Download e.coli-study material-2012

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ESCHERICHIA COLI
ESCHERICHIA COLI
The name of the genus is given after Escherich who first isolated the bacillus from faeces of infant suffering
from diarrhea. Unlike other coliforms, E. coli is a parasite inhabiting only in human and animal intestine. E. coli
is the type species of the genus Escherichia but several other spp. are present.
Morphology
E. coli is a Gram-negative, nonsporing, non- capsulated bacillus measuring 1-3i.m x 0.4 — 0.7m. Most strains
are motile by peritrichate flagella, some strains may be nonmotile.
Cultural character
They are aerobic and facultative anaerobes and grow on ordinary culture media (nutrient broth, nutrient agar)
at 37°C in 8 to 24 hours. On MacConkey’s agar medium the colonies are rose- pink due to lactose fermentation
and some strains show beta-haemolytic on blood agar.
Biochemical reaction
They break down most of the sugars glucose, mannitol, sucrose, maltose) with production of acid and gas. E.
coli is a typical coliform. Indole reaction and methyl red reaction are positive but V.P. and citrate utilization
tests are negative (IMViC= + + - -).
Resistance
Esch. coil has a slightly higher resistance to heat than other coliforms. They are excreted in faeces of man and
animal in very large numbers and contaminate the soil very widely and survive in the environment without
growth for several weeks or months.
Antigenic structure
1. Somatic antigen (0 antigen)
These are heat stable and over 164 different 0 antigens are described, which .are designated as 1,2,3 and so
on.
2. Surface antigen (K antigen)
The term K antigen was first used collectively for the surface or capsular polysaccharide antigens that cause 0
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ESCHERICHIA COLI
inagglutinability. These are looked upon as “capsular” structures, but they are not demonstrable by light
microscopy. These are destroyed by heating at 100°C to 121°C for one hour.
3. Flagella antigen (H antigen)
These are thermolabile and till now 50 H antigens are described and all of them are monophasic. The
organisms have to be grown in semisolid agar for H antigen determination.
4. Fimbrial antigen (F antigen)
These are thermolabile proteins and heating the organisms at 100°C detaches its fimbriae. These antigens may
cause confusion in serum agglutination tests when the suspension is fimbriate because many human sera
contain fimbrial antibody. The F antigen has no significance in antigenic classification of Esch. coli.
Toxins and toxic products
Some strains of Esch. coli produce enterotoxin, haemolysin and verocytotoxin.
Pathogenesis
Esch, coli forms a part of normal intestinal flora of man and animal, and the commensal strains belong to
several 0 groups. There are a great variety of strains of Esch. coli Which include purely commensal strains as
well as strains possessing virulence determinants that cause wide spectrum of diseases in all the age groups of
human beings and
animals. The virulent strains of Esch. coil act as specific pathogens of the gut (enteritis) and of extra-intestinal
sites (urinary tract infection, wound infection).
Clinical infections caused by Esch. coil.
1. Urinary tract infection
2. Septic infections of wound
3. Diarrhoeas
4. Dysentery
5. Septicaemia
6. Penumonia
7. Neonatal meningitis
8. Abscesses in a variety of organs.
1. Urinary tract infection
Esch. coli are the predominant coliform bacteria responsible for 60-80% infections of the urinary tract.
Members of a quite small number of Esch. coli 0 serotypes form the majority of isolates from urinary tract
infection (UTI). These bacteria are also normally present in the colon of both infected and non-infected
individuals.
(a) Polysaccharides of 0 and K-antigens protect the bacteria from the bactericidal effects of complements
and phagocytes in absence of specific antibodies. Strains possessing Ki or KS antigen appear to be more
virulent.
(b) Fimbriae specially mediate the adherence of the organism to the uro-epithelial cells. Urinary
obstruction due to enlarged prostate, calculi and pregnancy are important predisposing factors in UTI.
The bacterial flora of colon is the reservoir of infection and infection often occurs in an ascending
manner via the urethra from the perineum. Colonization of pen-urethral area by potential pathogen
may be a prerequisite for UTI. Females are more frequently affected due to shortness of urethra. In
newborn, the infection may occur by haematogenous spread.
2. Diarrhoea and dysentery
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ESCHERICHIA COLI
The Esch. coil agents causing diarrhoeal diseases are divisible into four groups .They produce diarrhoea
with different pathogenic mechanisms.
(i) Enteropathogenic Esch. coil (EPEC): They cause enteritis in infants, most prevalent in tropical countries. Outbreaks of infantile enteritis may occur in hospital with high mortality.
(ii) Enterotoxigenic Esch. coil (ETEC): The strains produce a heat-labile enterotoxin (LT) or a heat-stable
enterotoxin (ST) or both. They also contain colonization factors (CFA) which are specific for the host animal
species.
(iii) Enteroinvasive Esch. coli (EIEC) : These strains produce illness in patients of all ages which is identical to
shigella dysentery.
(iv) Vero cytotoxin-producing Esch. coil (VTEC) VTEC strains cause haemorrhagic colitis (HC) and haemolytic
uremia syndrome (HIUS) .
3. Pyogenic infections
E. coil may cause wound infection, peritonitis, biliary tract infection, septicemia and neonatal meningitis.
Neonates are prone to infection due to lack of 1gM antibodies.
Laboratory diagnosis
Specimens: Specimens to be collected depending on the type of lesion; fresh sample of midstream urine in
U.T.I., faeces or rectal swab in acute diarrhoea, pus from wound in sterile cotton swab, C.S.F. in pyogenic
meningitis etc.
The specimen should be cultured and examined immediately after collection and in case of unavoidable delay,
it may be stored at 4°C for a few hours before culture.
Microscopy
Centrifuged deposit of urine, CSF, pleural fluid & other body fluids show fair number of pus cells and bacteria.
Gram’s staining of the deposit shows short gram-negative bacilli in a background of large number of pus cells.
However, treatment of faeces with fluorescent labeled 0 group antisera is helpful for making an early
provisional diagnosis of infantile diarrhoea. It is important that 2 specimens should be collected from each
patient, one for microscopy and other for culture; alternatively culture should be done first followed by
microscopy from the residual portion of a single specimen.
Culture
Fresh specimen should be inoculated directly on MacConkey’s agar plate and blood agar plate and incubated
overnight at 37°C, when delay of 1-2 hours is unavoidable, specimen should be preserved in refrigerator.
MacConkey’s agar shows pink, circular, convex; smooth no viscous colonies with clear-cut margin.
Biochemical reaction
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ESCHERICHIA COLI
Most strains of Esch. coil produce acid and gas from large number of carbohydrates. Strains include some late
lactose as well as some nonlactose fermenters strains. Few strains are anaerogenic.
Enteropathogenic Esch. Coli (EPEC)
The ability of EPEC strains to cause enteritis (diarrhoea) in babies and adults has been confirmed by oral
administration of the organisms. Most strains belonging to EPEC serogroups do not produce LT, ST or VT and
are noninvasive. However, a few EPEC strains (0114, 0128) have been shown to produce enterotoxin.
The term EPEC is best reserved for EPEC strains which do not release LT, ST or VT.
EPEC strains colonize the mucosa of the upper part of small intestine, but the mechanism of adhesion is less
known. It is seen by electron microscope that EPEC are intimately associated with the mucosal surface and
partly surrounded by cup like projections (pedestals) of the enterocyte surface leading to loss of brush border
microvilli at the site of attachment.
EPEC diarrhoea is a major cause of infant mortality in tropical countries due to poor sanitation and hygiene. In
nurseries and neonatal units, the infection, is transmitted directly from case to case and via environmental
contamination. Sometimes mother serves as the source of infection.
Enterotoxigenic Esch. coil (ETEC)
ETEC strains are frequently seen in geographical regions where hygiene and nutrition are good, but occasional
outbreaks of diarrhoea in hospital or in the community occur irrespective of age of the patient.
Patho genesis
Enterotoxin producing Esch. coli have dual capacity to adhere to small-bowel epithelial cells and to produce
one or more enterotoxins, either ST or LT or both. Only enterotoxin is not sufficient to cause diarrhoea.
The illness that follows ETEC infection ranges from fulminating cholera like disease (found in Indian subcontinent) to mild, watery diarrhoea. Persons of all ages are affected.
Sources and mode of infection of ETEC diarrhoea are not well understood but in one study in Bangladesh
drinking water was the source of infection. Prevention involves careful hygienic practices with special care to
ingestion of clear water (boiled for 2 minutes), properly cooked food and prophylactic use of doxycycline.
Enteroinvasive Esch. coil (EIEC)
EIEC are far less pathogenic than ETEC and cause about 5% of all diarrhoea in areas of poor hygiene. EIEC may
cause a syndrome similar to shigellosis but the illness is of shorter duration.
EIEC do not produce enterotoxins, but invade the intestinal epithelial cells multiply and eventually cause an
ulceration of the bowel. The most common serogroup is 0124. Incubation period ranges from 8-24 hours and
symptoms include pyrexia with mucoid stools containing blood.
Pathogenesis
EIEC were first isolated from stools of older children and adults with shigella dysentery disease. Both Shigella
and EIEC cause dysentery by invasive mechanism. They invade intestinal epithelial cells and multiply
intracellularly which results in tissue destruction and consequent inflammation and ulceration producing
symptoms of bacillary dysentery. Virulence of Shigella and EIEC is dependent on chromosomal and plasmid
genes, i.e. a plasmid coded outer-membrane protein.
Epidemiology
EIEC infections are usually food borne but cross infection is also known. Occasional outbreaks have been
reported from UK and USA.
Vero cytotoxigenic Esch. coli (VTEC)
The importance of VTEC became evident only when their association with two diseases of previously unknown
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ESCHERICHIA COLI
etiology were established; haemorrhagic colitis (HC) and haemolytic uremia syndrome (HUS). The cytotoxin
produced by VTEC is called verotoxin because of its toxic effect on Vero cells in tissue culture.
Pathogenesis
A small number of Esch. coli strains most commonly 0157 : H7, cause widely distributed sporadic cases of HC
in USA. Persons of all age groups are affected. Food, particularly undercooked meat is an important source of
infection. All isolates are non-invasive. They act by two mechanisms.
(a) Adhesion: Adhesion of VTEC to intestinal mucosa is mediated by serologically distinct fimbrial antigens
on the bacterial cell surface.
(b) Verocytotoxin (VT): When VT was first described, it was found to be very similar to Shiga toxin
(produced by strains of Sh. dysenteriae type 1) in terms of physical, biological and antigenic properties.
VT is phage encoded. VTEC cause disease not by invasion but by a direct effect of the verocytotoxin on colonic
mucosal cells, and generally cause few systemic symptoms.
Manifestations
The clinical illness is usually heralded by cramps abdominal pain and watery diarrhoea. This is followed within
24 hour by frankly bloody stools (HC). Ascending and proximal transverse colons are mainly affected. Unlike
Shigella and Campylobacter dysentery, the patient is usually afebrile. The illness lasts for about a week. With
the exception of person developing HUS, the illness is usually self limited.
HUS is characterized by acute renal failure, micro-angiopathic haemolytic anaemia and thrombocytopenia.
Although, HUS may occur at any age, but infants and children are more frequently affected. Typical HUS
follows HC while atypical HUS occurs without diarrhoea! phase.
Epidemiology
VTEC infections are mainly due to strains of serogroup 0157. Outbreaks have occurred in communities, nursing
homes for elderly persons and also in day-care centers for young children.
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PASSWORD:E.coli
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