Agriculture Reference
In-Depth Information
published by our research group in 2004 and 2006 (Mukherjee and others 2004,
2006b). From more than 2,600 samples of fresh fruits and vegetables collected directly
at the fi eld, we detected
Salmonella
contamination in only 2 samples grown by farmers
who claimed to be organic, but were not certifi ed. None of the organic and conven-
tional produce tested positive for
E. coli
O157:H7.
The U.S. Food and Drug Administration conducted two surveys: one on domestic
and the other on imported fruits and vegetables (FDA 2002, 2003). Approximately
1,000 samples of broccoli, cantaloupe, different types of leafy vegetables, strawber-
ries, and tomatoes were collected from 21 different countries, and analyzed for the
presence of
Shigella
,
Salmonella,
and
E. coli
O157:H7. As many as 44 samples (4%
of the total sample lot) tested positive for
Shigella
and
Salmonella
. Almost all of
these contaminated samples were either cantaloupe or leafy vegetables and 7.3% of
the cantaloupes and 4.9% of leafy vegetables tested positive for either one or both
of these two pathogenic bacteria. Only one sample of strawberries and none of the
broccoli and tomato samples tested positive for pathogens. None of the 1,003
imported produce samples had
E. coli
O157:H7 contamination. The survey on
domestic fresh produce included samples of the same produce types from 18 states.
Eleven samples (1% of the total sample lot) tested positive for
Shigella
and
Salmonella
. Out of these 11 contaminated samples, 5 were cantaloupe (3% of the
cantaloupes) and 6 were leafy vegetables (1% of the leafy vegetables). Like the
survey on imported produce, none of the domestically produced tomatoes were con-
taminated with pathogens, and none of the 1,028 domestic produce samples tested
positive for
E. coli
O157:H7.
Overall, the occurrence of foodborne pathogens in produce testing studies is rela-
tively rare. From the 11 studies listed in Table 4.4,
Salmonella
was detected in 6 of
them, but only 2 studies reported prevalence values greater than 3%. The detection of
E. coli
O157:H7 in fresh vegetables is an extremely rare event. The fi rst documented
isolation of this pathogenic organism from a vegetable sample was accomplished by
the Minnesota Department of Agriculture from a sample of bagged lettuce implicated
in an outbreak that affected 21 individuals in Minnesota and Oregon (MDH 2005). The
only produce survey that has reported the detection of this serotype was a study con-
ducted in Washington (state) when three samples of sprouts tested positive (Samadpour
and others 2006). In addition, as many as 13 samples of spinach linked to human cases
yielded positive isolations during a large national outbreak in 2006 (CDC 2006).
Reliability of
Escherichia coli
as an Indicator for Fecal Contamination
in Produce
Ideally, a fecal indicator bacterium should demonstrate occurrence only in intestinal
environments and should also possess other criteria for bacterial indicators, such as
easy and reliable detection even when present in small numbers (Jay and others 2005).
Fecal coliforms and
Escherichia coli
have been used as a fecal indicator in water by
the EPA since the early 1900s (Doyle and Erickson 2006; EPA 1904). During much
of the 20th century, use of these bacteria as a fecal indicator has been extended to
foods. Over the years, as detection of
E. coli
became rapid and more reliable, this
bacterium has been more frequently used as direct indication of fecal contamination
in foods. However, in recent years, some reports have questioned the reliability of
E. coli
as a fecal indicator in foods, because this bacterium has been documented to
