714 Part VII: Food Safety
There is speculation that the low number of cells
required for infection by E. coliO157:H7 is related
to the acid resistance of the organism. It is resistant
to synthetic gastric fluid (pH 1.5) for longer than the
general stomach clearance time of 3 hours, so that a
large percentage of the bacteria survive passage to
the intestine (Park et al. 1998).
As well, E. coliO157:H7 is resistant to drying
and fermentation processes used to treat many deli
meats such as salamis. These meats are not cooked,
but drying and fermentation is considered a valid
process to kill any bacteria present. In 1994, how-
ever, the United States experienced an outbreak of
E. coliO157:H7 resulting from contamination of
salami, proving that this pathogen is resistant to the
fermentation process (Meng and Doyle 1997).
E. coliO157:H7 has been isolated from such di-
verse sources as ground beef, lettuce, raw cider, raw
milk, and untreated water (Altekruse et al. 1997). It
is commonly found in the intestinal flora of cattle,
and may enter the food chain from this source
(O’Brien and Kaper 1998). For example, if fields are
fertilized with contaminated manure, then the path-
ogen can be absorbed into plant tissue (e.g., lettuce),
where it is no longer accessible to removal by wash-
ing (Solomon et al. 2002).
The pathogenicity of the bacteria is attributed to
several virulence factors, including one or more shi-
ga toxins, a hemolysin, the adhesin intimin, and the
O-antigen (Osek 2003). Its key virulence factor is
the shiga toxin (Stx), which is carried by several dif-
ferent cryptic prophages with wide host ranges (Kim
et al. 2001, Perna et al. 2001). These prophages are
lysogenic and, because they are cryptic, have lost
the ability to enter a lytic cycle of infection. E. coli
O157:H7 may have evolved as a result of the in-
creased use of antibiotics, leading to an increase in
antibiotic resistant phage transfers (Tauxe 2002).
Shiga toxins produced by E. coliO157:H7 exac-
erbate the intestinal and systemic lesions caused by
the bacteria in the human host (O’Brien and Kaper
1998). Lesions in the intestine lead to bloody diar-
rhea, while in the kidney they are associated with
HUS. Stx is formed of one A polypeptide and five B
polypeptides. The A polypeptide is an N-glycosidase
responsible for inhibiting protein synthesis by de-
purinating an adenine residue in the 28S rRNA of
the host cell, while the B pentamer allows the toxin
to attach to globotriosylceramide (Gb3), a glycopro-
tein abundant in the cortex of the human kidney
(Park et al. 1998).
Seventy-six percent of E. coliO157:H7 strains
isolated from infected individuals have been shown
to produce a mixture of shiga toxins 1 (Stx1) and 2
(Stx2); 20% produced only Stx 2, and 3% displayed
only Stx 1 (Meng and Doyle 1997). Stx1 is highly
conserved with the Shiga disentiriaeshiga toxin,
while Stx2 has at least five subgroups (Acheson and
Keusch 1999). There is some evidence that Stx2 is
more important than Stx1 in causing the progression
of hemolytic uremic syndrome (Meng and Doyle
1997).YERSINIA ENTEROCOLITICAYersinia enterocoliticais a Gram-negative microor-
ganism found ubiquitously in the environment. The
majority of isolates found from human and environ-
mental samples are nonpathogenic. Little is known
about the epidemiology of this organism, although
the strongest link to the source of the pathogenic
strains is in pigs. Most of the isolates recovered
from pigs are the same as those found within hu-
mans (Fredriksson-Ahomaa and Korkeala 2003).
Outbreaks of Y. enterocolitica–mediated illness have
also been associated with water and milk consump-
tion (Gugnani 1999).
Y. enterocolitica has been recognized as an
emerging foodborne pathogen since the late 1980s.
Infection with the pathogen results in diarrhea, fe-
ver, headache, rigors, and vomiting and is often as-
sociated with infection of the intestinal lymph nodes
(Gugnani 1999, Takeda et al. 1999).
One of the major public health challenges in deal-
ing with Y. enterocoliticais in detecting the organ-
ism, because of its low numbers compared with
background flora commonly found in contaminated
samples. Direct isolation, even on selective media, is
often not successful. Y. enterocoliticadoes have psy-
chrotropic properties, or an ability to grow at tem-
peratures lower than 5°C (Woteki and Kineman
2003). In fact, one of the ways used to isolate this
organism has been enrichment on selective media
for a number of weeks at 4°C (Fredriksson-Ahomaa
and Korkeala 2003). There are also some recent
developments in the use of molecular-based meth-
ods that may help to alleviate the detection problem
(Fredriksson-Ahomaa and Korkeala 2003).
Of the 57 serotypes and six biotypes of Y. entero-
colitica known (Takeda et al. 1999), only those
strains possessing virulence factors are associated
with illness. The pathogen produces a heat-stable