Escherichia coli

Introduction
Escherichia coli, are part of the normal flora and incidentally cause
disease

Morphology & Identification

Typical Organisms: are short gram-negative rods.

Culture
E coli form circular, convex, smooth colonies with distinct edges. Some
strains of E coli produce hemolysis on blood agar.

Growth Characteristics
E coli typically produces positive tests for indole, lysine decarboxylase,
and mannitol fermentation and produces gas from glucose. An isolate
from urine can be quickly identified as E coli by its hemolysis on blood
agar, typical colonial morphology with an iridescent "sheen" on
differential media such as EMB agar, and a positive spot indole test. Over
90% of E coli isolates are positive for β-glucuronidase using the substrate
4-methylumbelliferyl-β-glucuronide (MUG). E coli can be confirmed
with a positive MUG test.
 Antigenic structure
E. coli has the antigens common to all Enterobacteriaceae.

O antigens: Somatic antigen are the most external part of the cell wall
lipopolysaccharide. Some O-specific polysaccharides contain unique
sugars. O antigens are resistant to heat and alcohol and usually are
detected by bacterial agglutination.
Most O antigen are cross-reactive like shigellae share one or more O
antigens with E coli. E coli may cross-react with some Providencia,
Klebsiella, and Salmonella species.

Occasionally, O antigens may be

associated with sp. diseases, eg,
specific O types of E coli are
found in diarrhea and in
urinary tract infections.
 K antigens are external to O antigens and may interfere with
agglutination by O antisera. They are polysaccharides. They may be
associated with virulence (eg, E coli strains producing K1 antigen are
prominent in neonatal meningitis, and K antigens of E coli cause
attachment of the bacteria to epithelial cells prior to gastrointestinal or
urinary tract invasion).

 H antigens are located on flagella and are denatured or removed by

heat or alcohol. They are preserved by treating motile bacterial variants
with formalin. The determinants in H antigens are a function of the
amino acid sequence in flagellar protein (flagellin).

 There are many examples of overlapping antigenic structures between

Enterobacteriaceae and other bacteria. Most Enterobacteriaceae share
the O14 antigen of E coli.
 The antigenic classification of Enterobacteriaceae often indicates the
presence of each specific antigen. Thus, the antigenic formula of an E
coli may be O55:K5:H21
Antigenic Structure……Colicins (Bacteriocins)

 Many gram-negative organisms produce bacteriocins. These

virus-like bactericidal substances are produced by certain
strains of bacteria active against some other strains of the
same or closely related species.

 Their production is controlled by plasmids.

 Colicins are produced by E coli, marcescens by serratia, and

pyocins by pseudomonas.

 Bacteriocin-producing strains are resistant to their own

bacteriocin; thus, bacteriocins can be used for "typing" of
organisms.
Pathogenesis & Clinical Findings

Urinary tract infection (UTI)

E coli is the most common cause of UTI & accounts for

approx. 90% of UTI in young women. The symptoms and
signs include urinary frequency, dysuria, hematuria, and pyuria.
Flank pain is associated with upper tract infection. None of these
symptoms is specific for E coli infection. UTI can result in
bacteremia & sepsis.

Nephropathogenic E coli typically produce a hemolysin. Most

of the infections are caused by E coli of a small number of O
antigen types. K antigen appears to be important in the
pathogenesis of upper tract infection.
E coli-Associated Diarrheal Diseases

E coli that cause diarrhea are extremely common worldwide.

These E coli are classified by the characteristics of their virulence
properties, and each group causes disease by a different
mechanism. The small or large bowel epithelial cell adherence
properties are encoded by genes on plasmids. Similarly, the toxins
often are plasmid- or phage-mediated. Five types of infections
occur
1. Enterotoxigenic E. coli (ETEC)
2. Enteropathogenic E. coli (EPEC)
3. Enterohemohaggic e, coli (EHEC)
4. Enteroinviasive E. coli (EIEC)
5. Enteragreegative E. coli (EAEC)
Enterotoxigenic E coli (ETEC)
 Common cause of "traveler's diarrhea" and imp. cause of
diarrhea in infants in developing countries. ETEC colonization
factors promote its adherence to epithelial cells of small bowel.

 Some strains of ETEC produce a plasmid encoded heat-labile

exotoxin (LT) (MW 80,000). Its subunit B attaches to the GM1
ganglioside at the brush border of epithelial cells of the small
intestine and facilitates the entry of subunit A (MW 26,000) into
the cell, where the latter activates adenylyl cyclase. This
markedly increases the local concentration of cyclic adenosine
monophosphate (cAMP), which results in intense and prolonged
hypersecretion of water and chlorides and inhibits the
reabsorption of sodium. The gut lumen is distended with fluid,
and hypermotility and diarrhea ensue, lasting for several days.

 LT is antigenic and cross-reacts with the enterotoxin of Vibrio

cholerae. LT stimulates the production of neutralizing antibodies
in the serum of persons previously infected with ETEC.
 Persons residing in areas where such organisms are highly prevalent
(eg, in some developing countries) are likely to possess antibodies
and are less prone to develop diarrhea on reexposure to the LT-
producing E coli.

 Some strains of ETEC produce the heat-stable enterotoxin STa

(MW 1500–4000), which is under the genetic control of a
heterogeneous group of plasmids. STa activates guanylyl cyclase in
enteric epithelial cells and stimulates fluid secretion. Many STa-
positive strains also produce LT. The strains with both toxins
produce a more severe diarrhea.

 The plasmids carrying the genes for enterotoxins (LT, ST) also may
carry genes for the colonization factors. It is possible that virtually
any E coli may acquire a plasmid encoding for enterotoxins.

 Care in the selection & consumption of foods is highly

recommended to help prevent traveler's diarrhea. Antimicrobial
prophylaxis can be effective but may result in antibiotic resistance
Once diarrhea develops, antibiotics shortens the duration of disease.
 Enteropathogenic E. coli (EPEC)

 It is an imp. cause of diarrhea in infants, especially in developing

countries & was associated with in nurseries in developed
countries.

 EPEC adhere to the mucosal cells of the small bowel.

Chromosomally mediated factors promote tight adherence. There
is loss of microvilli (effacement), formation of filamentous actin
pedestals or cup-like structures, and, occasionally, entry of the
EPEC into the mucosal cells. Characteristic lesions can be seen
on electron micrographs of small bowel biopsy lesions.

 The result of EPEC infection is watery diarrhea, which is usually

self-limited but can be chronic. EPEC diarrhea has been
associated with multiple specific serotypes of E. coli; strains are
identified by O antigen & rarely by H antigen typing.

 The duration of the EPEC diarrhea can be shortened and the

chronic diarrhea cured by antibiotic treatment.
 Enterohemorrhagic E coli (EHEC) produces verotoxin, named
for its cytotoxic effect on Vero cells, a line of African green monkey
kidney cells. There are at least two antigenic forms of the toxin.

 EHEC has been associated with hemorrhagic colitis, a severe form

of diarrhea, and with hemolytic uremic syndrome, a disease
resulting in acute renal failure, microangiopathic hemolytic anemia
(loss of RBCs through destruction caused by factors in the small
blood vessels), and thrombocytopenia (low amount of platelets).

 Verotoxin has many properties that are similar to the Shiga toxin
produced by some strains of Shigella dysenteriae type 1; however,
the two toxins are antigenically and genetically distinct.

 Of the E. coli serotypes that produce verotoxin, O157:H7 is the

most common and is the one that can be identified in clinical
specimens. EHEC O157:H7 does not use sorbitol, unlike most
other E coli, and is negative on sorbitol MacConkey agar (sorbitol is
used instead of lactose); O157:H7 strains also are negative on
MUG tests. Specific antisera are used to identify the O157:H7
strains. Many cases of hemorrhagic colitis can be prevented by
properly cooking ground beef.
Enteroinvasive E coli (EIEC) produces a disease very similar to
shigellosis. The disease occurs most commonly in children in
developing countries and in travelers to these countries. Like shigella,
EIEC strains are nonlactose or late lactose fermenters and are
nonmotile. EIEC produce disease by invading intestinal mucosal
epithelial cells.

Enteroaggregative E coli (EAEC) causes acute and chronic

diarrhea (> 14 days in duration) in persons in developing countries.
These organisms also are the cause of food-borne illnesses in
industrialized countries. They are characterized by their characteristic
pattern of adherence to human cells. EAEC produce ST-like toxin and
a hemolysin.
Sepsis

When normal host defenses are inadequate, E coli may reach the
bloodstream and cause sepsis. Newborns may be highly susceptible to E
coli sepsis because they lack IgM antibodies. Sepsis may occur
secondary to urinary tract infection.

Meningitis

E coli and group B streptococci are the leading causes of meningitis in

infants. Approximately 75% of E coli from meningitis cases have the K1
antigen. This antigen cross-reacts with the group B capsular
polysaccharide of N meningitidis. The mechanism of virulence
associated with the K1 antigen is not understood.
Diagnostic Laboratory Tests

Specimens
Specimens included urine, blood, pus, spinal fluid, sputum, or other
material, as indicated by the localization of the disease process.

Smears: Gram-negative rods

Culture
Specimens are plated on both blood agar and differential media. With
differential media, rapid preliminary identification of gram-negative
enteric bacteria is often possible.
 Treatment
No single specific therapy is available. The sulfonamides,
ampicillin, cephalosporins, fluoroquinolones, and aminoglycosides
have marked antibacterial effects against the enterics, but variation in
susceptibility is great, and laboratory tests for antibiotic sensitivity
are essential. Multiple drug resistance is common & controlled by
transmissible plasmids.

Various means have been proposed for the prevention of traveler's

diarrhea, including daily ingestion of bismuth subsalicylate
suspension (it can inactivate E coli enterotoxin in vitro) and regular
doses of tetracyclines or other antimicrobial drugs for limited
periods. Because none of these methods are entirely successful or
lacking in adverse effects, it is widely recommended that caution be
observed in regard to food and drink in areas where environmental
sanitation is poor and that early and brief treatment (eg, with
ciprofloxacin) be substituted for prophylaxis.
Epidemiology, Prevention, & Control

E. coli establish themselves in the normal intestinal tract within a few

days after birth and from then on constitute a main portion of the
normal microbial flora. Control measures are not feasible as far as the
normal endogenous flora is concerned.

Enteropathogenic E coli serotypes should be controlled like

salmonellae.

Some of the enterics constitute a major problem in hospital

infection. Their control depends on hand washing, rigorous asepsis,
sterilization of equipment, disinfection, restraint in intravenous
therapy, and strict precautions in keeping the urinary tract sterile (ie,
closed drainage).