GRAM NEGATIVE RODS
It comprises the following bacterial groups
b. Non-lactose fermenters
2. Oxidase Positive
These are pathogens that belong to the large group
of Gram negative rods.
They are naturally found in the intestinal tract, in
soil and water.
General Characteristics of Enterobacteria
Often referred to as coliforms or enterobacilli
Found as normal flora in intestinal tract of humans
Enterobacteria are aerobes and facultative
anaerobes, nonsporing and motile or non-motile.
They are oxidase negative,reduce nitrate to nitrite,
and ferment glucose with acid production(some
also produce gas).
Grow over a wide range of temperature in ordinary
Release endotoxin from their cell wall.
Some release exotoxin.
When lyzed, enterobacteria release endotoxin from
their cell wall (feature of all Gram negative rods).
Most of them have possessed three types of
antigens which are used in serotyping, particularly
salmonellae, shigellae, and E.coli.
Cross-reactions however can occur, due to a sharing
O antigens: These are found in the bacterial cell
They are heat stable.
K antigens: These are capsular polysaccharide
They surround the cell wall and can therefore
interfere with the testing of underlying O antigens
(K antigens can be heat-inactivated, enabling O
antigens to be detected).
The K antigens of some salmonellae, e.g. S. Typhi,
are called Vi antigens.
These are flagellar protein antigens possessed by
They are heat labile (destroyed at 60–100 ºC).
Salmonella serovars often produce two different
antigenic types of H antigens, called phase 1 and
E. coli causes:
Urinary tract infections. E. coli is the commonest
pathogen isolated from patients with cystitis.
Recurring infections are common in women.
Infections of wounds, peritonitis, sepsis and
endotoxin induced shock.
Meningitis and bacteraemia in neonates.
E. coli capsular type K1 is associated with neonatal
Diarrhoeal disease: infantile gastroenteritis,
traveller’s diarrhoea, dysentery, and haemorrhagic
diarrhoea which may progress to haemolytic
E. coli strains associated with diarrhoeal disease
ETEC (Enterotoxigenic E. coli): Causes watery
(secretory) diarrhoea due to the production of
plasmid mediated toxins (LT, ST) in infants and
adults, particularly in developing countries.
It is often referred to as traveller’s diarrhoea.
EPEC (Enteropathogenic E. coli): Causes
vomiting,fever, and prolonged diarrhoea mainly in
infants (less than 2 y). Due to bacteria adhering to
epithelial cells, multiplying and causing lesions.
EIEC (Enteroinvasive E. coli): Causes dysentery
(similar to shigellosis), fever and colitis, with blood,
mucus, and many pus cells in faecal specimens.
Due to bacteria invading and multiplying in
EHEC (Enterohaemorrhagic E. coli): Causes life-
threatening haemorrhagic diarrhoea (colitis) in all
ages, without pus cells, and often without fever.
It can progress to haemolytic uraemic syndrome
with renal failure.
EaggEC (Enteroaggregative E. coli): Causes chronic
watery diarrhoea and vomiting, mainly in children.
Due to the bacteria adhering to tissue cells often in
Specimens: Depending on site of infection,
specimens include urine, pus, faeces, cerebrospinal
fluid (infants), and blood for culture.
E. coli is a Gram negative usually motile rod.
Inactive strains are nonmotile. A minority of strains
E. coli is an aerobe and facultative anaerobe.
Optimum temperature for growth is 36–37 ºC with
most strains growing over the range 18–44 ºC.
Blood agar: E. coli produces 1–4 mm diameter
colonies after overnight incubation.
The colonies may appear mucoid.
Some strains are haemolytic.
MacConkey agar and CLED agar: E. coli ferments
lactose, producing smooth pink colonies on
MacConkey agar and yellow colonies on CLED agar.
Some strains (e.g. inactive strains) are late or non-
XLD and DCA agar: Yellow colonies are produced on
XLD agar. Growth of E. coli is usually inhibited on
KIA (Kligler iron agar): Most strains of E. coli
produce an acid deep and an acid slope with gas
production and no H2S blackening (similar to other
lactose fermenting coliforms).
Most strains of E. coli are;
Lysine decarboxylase (LDC) positive.
Reduce nitrate to nitrite, giving a positive urine
Citrate and H2S negative.
Antimicrobials that are used to treat E. coli urinary
and other infections include those with activity
against Gram negative organisms such as
cotrimoxazole,nalidixic acid, nitrofurantoin,
cephalosporins, and aminoglycosides.
In the treatment of E. coli diarrhoea, the use of
antibiotics is in general only of minor importance.
Rehydration of the patient is always the most
important measure taken.