Agriculture Reference
In-Depth Information
Under Controlled Conditions
Studies have been done on the contamination of both leafy vegetables and root crops
grown in manure-amended soil with pathogens such as
E. coli
O157:H7 and
Salmonella.
Either in a growth chamber or greenhouse, the contamination of onions and carrots by
E. coli
O157 and radishes and arugula by
S
. Typhimurium occurred on the vegetable
surface at harvest time ranging from 7 to 17 weeks after manure application (Natvig
and others 2002; Islam and others 2004d). Franz and others (2005) found only one root
sample of lettuce grown in loam soil amended with manure positive with
E. coli
O157:H7. However, Johannessen and others (2005) failed to detect the pathogen in the
roots, outer leaves, and edible parts of the lettuce, but found the pathogen survived in
soil for at least 8 weeks but not 12 weeks. In that study, analyzing lettuce samples for
pathogen was done only at the harvest, which may have missed the detection of early
lettuce contamination by manure-amended soil. In addition, a low inoculation level of
pathogen may be another contributing factor for failing to detect pathogen on lettuce,
because low inocula may reduce the probability for the bacteria to compete with indig-
enous microorganisms and colonize the rhizosphere (Solomon and others 2002).
Most importantly, as long as the pathogen can be detected in soil, there is a pos-
sibility for produce being contaminated, especially when the washing step is not
thoroughly performed.
Under Field Conditions
Using two avirulent strains, Islam and others (2004a-c, 2005) performed a series of
experiments investigating the persistence of
E. coli
O157:H7 and
S
. Typhimurium on
various plants grown in manure compost-amended soil or irrigated with contaminated
water under fi eld conditions. At an initial inoculation level of ca. 10
7
CFU/g,
E. coli
O157:H7 was detected at harvest on carrots, lettuce, and parsley for 126, 77, and 177
days, respectively, whereas
S
. Typhimurium was detectable at harvest on radishes,
carrots, lettuce, and parsley for 84, 203, 63, and 231 days, respectively. These studies
demonstrated that the extent of pathogen survival on produce varied among types of
plants, bacterial species, types of soil, and compost used for growing vegetables
although the contamination route via irrigation water or manure compost did not affect
the survival curves of both strains. Van Renterghem and others (1991) reported that
L. monocytogenes
was detected 3 months later in 3 out of 6 radish samples, which
were sown in soil inoculated with the pathogen in pig manure. However, a fi eld study
using manure naturally contaminated with
E. coli
O157:H7 failed to prove that patho-
gen contamination occurred from manured soil to lettuce (Johannessen and others
2004). As suggested by the researchers, heavy rainfall soon after fertilizer application
possibly washed away
E. coli
O157:H7 from the lettuce fi eld. Further research on
determining the minimal level of pathogen in soil required for produce contamination
may help to assess the risk of using contaminated manure or compost for produce
production in the fi eld.
Internalization via Roots
A number of studies have explored the possibility of enteric bacteria colonizing and
being internalized in plants such as lettuce, tomato, spinach, and sprouts (Guo and
