382 ◾ Fundamental Food Microbiology
following picking, heat treatment before preparing salad: Any one or more of these steps could
have been used to avoid this incident. (Note: It is illegal to bring food into the United States from
other countries without prior permission of regulatory agencies.)
enterotoxigenic Escherichia coli (eteC) 15,16
Importance
One pathogenic Esc. coli subgroup that correlates well with toxico-infection belongs to the enterotoxigenic Esc. coli (ETEC) type. They produce diarrheal diseases when ingested in large numbers
through contaminated foods and water. The symptoms are more like those in cholera. The incidence is high in many developing countries and is directly related to poor sanitation.
Characteristics
Many serotypes in both subgroups are involved in human gastroenteritis. They are gram-negative
small curved rods, nonsporulating, and motile (nonmotile strains can be present). The strains
are facultative anaerobes and can grow effectively in both simple and complex media and many
foods. Growth occurs between 10°C and 50°C with optimum at 30°C–37°C. Some strains can
grow below 10°C. Rapid growth occurs under optimum conditions. Growth-limiting factors are
low pH (below 5.0) and low A W (below 0.93). The cells are sensitive to low-heat treatment, such
as pasteurization.
Habitat
All strains in the ETEC subgroup 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 and contaminate soil, water, and food. In animals, some serotypes may not produce disease
symptoms.
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 (Figure 26.3), similar to cholera toxin (CT) produced by Vib. Cholerae. LT has two
serogroups, LT-I (86 kDa) and LT-II, which are antigenically distinct. LT induces fluid secretion
by epithelial cells of the small intestine. ST is a heat-stable 2 kDa peptide and is nonantigenic. It
also increases fluid secretion by intestinal cells but through a different mode of action. 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.
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
following picking, heat treatment before preparing salad: Any one or more of these steps could
have been used to avoid this incident. (Note: It is illegal to bring food into the United States from
other countries without prior permission of regulatory agencies.)
enterotoxigenic Escherichia coli (eteC) 15,16
Importance
One pathogenic Esc. coli subgroup that correlates well with toxico-infection belongs to the enterotoxigenic Esc. coli (ETEC) type. They produce diarrheal diseases when ingested in large numbers
through contaminated foods and water. The symptoms are more like those in cholera. The incidence is high in many developing countries and is directly related to poor sanitation.
Characteristics
Many serotypes in both subgroups are involved in human gastroenteritis. They are gram-negative
small curved rods, nonsporulating, and motile (nonmotile strains can be present). The strains
are facultative anaerobes and can grow effectively in both simple and complex media and many
foods. Growth occurs between 10°C and 50°C with optimum at 30°C–37°C. Some strains can
grow below 10°C. Rapid growth occurs under optimum conditions. Growth-limiting factors are
low pH (below 5.0) and low A W (below 0.93). The cells are sensitive to low-heat treatment, such
as pasteurization.
Habitat
All strains in the ETEC subgroup 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 and contaminate soil, water, and food. In animals, some serotypes may not produce disease
symptoms.
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 (Figure 26.3), similar to cholera toxin (CT) produced by Vib. Cholerae. LT has two
serogroups, LT-I (86 kDa) and LT-II, which are antigenically distinct. LT induces fluid secretion
by epithelial cells of the small intestine. ST is a heat-stable 2 kDa peptide and is nonantigenic. It
also increases fluid secretion by intestinal cells but through a different mode of action. 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.
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