Agriculture Reference
In-Depth Information
A range of maximum survival times from 28 days at room temperature to 500 days
in frozen soil have been reported for
E. coli
O157:H7. In should be noted, however,
that more than four reports were able to recover pathogenic organisms long after
120 days.
In general, it appears that the survival of
Salmonella
and
E. coli
O157:H7 was
stimulated by increasing moisture in soil and by lower temperature. In one study,
E. coli
and
Salmonella
Typhimurium inoculated in moist grey loam topsoil showed
greater persistence than that in drier soil (Chandler and Craven 1980). On average
Salmonella
seems to adapt better than
E. coli
O157:H7 serotype to the harsh, competi-
tive soil environment, and it survives longer (Warriner 2005). Foodborne pathogens
also persist longer in the rhizosphere, compared to bulk soil alone. Some researchers
have suggested that plant roots provide better protection for the pathogens against
protozoa (Maule 2000).
Several studies have focused on survival of foodborne pathogens and fecal indica-
tor bacteria in soil fertilized with animal wastes inoculated with laboratory grown
cultures. In one study, fecal indicator
E. coli
and other enteric bacteria belonging to
the enterococci group were inoculated in bovine manure (Lau and Ingham 2001).
E.
coli
showed greater survival than enterococci, in two different types of soils in
Wisconsin amended with bovine manure. In another study, in Denmark,
S. enterica
serovar Typhimurium DT12 was inoculated in animal waste slurry from a piggery,
and the contaminated wastes were disposed of by spreading them on agricultural soil
(Baloda and others 2001). The pathogen remained detectable for 2 weeks in the soil.
That study emphasized the signifi cance of improper agricultural practices that may
lead to transmission of human pathogens into agricultural soil, and then to human
foods like produce cultivated in such soils.
In another fi eld study, a laboratory grown
E. coli
O157:H7 culture was inoculated
in commercially manufactured composts, made from poultry and cattle manure, and
these inoculated composts were mixed into agricultural soil plots (Islam and others
2005). The pathogen was detected in the manure-amended soil for as long as 196 days,
and in onions grown in those plots for almost 170 days. Hutchison and colleagues
(2004) reported survival of
E. coli
O157:H7 and
Salmonella
when untreated and
inoculated livestock wastes were applied on the top of agricultural soil. In this latter
study, when untreated pig and cattle manure was left on the top of agricultural soil
for at least a week before incorporation into the soil, the D-values for
Salmonella
decreased by 1 to 3 days. For
E. coli
O157:H7, the D-values decreased by 5 days
when incorporation of untreated cattle manure and pig slurry into agricultural soil was
delayed by at least a week.
The majority of survival studies have been conducted using inoculation with
strains grown in the laboratory, and the different inoculum preparations may also
explain the wide variability of results found in the literature. Among the few studies
that have monitored the survival of naturally occurring pathogenic bacteria, a docu-
mented case of environmental survival of an
E. coli
O157:H7 strain in soil was related
to an outbreak of gastroenteritis among participants in a boy scout camp (Howie and
others 2003; Strachan and others 2001). In that outbreak, 20 boys developed gastro-
enteritis after camping on a pasture fi eld previously used for grazing sheep.
Investigations of the fi eld reported that a low level of 1.8 log CFU/g of O157:H7
