Agriculture Reference
In-Depth Information
for at least 56, 152, and 193 days at 5 °, 15 °, and 21 °C, respectively (Jiang and others
2002 ).
Salmonella
persisted in hog manure
amended loamy sand and clay soils for
more than 180 days during the simulated summer-winter season (25, 10, 4,
−
18 ° C)
as compared with for less than 160 days either in a spring-summer temperature (4,
10, 25, 30 °C) or winter to summer (
−
18, 4, 10, 25 °C) regimens (Holley and others
2006). Similar results for
S.
Typhimurium in manure-amended soil were reported by
Natvig and others (2002), who recommended applying manure in late fall to ensure
that harvested vegetables were not contaminated with
S
. Typhimurium since the
repeated freeze-thaw cycles could inactivate the pathogen.
−
Infl uence of Microbial Activity in Soil
Studies have revealed that soil with less microbial activity allows the extended sur-
vival of manure-borne enteric pathogens due to less competition from the indigenous
soil microfl ora (FDA 2001; Jiang and others 2002, 2004). You and others (2006) found
that
Salmonella
Newport in soil contaminated with dairy manure were detectable
through direct plating for 107 and 158 days in manure mixed with nonsterilized soil
and manure mixed with sterilized soil, respectively. Nutrient levels vary among dif-
ferent types of soil and so do the diversity and numbers of soil microbial populations.
The number of
E. coli
increased by ca. 1.5-2 logs CFU/g during the fi rst 2 weeks of
manure incorporation but declined signifi cantly greater in loamy sand soil than in silty
clay loam soil (Lau and Ingham 2001).
Presence of the Rhizosphere
The rhizosphere, the soil environment surrounding the root, is a complex ecosystem
where the interactions among soil, roots, and microbes take place (Brimecombe and
others 2007). The rhizosphere is rich in organic compounds released by plant roots
and microorganisms, which may affect the survival and growth of enteric bacteria
introduced from manure. Using a soil core model, Gagliardi and Karns (2002) have
shown that
E. coli
O157:H7 was able to survive for 25-47 days, 47-96 days, and 92
days, in fallow soil, on rye roots, and on alfalfa roots, respectively. The authors also
showed that the metabolic activity of microbial community in soil microcosms was
enhanced from rye and alfalfa roots after 14 days and by 63 days, respectively.
Recently, a study by Klerks and others (2007) demonstrated that
S. enterica
serovars
moved toward to the root exudates produced by the lettuce cultivar
Tamburo
via
chemotaxis. Apparently, the persistence of enteric pathogens in the rhizosphere is the
outcome of interaction among pathogens, soil microorganisms, soil, and plant roots.
Types of Animal Wastes
Animal wastes of different sources vary signifi cantly in organic loads, pH, texture,
microbial population, and solid contents (CAST 1996). Studies have revealed that the
changes in diet can affect the shedding of pathogens by animals and subsequent per-
sistence of the pathogens in manure and in manure-amended soil (Franz and others
2005). The populations of both
E. coli
O157:H7 and
S
. Typhimurium declined faster
in the manure derived from straw diet than in the manure from grass silage plus maize
silage (Franz and others 2005). Several studies investigated the persistence of
E. coli
and
L. monocytogenes
pathogens in soil-fertilized with a variety of wastes such as
