WORLD OF MICROBIOLOGY AND IMMUNOLOGY Enterovirus infections189•
The diagnosis of enterobacterial infections can be com-
plicated by the fact that viruses, protozoa, and other kinds of
bacteria can also cause similar symptoms. The location of some
of the symptoms can help determine the nature of the infection.
For example, if nausea and vomiting is involved, then the
enterobacterial infection could well be centered in the small
intestine. If a fever is present, then dysentery is more likely.
The treatment for many enterobacterial infections is the
administration of the suitable antibiotic or combination of
antibioticsthat the isolated organism is determined to be sus-
ceptible to. As well, and every bit as important, is the admin-
istration of fluids to prevent dehydration because of the
copious loss of fluids during diarrhea. The dehydration can be
extremely debilitating to infants and the elderly.See alsoE. coli O157:H7 infection; Invasiveness and intracel-
lular infectionsEEnterotoxin and exotoxinNTEROTOXIN AND EXOTOXIN
Enterotoxin and exotoxin are two classes of toxin that are pro-
duced by bacteria.
An exotoxin is a toxin that is produced by a bacterium
and then released from the cell into the surrounding environ-
ment. The damage caused by an exotoxin can only occur upon
release. As a general rule, enterotoxins tend to be produced by
Gram-positive bacteria rather than by Gram-negative bacteria.
There are exceptions, such as the potent enterotoxin produced
by Vibrio cholerae. In contrast to Gram-positive bacteria,
many Gram-negative species posses a molecule called
lipopolysaccharide. A portion of the lipopolysaccharide, called
the lipid A, is a cell-associated toxin, or an endotoxin.
An enterotoxin is a type of exotoxin that acts on the
intestinal wall. Another type of exotoxin is a neurotoxin. This
type of toxin disrupts nerve cells.
Many kinds of bacterial enterotoxins and exotoxins
exist. For example, an exotoxin produced by Staphylococcus
aureusis the cause of toxic shock syndrome, which can pro-
duce symptoms ranging from nausea, fever and sore throat, to
collapse of the central nervous and circulatory systems. As
another example, Staphylococcus aureus also produces
enterotoxin B, which is associated with food-borne illness.
Growth of the bacteria in improperly handled foods leads to
the excretion of the enterotoxin. Ingestion of the toxin-con-
taminated food produces fever, chills, headache, chest pain
and a persistent cough. This type of illness is known as a food
intoxication, to distinguish it from bacterial food-borne illness
that results from growth of the bacteria following ingestion of
the food (food poisoning).
Enterotoxins have three different basis of activity. One
type of enterotoxin, which is exemplified by diphtheriatoxin,
causes the destruction of the host cell to which it binds.
Typically, the binding of the toxin causes the formation of a
hole, or pore, in the host cell membrane. Another example of
a pore-forming exotoxin is the aerolysin produced by
Aeromonas hydrophila.A second type of enterotoxin is known as a superantigen
toxin. Superantigen exotoxins work by overstimulating the
immune response, particularly with respect to the T-cells.
Examples of superantigen exotoxins include that from
Staphylococcus aureusand from the “flesh-eating” bacterium
Streptococcus pyogenes.
A third type of enterotoxin is known as an A-B toxin. An
A-B toxin consists of two or more toxin subunits that work
together as a team to exert their destructive effect. Typically,
the A subunit binds to the host cell wall and forms a channel
through the membrane. The channel allows the B subunit to
get into the cell. An example of an A-B toxin is the enterotoxin
that is produced by Vibrio cholerae.
The cholera toxin disrupts the ionic balance of the host’s
intestinal cell membranes. As a result, the cells of the small
intestine exude a large amount of water into the intestine.
Dehydration results, which can be lethal if not treated.
In contrast to the destructive effect of some exotoxins,
the A-B exotoxin (an enterotoxin of Vibrio choleraedoes not
damage the structure of the affected host cells. Therefore, in
the case of the cholera toxin, treatment can led to a full
resumption of host cell activity.See alsoAnthrax, terrorist use of as a biological weapon;
Bacteria and bacterial infectionEEnterovirus infectionsNTEROVIRUS INFECTIONS
Enteroviruses are a group of virusesthat contain ribonucleic
acidas their genetic material. They are members of the picor-
navirus family. The various types of enteroviruses that infect
humans are referred to as serotypes, in recognition of their dif-
ferent antigenic patterns. The different immune response is
important, as infection with one type of enterovirus does not
necessarily confer protection to infection by a different type of
enterovirus. There are 64 different enterovirus serotypes. The
serotypes include polio viruses, coxsackie A and B viruses,
echoviruses and a large number of what are referred to as non-
polio enteroviruses.
The genetic material is enclosed in a shell that has 20
equilateral triangles (an icosahedral virus). The shell is made
up of four proteins.
Despite the diversity in the antigenic types of
enterovirus, the majority of enterovirus cases in the United
States is due to echoviruses and Coxsackie B viruses. The
infections that are caused by these viruses are varied. The par-
alytic debilitation of polio is one infection. The importance of
polio on a global scale is diminished now, because of the
advent and worldwide use of polio vaccines. Far more com-
mon are the cold-like or flu-like symptoms caused by various
enteroviruses. Indeed, the non-polio enteroviruses rival the
cause of the “common cold,” the rhinovirus, as the most com-
mon infectious agent in humans. In the United States, esti-
mates from the Centers for Disease Controlare that at least ten
to fifteen million people in the United States develop an
enterovirus infection each year.womi_E 5/6/03 2:12 PM Page 189