Agriculture Reference
In-Depth Information
are ideal for a pathogen (warm temperature, high humidity, adequate nutrients) (Brandl
2006 ). Sophisticated fl uorescence microscopy experiments have revealed specifi c
locations on leaves and roots where subcuticlar cells, root hairs, or breaks in the tissue
(e.g., lateral root formation) provide sites and nutrients for harboring opportunistic
pathogen cells. Aggregation of enteric pathogen cells with one another and with plant
epiphytic or plant pathogen microfl ora suggest that active and complex interactions
may occur on plants in the fi eld, resulting possibly in interactions/contamination very
diffi cult to remove by normal washing or sanitizing methods (Brandl 2006). In addi-
tion, there appears to be emerging support for the hypothesis that some human patho-
gen cells on plants may become internalized through different routes of entry on roots,
shoots, and fl owers (Guo and others 2001; Solomon and others 2002; Warriner and
others 2003; Dong and others 2003; Franz and others 2007; Doyle and Erickson 2008;
Schikora and others 2008). Indeed, recent reports examining the plant response to
potential human pathogens in model plant systems (
Arabidopsis thaliana
mutants and
gene expression arrays) indicate that genes and gene pathways are upregulated simi-
larly to plant resistance responses to plant pathogens (Dong and others 2003; Thilmony
and others 2006; Schikora and others 2008). Thus, the potential for some human
pathogens to be endopathogenic for some plant hosts in a preharvest environment
raises obvious concerns regarding postharvest treatments for decontamination.
Reviews of different mechanisms that plant epiphytes and pathogens and human
enteric pathogens use to attach to plants (Mandrell and others 2006; Solomon and
others 2006) and an excellent review of the general biology, ecology, and fi tness
characteristics of human enteric pathogens on plants have been published previously
(Brandl 2006). Further details about the molecular interactions that can occur between
bacterial human pathogens (e.g., fl agellin, fi mbriae, pili, curli, outer membrane pro-
teins) and plants (generally undefi ned), and the microbial ecology on plants that may
enhance or control pathogen survival are provided in these reviews and also chapters
elsewhere in this topic.
Conclusions
The increased incidence of produce-related outbreaks tracked to specifi c regions, and
E. coli
O157:H7 outbreaks in particular, has stimulated questions about what might
have changed over the last decade to explain this increase. Is it related to growing
(fertilization, water, shallow tilling, seeds, cultivars) or production practices (cutting,
transport, bagging, atmosphere), changes in the pathogens (increased fi tness in
animals, water), livestock (transport, incidence of pathogens), or better detection
(methods, public health system, media)? Clearly, some of these questions raise issues
that would be considered higher risk factors than others and worthy of prioritizing
for research.
Most people can appreciate that animals or feces on or near fresh produce fi elds
are major potential risk factors, probably worthy of attempts to prevent continued
intrusion. Lacking convincing evidence of pathogen carriage by a suspect animal
species, however, becomes problematic for making informed decisions about mitiga-
tion approaches (predation, fencing, testing). Indeed, lack of defi nitive proof of sources
of pathogens has created a signifi cant confl ict between conservationists, environmen-
talists, and growers on one side versus those in the produce industry responsible for
