Page 164 - Review of Medical Microbiology and Immunology ( PDFDrive )

mebooksfree.com
mebooksfree.com
mebooksfree.com

mebooksfree.com
mebooksfree.com
mebooksfree.com
mebooksfree.com
mebooksfree.com
mebooksfree.com mebooksfree.com mebooksfree.com CHAPTER 18 Gram-Negative Rods Related to the Enteric Tract 153 mebooksfree.com
PATHOGENS BOTH WITHIN & OUTSIDE THE ENTERIC TRACT

ESCHERICHIA
cause diarrhea. The toxins are strikingly cell-specific; the
cells of the colon are not susceptible, probably because
Diseases
they lack receptors for the toxin. Enterotoxigenic strains
Escherichia coli is the most common cause of urinary tract
enterotoxins.
infection and gram-negative rod sepsis. It is one of the two
important causes of neonatal meningitis and the agent of E. coli (ETEC) can produce either or both of two
(1) The heat-labile toxin (LT) acts by stimulating ade-
most frequently associated with “traveler’s diarrhea,” a
nylate cyclase. Both LT and cholera toxin act by catalyzing
mebooksfree.com
mebooksfree.com mebooksfree.com mebooksfree.com the cyclase. This irreversibly activates the cyclase. The mebooksfree.com
mebooksfree.com
watery diarrhea. Some strains of E. coli are enterohemor-
the addition of adenosine diphosphate-ribose (a process
rhagic and cause bloody diarrhea.
called ADP-ribosylation) to the G protein that stimulates
Important Properties
resultant increase in intracellular cyclic adenosine mono-
phosphate (AMP) concentration stimulates cyclic AMP–
Escherichia coli is a straight gram-negative rod (see
dependent protein kinase, which phosphorylates ion
Figure 18–1), in contrast to the curved gram-negative rods
transporters in the membrane. The transporters export
of the genera Vibrio, Campylobacter, and Helicobacter.
ions, which cause an outpouring of fluid, potassium, and
Escherichia coli is the most abundant facultative anaer-
obe in the colon and feces. It is, however, greatly outnum-
resulting in watery diarrhea. Note that cholera toxin has the
bered by the obligate anaerobes such as Bacteroides.
same mode of action.
Escherichia coli ferments lactose, a property that distin-
(2) The other enterotoxin is a low-molecular-weight,
guishes it from the two major intestinal pathogens, Shigella chloride from the enterocytes into the lumen of the gut,
heat-stable toxin (ST), which stimulates guanylate cyclase.
and Salmonella. It has three antigens that are used to iden-
mebooksfree.com mebooksfree.com mebooksfree.com mation, do not invade the intestinal mucosa, and cause a mebooksfree.com
mebooksfree.com
mebooksfree.com
tify the organism in epidemiologic investigations: the O, or
The enterotoxin-producing strains do not cause inflam-
cell wall, antigen; the H, or flagellar, antigen; and the K, or
capsular, antigen. Because there are more than 150 O, 50 H,
watery, nonbloody diarrhea. However, certain strains of E.
and 90 K antigens, the various combinations result in
coli are enteropathic (enteroinvasive) and cause disease not
more than 1000 antigenic types of E. coli. Specific sero-
by enterotoxin formation but by invasion of the epithelium
types are associated with certain diseases (e.g., O55 and
of the large intestine, causing bloody diarrhea (dysentery)
O111 cause outbreaks of neonatal diarrhea).
accompanied by inflammatory cells (neutrophils) in the
stool.
Pathogenesis
Certain enterohemorrhagic strains of E. coli (i.e., those
The reservoir of E. coli includes both humans and animals.
The source of the E. coli that causes urinary tract infec-
rhea by producing an exotoxin called Shiga toxin, so called
tions is the patient’s own colonic flora that colonizes the with the O157:H7 serotype) (STEC) also cause bloody diar-
because it is very similar to that produced by Shigella spe-
mebooksfree.com
mebooksfree.com
mebooksfree.com mebooksfree.com mebooksfree.com bacteriophages. Shiga toxin is also called verotoxin because mebooksfree.com
urogenital area. The source of the E. coli that causes neo-
cies. Shiga toxin acts by removing an adenine from the
natal meningitis is the mother’s birth canal; the infection is
large (28S) ribosomal RNA, thereby stopping protein syn-
acquired during birth. In contrast, the E. coli that causes
thesis. Shiga toxin is encoded by temperate (lysogenic)
traveler’s diarrhea is acquired by ingestion of food or water
contaminated with human feces. Note that the main reser-
it has a cytopathic effect on Vero (monkey) cells in
voir of enterohemorrhagic E. coli O157 is cattle and the
culture.
organism is acquired in undercooked beef, for example,
These O157:H7 strains are associated with outbreaks of
hamburgers.
bloody diarrhea following ingestion of undercooked ham-
Escherichia coli has several clearly identified compo-
burger, often at fast-food restaurants. The bacteria on the
nents that contribute to its ability to cause disease: pili, a
capsule, endotoxin, and three exotoxins (enterotoxins), two
those in the interior, which is undercooked, survive. Also,
that cause watery diarrhea and one that causes bloody diar-
direct contact with animals (e.g., visits to farms and petting
rhea and hemolytic–uremic syndrome. surface of the hamburger are killed by the cooking, but
zoos) has resulted in bloody diarrhea caused by O157:H7
mebooksfree.com
mebooksfree.com
mebooksfree.com mebooksfree.com mebooksfree.com strains also have a life-threatening complication called mebooksfree.com
strains. E. coli O157 has a low ID of approximately 100
50
Intestinal Tract Infection
organisms.
The first step is the adherence of the organism to the cells
Some patients with bloody diarrhea caused by O157:H7
of the jejunum and ileum by means of pili that protrude
from the bacterial surface. Once attached, the bacteria
hemolytic–uremic syndrome (HUS), which occurs when
Shiga toxin enters the bloodstream. This syndrome consists
synthesize enterotoxins (exotoxins that act in the enteric
of hemolytic anemia, thrombocytopenia, and acute renal
tract), which act on the cells of the jejunum and ileum to
mebooksfree.com mebooksfree.com mebooksfree.com mebooksfree.com mebooksfree.com mebooksfree.com