Fundamental Food Microbiology, Third Edition - Fuad Fathir

402 FUNDAMENTAL FOOD MICROBIOLOGY
C. Habitat
All strains in both subgroups can establish in the small intestine of humans without
producing symptoms. The carriers can shed the organisms in feces and can contaminate
food and water directly or indirectly. Many animals, including domestic ones,
can also harbor different serotypes of both subgroups and contaminate soil, water,
and food. In animals, some serotypes may not produce disease symptoms.
D. Toxins and Toxin Production
The strains in the ETEC subgroup produce two types of enterotoxins: one is heat
labile (LT) and the other is heat stable (ST). A strain can produce either LT or ST,
or both. LT toxin is an antigenic protein, similar to cholera toxin produced by Vib.
cholerae, that induces fluid secretion by epithelial cells of the small intestine. ST is
a heat-stable protein, lower in molecular weight than LT, and is nonantigenic. It also
increases fluid secretion by intestinal cells, but through a different mode of action.
Strains in EPEC subgroups were previously thought not to produce enterotoxins
such as those by ETEC serotypes. However, some studies have shown that several
serotypes produce LT toxin, and several others produce toxins different from LT and
ST of ETEC serotypes. In addition to LT and ST enterotoxins, ETEC serotypes can
also produce additional factors that enable the cells to colonize, multiply, and initiate
infection. The genetic determinants of the enterotoxins in ETEC are plasmid linked
and can be transferred to other Esc. coli strains. 3,12
The production of enterotoxins by ETEC strains is influenced by media composition,
culture age, and alteration of a culture during growth. Optimum production
occurs in a nutritionally rich medium at pH 8.5. Aeration of broth facilitates good
toxin production. The toxins are generally detected in a growing culture within 24
h at 35�C. However, the toxin can be produced by the cells growing at 25 to 40�C. 3,12
E. Disease and Symptoms
EPEC strains were initially thought to be associated with infant diarrhea in many
tropical and developing countries, causing high mortality. In contrast, ETEC strains
are regarded as the cause of traveler’s diarrhea, a nonfatal diarrheal disease. However,
some consider that various EPEC strains can also cause traveler’s diarrhea. Ingestion
of a high level (10 6–9 cells) of viable cells of the organisms by adults is necessary
for the symptoms to occur within 24 to 72 h. Symptoms include mild to severe
diarrhea that lasts for 24 to 30 h. In severe cases, dehydration, prostration, and shock
may accompany the diarrhea. Not all individuals show symptoms; those who develop
symptoms may shed the organisms in feces after recovery. 3,12
F. Food Association
Many types of foods, including meat products, fish, milk and dairy products, vegetables,
baked products, and water have been associated with gastroenteritis of Esc.
coli origin in many countries. 3,12 These include serotypes from both EPEC and ETEC