Gram negative bacilli

Other
Gram negative Bacilli Oxidase Oxidase
Positive Negative
Pseudomonas
Vibrio
Neisseria (diplococci)
Brucella
Pasturella

Enterobacteriaceae
Features common to all members of this heterogeneous family are:
1- Anatomic location.
2- Gram-negative bacilli.
3- They are all facultative anaerobes; (growth is rapid).
4- Grow on simple media.
5- They are oxidase negative.
6- They are divided into two groups according to their effect on lactose.

Enterobacteriaceae

Lactose fermenter Non-lactose fermenter

* Escherichia coli * Salmonella
* Klebsiella * Shigella
* Citrobacter * Proteus
* Enterobacter

7- All members ferment glucose with acid production (fermentation of other sugars
varies).
8- They reduce nitrates to nitrites as part of their energy-generating processes.
9- All have LPS endotoxin.
10- Some Enterobacteriaceae also produce protein exotoxins (Cytotoxins kill cells while
Enterotoxins cause diarrhea)

Lactose fermenter

Escherichia coli (E. coli)

E. coli is a normal inhabitant of the intestinal tract of man and animals. Some strains
cause intestinal and extra intestinal disease in man.
1- Morphology:
 Gram – negative bacilli.
 Motile ( Peritrichous flagella).
 Some strains are capsulated, especially those from extra-intestinal infections.
2- Culture:
 Facultative anaerobes.
 Grow on simple media.
 On MacConkey agar, they produce rose – pink colonies due to lactose
fermentation.
3- Antigenic structure:
 Somatic “O” antigen (LPS) > 180 have been described.
 Flagellar “H” antigen > 50 have been identified.
 Capsular “K” antigen.

The antigenic formula for serotypes is described by linking the letter (O, K, or H)
and number of the antigens present (eg, O111:K76:H7).
4- TOXINS
E. coli can produce every kind of toxin found among the Enterobacteriaceae
such as:

 Alpha – hemolysin: is pore-forming cytotoxin The toxin causes leakage of

cytoplasmic contents and eventually cell death.

 Shiga toxin: is produced by Shigella and E. coli and are of the AB type
prevents protein synthesis, leading to cell death.

 Labile toxin (LT): is also an AB toxin. The structure and biological effect of LT
is very similar to cholera toxin, the result is the stimulation of chloride
secretion out of the cell and the blockage of NaCl absorption The net effect is
the accumulation of water and electrolytes into the bowel lumen.

 Stable toxin (ST): is a small (17- to 18-amino acid) peptide, ST causes an

LT-like net secretion of fluid and electrolytes into the bowel lumen.

5- Diseases caused by E.coli:

A) Intestinal diseases:
There are 5 groups of E.coli associated with intestinal diseases:

1- Enterotoxigenic E. coli(ETEC): an important cause of traveler’s diarrhea.

2- Enteropathogenic(EPEC)or Enteroadherent E. coli: attach to mucosal
cells in the small intestine by outer membrane protein that called intimin,
causing destruction of microvilli. Watery diarrhea results .cause of
neonatal enteritis, especially in underdeveloped countries and in nursery
outbreaks.
3- Enteroinvasive E. coli(EIEC): which causes a dysentery-like disease and
invades the host cell and provokes a significant inflammatory response.
4- Enteroaggressive E.coli [EAEC]: Cause acute and chronic diarrhoea in
developing countries. Little is known about their virulence factor.
 5- Enterohemorrhagic E. coli(EHEC):(Vero cytotoxin-producing E.coli
VTEC) which causes hemorrhagic colitis and has been associated with the
hemolytic-uremic syndrome esp. in children.

Hemolytic-uremic syndrome (HUS):

- (HUS) is defined by the following:
 Acute renal insufficiency.
 Microangiopathic hemolytic anemia.
 Thrombocytopenia.
- EHEC, serotype O157:H7 is the most common cause of HUS.
- EHEC produces verotoxin that destroys microvilli, then verotoxin enters
blood, causing vascular endothelial damage and increased platelet
aggregation. Thus leading to formation thrombi causing ischemic damage
to colon (Hemorrhagic colitis), kidneys (haemolytic uremic syndrome).

Laboratory Diagnosis:
 E. coli O157:H7 does not ferment sorbitol, which serves as an important
criterion that distinguishes it from other strains of E. coli.

The isolation of enterotoxigenic or enteropathogenic E. coli from patients

with diarrhea is not a routine diagnostic procedure.
 EPEC -- culture on MacConkey agar and molecular method.
 ETEC --produced HL , HS toxins (vibirio like).
- Traditional tissue culture.
- ELISA for LT.
- Biken test (precipitint test) performed directly on bacterial colonies.
- Latex particle agglutination for screening.
 EIEC:
- Sereny test cause conjuntivitis in guinea pigs now adays is replaced by
tissue culture on HEP2 or Hela cells.
- PCR for gene encoding the invasion plasmid antigen (ipaH).
 VTEC: (Shiga-like toxin)
- DNA probes for the gene encoded VT1, VT2.
- Sorbitol instead of lactose O157 VTEC ferment sorbitol slowly.

B) Extra – intestinal diseases:

1- Urinary tract infection:

E.coli is the commonest cause of urinary tract infections (60-90%).
Certain strains known as uropathogenic E.coil colonize the vagina and
periurethral region from where they ascend to the bladder or kidney causing
cystitis or pyelonephritis .
2- Neonatal meningitis:
Esch. Coli cause 40% of neonatal meningitis.
3- Wound sepsis
Diagnosis:
1- Sample:
According to the site of infection e.g.
- Urine [Mid stream urine].
- Pus.
- Stool.
- C.S.F.
2- Direct smear:
From any clinical specimen [with the exception of faeces ].

3- culture:
- On MacConkey agar
- Lactose fermenting pink colonies.

4- Biochemical reaction:

- Indol +ve
- Methyl red +ve
- Vogesproskauer -ve
- Citrate -ve

5- Serological test:

- It is done in cases of gastro – enteritis.

- Suspected colony + specific anti sera agglutination.

Treatment

 Treatment of E. coli infections depends on the site of disease and the

resistance pattern of the specific isolate. For example,
 An uncomplicated lower urinary tract infection can be treated for just 1 –3
days with oral trimethoprim-sulfamethoxazole or an oral penicillin, e.g.,
ampicillin.
 However, E. coli sepsis requires treatment with parenteral antibiotics (e.g., a
third-generation cephalosporin, such as cefotaxime, with or without an
aminoglycoside, such as gentamicin).
 For the treatment of neonatal meningitis, a combination of ampicillin and
cefotaxime is usually given.
 Antibiotic therapy is usually not indicated in E. coli diarrheal diseases.
However, administration of trimethoprim-sulfamethoxazole or loperamide
(Imodium) may shorten the duration of symptoms. Rehydratio n is typically
all that is necessary in this self-limited disease.

Prevention: There is no specific prevention for E. coli infections, such as active

or passive immunization.