Agriculture Reference
In-Depth Information
(distinctive red colonies), for example, moved in the river at approximately 2.5 km/hr
over the 2.9 km between the dosing and detection points. The dosed strain was detected
at a maximum of 500 cells/ml, which refl ected a signifi cant dilution (
1 0
8
- fold)
of the bacteria during transport (Wimpenny and others 1972). To achieve a compa-
rable amount of
E. coli
O157:H7 from “ high - shedder ” cattle feces (e.g., 10
6
cells/g),
for example, would require
>
1.7
×
200,000 kg of feces.
In a separate study in an elevated region within miles of leafy vegetable production,
transport of
E. coli
O157:H7 strains was tracked from a point source (small corral
with a few head of cattle) into a small stream (Cooley and others 2007). Indistinguishable
or related pathogen strains identifi ed by MLVA genotyping were isolated at the point
source and up to 135 m downstream (3 m lower altitude) from the point source.
However, water fl ow was relatively low prior to and at the time of sampling (Cooley
and others 2007).
Isolation and/or detection of pathogens in water at distant sites from a suspected
point source, therefore, might involve one or more of the following: large volumes of
feces and/or high-shedding animals, very sensitive detection of few pathogen cells,
multiple point sources with related strains, or transport mechanisms (e.g., cell-cell or
cell-particulate aggregates, mats, fl otation) different than those refl ected by laboratory
cultured microorganisms in tracer studies. Accurate tracer studies of pathogens in the
environment would be advantageous for understanding fate and transport mechanisms
relevant to produce contamination.
Pathogens in animal feces deposited on rangeland, feedlots, or dairy alleys, and into
storage ponds are exposed to dispersion, transport, and inactivation that could be
affected by soil and fecal matrices, particle sizes, buoyancy, microbial competitors/
predators or cooperators, and even climate (rainfall, temperature, UV exposure). It is
noteworthy that during the 2006 outbreak of
E. coli
O157:H7 associated with bagged
baby spinach, unusually high daily temperatures occurred at the time of planting: July
22 - 25, 2006: max. daily 100 - 110 ° F (37.7 - 43.3 ° C); ave. daily 77 - 85 ° F (25 - 29.4 ° C),
and approximately 5-6 days prior to harvest (CalFERT 2007b,c). This unusual condi-
tion stimulates questions regarding when contamination occurred in the crop cycle and
whether high temperatures may have enhanced survival or growth of pathogen in the
preharvest environment. For example,
E. coli
O157 has been shown to survive and
increase in number with increasing temperature (10-30 °C) in natural freshwater
microcosms containing low concentrations of organic carbon (Vital and others 2008).
The direct correlation between pathogen growth and water temperature is consistent
with enteric bacteria that have evolved to grow optimally at body temperatures.
>
Survival of Human Pathogens on Preharvest Plants
Outbreaks associated with preharvest contaminated produce confi rm that enteric bac-
teria are capable of attaching somewhere on the plants and remaining viable (Tables
1.1 and 1.3). Field studies with nonpathogenic varieties of
E. coli
O157:H7 and other
pathogens on plants under fi eld conditions confi rm that they can survive for weeks
and months depending upon the amount of bacteria applied and the treatment condi-
tions (Tables 1.2 and 1.3). Laboratory studies indicate that
E. coli
O157:H7 and
Salmonella
applied to a variety of plant roots, leaves, and seeds can attach tenaciously
(resisting sanitization) and survive, but also in some instances grow when conditions
