Agriculture Reference
In-Depth Information
persistence in soil, and the processes involved in exposure of seedlings or mature
plants to pathogen are unclear. In the absence of an effective “kill step” for postharvest
produce, it remains important to identify sources of pathogens and their fate and
transport in produce environments; this may assist in development of strategies for
preventing contamination of produce destined for the ready-to-eat market.
Fate and Transport of Human Pathogens in the Environment
It has been diffi cult to determine the primary source of preharvest produce contamina-
tion; however, nearby livestock, poultry, or other farm animals are obvious potential
point sources for further dissemination in the environment, and linked possibly to
produce (Table 1.3). Potential mechanisms for dissemination of pathogens from con-
tained farms or feedlots are movement of livestock to new locations, wildlife intrusion,
water runoff/fl ooding (Table 1.4), dust/bioaerosols, manure/compost/compost-tea fer-
tilizers, and possibly other intra- and interfarm human activities (farm vehicles and
equipment).
For pathogens to be transported outside an animal host, they must remain fi t enough
to survive (and possibly grow) until they encounter an environment favorable for
growth. Findings from previous studies measuring the survival of pathogenic
E. coli
and
Salmonella
in manure, soil, and water are relevant to hypotheses about how pre-
harvest contamination occurs. Table 1.5 is a list of selected studies that provide a
comparison of measured fi tness characteristics of
E. coli
O157,
E. coli
O157:H7, and
Salmonella
in environments relevant to fresh produce contamination, including
manure, soil, manure-amended soil, and water. It is worth noting that some of these
studies report the incidence of pathogens in their natural state in relevant environmen-
tal samples, whereas others involved spiking samples with marked strains and then
monitoring their incidence over a period of time.
Each study listed in Table 1.5 involved different locations and experimental condi-
tions; however, it is noteworthy that outcomes generally were consistent. For example,
in nearly all studies,
E. coli
O157 or
E. coli
O157:H7 remains detectable in some
samples for
30 days, but longer than 6 months in other samples (Table 1.5; cow water
trough, sheep manure, manure-amended soil).
Salmonella
cells were detectable for
similar periods of time (e.g., soil, manure-amended soil), but an outbreak strain was
detectable for
>
1500 days in soil samples from an almond orchard linked to the out-
break (see below). Similarly, multiple strains of
E. coli
O157 were isolated for months
from biofi lms on fl int shingles immersed in stream beds exposed to runoff from farm
animals positive for the pathogen (Cooper and others 2007).
These studies support the persistence theory and possible mechanisms of periodic
reintroduction of pathogens in agricultural environments. Conversely, a recent study
of potential pathogens isolated from livestock and then inoculated onto spinach and
lettuce in fi eld plots reported rapid die-off of a shigatoxin-negative strain of
E. coli
O157:H7; this was in contrast to the survival of a strain of
S.
Enteritidis for at least
14 days (Hutchison and others 2008). These contrasting results emphasize again the
variability of pathogen survival in complex environments, and the dependence of
results probably upon pathogen fi tness, experimental design (fi eld versus microcosm),
and other factors (spatial, temporal, indigenous fl ora, disease, etc.), any of which might
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