UNIVERSITY OF COLORADO

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UNIVERSITY OF COLORADO SCHOOL OF
MEDICINE DEPARTMENT OF PATHOLOGY RESIDENCY
TRAINING PROGRAM MICROBIOLOGY IV
Enterobacteriaceae Lactose Fermenters Case
Study Hemolytic Uremic Syndrome
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Hemolytic Uremic Syndrome
CASE HISTORY An 11-year-old male experienced
stomach cramps followed by bloody diarrhea a few
hours later. The patient was hospitalized.
Blood cultures were drawn on admission. The blood
platelets were very low. X-ray revealed
significant intestinal edema. Kidney output
dropped to zero. The child received 3 units of
fresh frozen plasma, the first of 11 plasma
exchanges administered over the next three weeks.
In the second week, pericarditis was diagnosed
and treated with pericardiocentesis, followed by
surgery to excise the pericardium. The patient
became hypotensive and convulsive. Intestinal
perforation occurred and surgery was performed to
correct this complication. After 5 additional
surgeries, the child gradually recovered.
Adapted from case originally presented by Dr.
James Beebe, Director Microbiology Laboratory,
Colorado Dept. Public Health. CACMLE
Teleconference, July 25, 2001.
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Appearance of the isolate from the admission
blood culture as seen on the surface of sheep
blood agar (left upper), Standard MacConkey agar
(left lower) and sorbitol MacConkey agar (right)
after 48 hours incubation at 35o C. The colonies
are entire, convex, and smooth. The colonies on
standard MAC are pink-red, and on sorbitol
MacConkey are non-pigmented. What presumptive
species identification might be made? What spot
test reaction might be helpful? What serogroup
does the reaction on sorbitol-MacConkey agar
suggest? Does this provide a definitive
identification? What types of stool specimens
should be screened with sorbitol/MAC?
Agar Plate Appearance of Blood Culture Isolate
ANSWERS
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Appearance of this isolate on Rainbow agar (a
product of Biolog, Inc., Hayward,
California). Observed are commensal E. coli
strains in the background (black pigmentation)
and a few scattered Shiga-toxin producing strains
(red pigmentation). Biochemical identification is
necessary as not all colonies having these
appearances are E. coli. Also, not all colonies
with a red pigmentation are serogroup 0157H7
What additional tests might be performed to
establish a definitive species identification of
this isolate? ANSWER
Colony Appearance on Rainbow Agar
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Identification using the Vitek GNI Card
The biocode number 3000701266 on the Vitek GNI
card codes out to Escherichia coli, 98 Hafnia
alvei, 2. An astute observer noted that the
sorbitol reaction was negative. Serotype 0157H7
was suspected.
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DIAGNOSIS Hemolytic Uremic Syndrome Etiologic
Agent Escherichia coli Serogroup 0157
Recapitulation on next page
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Escherichia coli Serogroup 0157 Recapitulation
Outbreaks of foodborne illness have been
associated with consumption of ground beef, roast
beef, raw milk, yogurt, and unpasteurized apple
juice. Person-to-person transmission occurs, as
well as water-borne and swimming-associated
infections. Cases peak in the warm months.
Primary reservoir is cattle, but deer and elk may
carry the agent as well. As few as 10 viable
cells can initiate infection in humans. Illness
is characterized by bloody diarrhea, typically
after 3-8 days incubation. After infection,
humans shed E coli 0157 for as long as 71 days.
Close observation is needed when treating with
antibiotics as illness may be exacerbated,
promoting the emergence of hemolytic uremic
syndrome. Hemolytic uremic syndrome is defined
by a traid of features (acute renal failure,
thrombocytopenia, and microantiopathic hemolytic
anemia) and is the leading cause of acute renal
failure in children. As presented here, a large
portion of EHEC isolates belong to serogroup
0157H7, which may be suspected as this serogroup
does not produce acid from sorbitol (appears
lactose-negative on sorbitol MacConkey agar).
Suspected isolates should be confirmed with
specific 0157H7 antisera.
Review of pathogenic E. coli on next page
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Escherichia coli may be categorized into major
groups according to serotypes and virulence
mechanisms. ETEC are the most common cause of
travelers diarrhea and have caused several
food-borne outbreaks in the United States. A
subset of the ETEC, serogroup 0157 H7, includes
strains that produce bloody diarrhea and often
hemolytic uremic syndrome. There are an
estimated 78,420 cases of ETEC in the United
States each year.
Review on next page
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Pathogenic E. coli Review
Enterotoxigenic E. coli (ETEC) for heat-labile
and/or heat-stable enterotoxins that produce a
secretory diarrhea (travelers diarrhea)
similar to that of Vibrio cholerae. Surface
attachment of the bacterial cells to intestinal
epithelial cells is prerequisite to toxin
production. Enteroinvasive E. coli (EIEC) are
capable of penetrating the intestinal epithelial
cells and producing an inflammatory diarrhea
similar to that caused by Shigella species. This
strain can be suspected when observing blood,
mucus, and segmented neutrophils in fecal smears.
Both ETEC and EIEC strains are recovered
primarily from patients who have traveled to
foreign countries. Enteropathogenic E. coli
(EPEC) cause non-bloody diarrheal syndromes
primarily in infants. The pathogenesis is
unclear however, the inflammatory reactions and
epithelial degenerative changes that are observed
in tissue sections may be secondary to adhesive
properties of the bacterium, believed to be
plasmid related.
BACK
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What presumptive species identification might be
made? Observed on the blood agar and MAC
plates are colonies representative of a
lactose-fermenting gram negative bacillus. The
absence of colony pigmentation on
Sorbitol-MacConkey agar suggests a 0157
serogroup. What spot test reaction might be
helpful? A positive spot indole reaction would
help confirm the identification of E. coli. As
mentioned before, the lack of acid production of
the colonies growing on Sorbitol
MacConkey agar serves as a screen for serogroup
0157, one important agent of hemolytic uremic
syndrome. Does this provide a definitive
identification? Approximately 80 of E. coli
stains are sorbitol negative. However, most
sorbitol negative glucose fermenters are E.
coli. In either case, serogrouping studies must
be performed for confirmation. What types of
stool specimens should be screened with
sorbitol/MAC? Obviously sorbitol/MacConkey agar
plates will not be set up on all stool
specimens. However, those that appear bloody, or
those obtained f rom a patient with a history of
passing bloody specimens should be screened.
Cefixime and tellurite are added in some
sorbitol/Mac formulas to make the medium even
more selective for recovery of E.coli 0157.
BACK
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E. coli Identification Recapitulation
In summary, above are key biochemical
characteristsics by which E. coli can be
identified, and by which 0157 serogroups can be
differentiated from non-0157 and commensal
strains. Note that acid production from sorbitol
and rhamnose, and a negative reaction for
glucuronidase are key presumptive biochemical
clues, and resistance to cefoxime and tellurite
are more definitive.
BACK