli
in air - dried
sandy and silty soils amended with municipal sludge (biosolids) declined more slowly
than in moist soils; up to 3 log differences were noted after 35 compared to 91 days
in the fi eld (Lang and Smith 2007). These studies are monitoring generic rather than
pathogenic
E. coli
; however, the results are informative about different feces (cow,
human), exposure to sun (UV) or moisture, and rates of resuscitation in rain—
important environmental factors affecting pathogens in the environment.
E. coli
O157
and
S. enterica
, and generic
E. coli
as fecal indicator bacteria, appear capable of
surviving months or even years under the appropriate environmental conditions and,
under optimal conditions, they grow 1 to 3 logs (Table 1.5). Indeed, in a recent study
of
Salmonella
in tomato crop debris, it appears this may be another aspect of the
preharvest environment worth considering as a site conducive to survival or growth
of pathogen for extended periods of time (Barak and Liang 2008). Tomato seeds
planted in soil with
Salmonella
-contaminated tomato crop debris resulted in plants
contaminated with
Salmonella
in the rhizoplane
>
>
phyllosphere.
Salmonella
survived
