WORLD OF MICROBIOLOGY AND IMMUNOLOGY Enterobacteriaceae187•
and kill these cells. The resulting abdominal pain and tender-
ness, with sudden and explosive bloody diarrhea, is called
dysentery. Other symptoms of the dysentery include dehydra-
tion, fever, and sometimes the establishment of a bowel mal-
function that can become chronic. The damage can be so
extensive that a complete perforation of the intestinal wall can
occur. Leakage of intestinal contents into the abdominal cav-
ity can be a result, as can a thickening of the abdominal wall.
Other trophozoites form cysts and are shed into the
external environment via the feces. These can spread the
infection to another human.
Drugs are available to treat the symptomatic and asymp-
tomatic forms of the infection.
In about 10 percent of people who are infected, some of
the trophozoites can enter the circulatory system and invade
other parts of the body, such as the liver, colon, and infre-
quently the brain. The reasons for the ability of the tropho-
zoites to establish infections in widespread areas of the body
are still not understood. The current consensus is that these
trophozoites must somehow be better equipped to evade the
immune responses of the host, and have more potent virulence
factors capable of damaging host tissue.
Infection can occur with no obvious symptoms being
shown by the infected person. However, these people will still
excrete the cysts in their feces and so can spread the infection
to others. In others, infection could produce no symptoms, or
symptoms ranging from mild to fatal.
Although the molecular mechanisms of infection of
Entamoeba histolyticaare still unclear, it is clear that infection
is a multi-stage process. In the first step the amoeba recog-
nizes the presence of a number of surface receptors on host
cells. This likely involves a reaction between the particular
host receptor and a complimentary molecule on the surface of
the amoeba that is known as an adhesion. Once the association
between the parasite and the host intestinal cell is firm, other
molecules of the parasite, which may already be present or
which may be produced after adhesion, are responsible for the
damage to the intestinal wall. These virulence factors include
a protein that can form a hole in the intestinal wall of the host,
a protein-dissolving enzyme (protease), a glycocalyxthat cov-
ers the surface of the protozoan, and a toxin.
Comparison of pathogenic strains of Entamoeba his-
tolyticawith strains that look the same but which do not cause
disease has revealed some differences. For example, the non-
pathogenic forms have much less of two so-called glycolipids
that are anchored in the microbe membrane and protrude out
from the surface. Their function is not known, although they
must be important to the establishment of an infection.
Completion of the sequencing of the genome of
Entamoeba histolytica, expected by 2005, should help identify
the function of the suspected virulence factors, and other, yet
unknown, virulence factors. Currently, little is known of the
genetic organization and regulation of expression of the
genetic material in the protozoan. For example, the reasons for
the variation in the infection and the symptoms are unclear.See alsoAmebic dysentery; ParasitesEEnterobacteriaceaeNTEROBACTERIACEAE
Enterobacteria are bacteriafrom the family Enterobacteri-
aceae, which are primarily known for their ability to cause
intestinal upset. Enterobacteria are responsible for a variety of
human illnesses, including urinary tract infections, wound
infections, gastroenteritis, meningitis, septicemia, and pneu-
monia. Some are true intestinal pathogens; whereas others are
merely opportunistic pests which attack weakened victims.
Most enterobacteria reside normally in the large intes-
tine, but others are introduced in contaminated or improperly
prepared foods or beverages. Several enterobacterial diseases
are spread by fecal-oral transmission and are associated with
poor hygienic conditions. Countries with poor water deconta-
mination have more illness and death from enterobacterial
infection. Harmless bacteria, though, can cause diarrhea in
tourists who are not used to a geographically specific bacterial
strain. Enterobacterial gastroenteritis can cause extensive fluid
loss through vomiting and diarrhea, leading to dehydration.
Enterobacteria are a family of rod-shaped, aerobic, fac-
ultatively anaerobic bacteria. This means that while these bac-
teria can survive in the presence of oxygen, they prefer to live
in an anaerobic (oxygen-free) environment. The
Enterobacteriaceae family is subdivided into eight tribes
including: Escherichieae, Edwardsielleae, Salmonelleae,
Citrobactereae, Klebsielleae, Proteeae, Yersineae, and
Erwineae. These tribes are further divided into genera, each
with a number of species.
Enterobacteria can cause disease by attacking their host
in a number of ways. The most important factors are motility,
colonization factors, endotoxin, and enterotoxin. Those enter-
obacteria that are motile have several flagella all around their
perimeter (peritrichous). This allows them to move swiftly
through their host fluid. Enterobacterial colonization factors
are filamentous appendages, called fimbriae, which are shorter
than flagella and bind tightly to the tissue under attack, thus
keeping hold of its host. Endotoxins are the cell wall compo-
nents, which trigger high fevers in infected individuals.
Enterotoxins are bacterial toxins which act in the small intes-
tines and lead to extreme water loss in vomiting and diarrhea.
A number of tests exist for rapid identification of enter-
obacteria. Most will ferment glucose to acid, reduce nitrate to
nitrite, and test negative for cytochrome oxidase. These bio-
chemical tests are used to pin-point specific intestinal
pathogens. Escherichia coli (E. coli), Shigella species,
Salmonella,and several Yersiniastrains are some of these
intestinal pathogens.
E. coliis indigenous to the gastrointestinal tract and
generally benign. However, it is associated with most hospital-
acquired infections as well as nursery and travelers diarrhea.
E. colipathogenicity is closely related to the presence or
absence of fimbriae on individual strains. Although most E.
coliinfections are not treated with antibiotics, severe urinary
tract infections usually are.
The Shigellagenus of the Escherichieae tribe can pro-
duce serious disease when its toxins act in the small intestine.
Shigellainfections can be entirely asymptomatic, or lead to
severe dysentery. Shigellabacteria cause about 15% of pedi-womi_E 5/6/03 2:12 PM Page 187