Fundamental Food Microbiology, Third Edition - Fuad Fathir

FOODBORNE INFECTIONS 375
contamination of food with Shi. sonnei. In general, food service establishments have
been implicated in more outbreaks, and poor personal hygiene has been the major
cause. The disease is more predominant during late spring to early fall. 12,22
B. Characteristics
The cells of the species are Gram-negative, nonmotile, facultative anaerobic rods.
They are generally catalase positive and oxidase and lactose negative. They ferment
sugars, usually without forming gas. On the basis of DNA homology, both Shigella
and Escherichia can be included in one genus, and because of many biochemical
similarities, the separation between the two genera is not clear. There are suggestions
that Shigella species could be pathogenic variants of Esc. coli. They are more like
Shiga-toxin-producing Esc. coli, with Shigella O antigen. 23
The strains grow between 7 and 46�C, with an optimum at 37�C. The cells are
not as fragile as once thought. They survive for days under different physical and
chemical stresses, such as refrigeration, freezing, 5% NaCl, and pH 4.5. They are
killed by pasteurization. The strains can multiply in many types of food when stored
at growth temperature range. 22,24
C. Habitat
The intestine of humans and some primates is the only habitat known. Humans can
carry the organism in the intestine, and shed it in the feces without showing any
symptoms. Following recovery from shigellosis, an individual can remain a carrier
for months. 22,24
D. Toxins
The strains are believed to carry plasmid-encoded invasive traits that enable the
shigellae cells to invade epithelial mucosa of the small and large intestines. Once
engulfed by the epithelial cells, they can produce an exotoxin that has an enterotoxigenic
property. The toxin is designated as Shiga toxin (ST). The invasive trait is
expressed at 37�C but not at 30�C. Shigellae cells growing at 30�C need a few hours
of conditioning at 37�C before they can invade intestinal epithelial cells. The
engulfed shigellae cells kill the epithelial cells and then attack fresh cells, causing
ulcers and lesions. 22,24
E. Disease and Symptoms
The infective dose is very low, ca. 10 1–3 cells in adults. Following ingestion of a
contaminated food, the symptoms occur in 12 h to 7 d, but generally in 1 to 3 d. In
case of mild infection, symptoms last for 5 to 6 d, but in severe cases, symptoms
can linger for 2 to 3 weeks. Certain individuals might not develop symptoms. An
infected person sheds the pathogens long after the symptoms have stopped. The
symptoms are the consequence of both invasiveness of epithelial mucosa and the
enterotoxin and include abdominal pain, diarrhea often mixed with blood, mucus
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