Agriculture Reference
In-Depth Information
others 2002; Itoh and others 1998). Current studies suggest that the enteric pathogens
originating from animal manure fertilizer gain entry into the growing plants through
the root systems and then migrate throughout the edible portion of the plants (Solomon
and others 2006). The successful internalization of human pathogens depends on a
number of factors such as strain specifi city, mobility of microorganisms, emergence
of lateral roots, indigenous microorganisms, temperature, humidity, plant variety and
defense mechanisms, and presence of root exudation (Cooley and others 2003, 2006;
Solomon and others 2002; Warriner and others 2003; Klerks and others 2007).
Both
E. coli
O157 and
Salmonella
have been reported to gain entry to the plant
under laboratory conditions using either hydroponic or soil models systems. Under
confocal laser scanning microscope, Solomon and others (2002) observed the inter-
nalization of
E. coli
O157:H7 inside lettuce, 45
m from the outer surface. The specifi c
interactions between
S. enterica
and the lettuce cultivar
Tamburo
are triggered by the
root exudates of lettuce, which is correlated for the degree of colonization but not
prevalence (Klerks and others 2007). However, two separate fi eld studies reported that
both
E. coli
O157:H7 and
Listeria innocua
failed to be internalized in the crisphead
lettuce and parsley, respectively (Johannessen and others 2005; Girardin and others
2005). Several other fi eld studies on the persistence of pathogen on produce did not
differentiate the sites of pathogen contamination (Islam and others 2004a-c, 2005;
Ingram and others 2004, 2005). Therefore, further study is needed to understand the
mechanisms for enteric pathogens interacting with plant roots and to assess whether
pathogen internalization occurs under fi eld conditions. This information is extremely
useful to develop intervention strategies for preventing produce contamination during
plant growth.
μ
Impact of Wildlife on Farms
Wildlife, such as birds, rodents, pigs, and deer, can be a vector for transmitting fecal
pathogens to produce fi elds without the grower's knowledge and, at various times
during the growing seasons, serving as sources of pathogen contamination of fresh
vegetables (Nielsen and others 2004). Wild pig fecal droppings were found near the
spinach - growing fi eld linked to the 2006-2007
E. coli
O157:H7 outbreak in California
(CalFERT 2007). Detection of low levels of indigenous
E. coli
for the extended period
of time after manure application to soil might be due to birds or mammal activity in
the fi eld (Ingham and others 2004).
Organic Production Practices
Organic produce may be more susceptible to fecal contamination due to the use of
animal manure as organic fertilizer. According to the guidelines set by the USDA
National Organic Program (NOP), raw animal manure must either be composted,
applied to land used for crops not intended for human consumption, or incorporated
into the soil at least 90 days for edible crops not in contact with soil, and 120 days
prior to the harvest of edible crops in contact with soil (NOSB 2002). There were a
few studies conducted to validate these rules scientifi cally. Although a greenhouse
study suggested that applying raw manure to soil in early spring and late fall for at
least 120 days prior to vegetable harvest should ensure that the vegetables would be
free of pathogen contamination (Natvig and others 2002), two fi eld studies by Ingham
