Agriculture Reference
In-Depth Information
and with increasing frequency in samples collected during and following harvests
(Aug-Dec) and following rain events (Uesugi and others 2007).
Salmonella
strains
isolated during the 5-year study were all phage type 30 and indistinguishable from
the clinical outbreak strains (or one band difference) by two-enzyme pulsed fi eld gel
electrophoresis (PFGE) analysis. Although it was probable that almonds became
contaminated by pathogens present in soil/dust where almonds were dropped and then
harvested by sweepers, the original source of the outbreak-related strain was never
identifi ed, nor were any suspect practices (Uesugi and others 2007).
The extended persistence of any pathogens in an agricultural environment, espe-
cially strains that have the potential to cause an outbreak, raises questions relevant to
other produce-related outbreaks. Is contamination periodic and cumulative or due to
major isolated contamination events? Do persistent strains refl ect selection and evolu-
tion of special fi tness characteristics in a specifi c environment (e.g., orchard environ-
ment; almond, leafy vegetable, tomato surface)? Is the incidence or concentration of
pathogens greater now than in the past? Does pathogen survival at low concentrations
in harsh soil conditions (dry, high UV) with subsequent resuscitation/amplifi cation
(rain/moisture, low UV) relate to virulence? Do certain wildlife species (e.g., mam-
malian, avian, amphibian) become colonized and high shedders of pathogen and
associated with persistent contamination? These and other questions stimulated by
recent outbreaks are diffi cult to answer, but they assist in focusing on areas for further
research.
Incidence of Generic
E. coli
on Produce
Increased concerns in the U.S. and other countries about produce-associated outbreaks
(Table 1.1) have stimulated initiation of multiple surveys of fresh produce for selected
pathogens, and also surveys of the incidence of generic
E. coli
as an indicator of fecal,
and potential pathogen, contamination. The results from some of these studies, includ-
ing recent surveys, are presented to indicate the general microbiological quality of
different types of produce grown in different regions conventionally or organically,
and tested at different stages of the pre- and postharvest cycle.
A survey of produce items (e.g., arugula, cantaloupe, cilantro, parsley, spinach)
collected between November 2000 to May 2002 from 13 farms in the southeastern
U.S. revealed
E. coli
levels ranging from 0.7 to 1.5 log CFU/g for fi eld or packing-shed
produce (Johnston and others 2005). All samples were negative for
L. monocytogenes
and
E. coli
O157:H7; however, 3 of 398 samples tested for
Salmonella
were positive
(0.7%). A similar survey by the same investigators comparing produce grown in the
southern U.S. and Mexico involved testing 466 produce items obtained from packing
sheds between November 2002 and December 2003. Levels of
E. coli
ranged between
0.7-1.9 and 0.7-4.0 log CFU/g for Mexican and southeastern U.S. produce, respec-
tively (Johnston and others 2006a). All samples were negative for
E. coli
O157:H7,
Salmonella,
and
Shigella
; however, three domestic cabbage samples were positive for
L. monocytogenes
(0.6% of total produce samples; 7% of cabbage samples).
A variety of fresh produce items grown conventionally or organically on farms in
Minnesota were picked between May and September 2002 and surveyed for microbio-
logical quality (Mukherjee and others 2004).
E. coli
incidence was 4.3, 11.4, and 1.6%
