Escherichia coli is a bacterium, which inhabits the intestinal tract of humans and other warm-blooded mammals.
It constitutes approximately 0.1% of the total bacteria in the adult intestinal tract.
Its name comes from the name of the person, Escherich, who in 1885 first isolated and characterized the bacteria.
Produces a heat-labile toxin (LT) and a heat stable toxin (ST): LT action is identical to cholera toxin. LT causes diarrhea by stimulating the activity of a membrane-bound adenylate cyclase, ATP is converted into cAMP.
cAMP induces the active secretion of Cl- and inhibit the absorption of Na, creating an electrolyte imbalance across the intestinal mucosa, resulting in the loss of large amounts of fluid and electrolytes from the intestine.
Produces diarrhea by several poorly understood mechanisms.
One mechanism is the production of adhesion factor which causes the adherence to the cells of the small intestine.
Produces toxin similar to Shigella dysenteria (Shiga-like toxin SLT-1 and SLT-2).
The toxin inhibits protein synthesis in the affected cells.
The disease is known as hemolytic colitis.
E.coliO157H7 representative of this type.
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.
Various Types of E. coli
non-pathogenic when found in the alimentary tract of man.
Cystitis, meningitis, vaginitis, wound infection.
Epidemic diarrhea of the newborn.
NOTE: It is now widely accepted that certain serological types of E. coli are responsible for outbreaks of infantile diarrhea in nurseries.
Antigenic structure: O-antigen, H-antigen and K antigen
Stained smears reveal gram negative rods
Triple Sugar Iron Agar A/AG
IMVIC reaction + + - -
On EMB : Green metallic sheen
Lysine decarboxylase +
Since not all strains are enteropathogenic, agglutination with specific antisera must be performed before issuing a report of isolating EPEC.
Antibiotic sensitivity testing must be performed. Aminoglycosides are effective most especially Amikacin.
Treating E. coli infections with antibiotics may actually place the patient in severe shock which could possibly lead to death. This is because more of the bacterium's toxin is released when the cell dies.ENTEROBACTER
Although this bacterium is part of the normal flora of the human intestinal tract, several species cause opportunistic infections of the urinary tract as well as other parts of the body.E. aerogenes and E. cloacae are two such pathogens that do not cause diarrhea, but that are sometimes associated with urinary tract and respiratory tract infections.
Enterobnacter aerogenes (Type species)
E. sakazakii (cause of diarrhea; associated with infant formula) new genus Cronobacter.
Formerly known as aerobacter.
Often confused with Klebsiella.
ornithine decaroboxylase (ODC) positive .
These tests are used to differentiate it from the similar genus of Klebsiella.
Short and plump rods occurring singly in pairs or in short Chains.
motile with a peritrichous flagella.
B. Culture Characteristics:
Culture medium: EMB, ENDO, Mac.
On EMB, E. aerogenes appear as pink, while on Mac., colonies appear as pink usually not surrounded by lines of precipitated bile.
The organism is facultative anaerobe, reduces nitrates to nitrite.
Does not produces indole from tryptophan.
Produce acetyle methyl carbinol (Voges-Proskauer).
Utilize Citrate as the source of carbon.
IMVIC reaction - - + +.
Produces A/AG (Yellow slant over yellow butt with gas).
Positive Motility on a semi-solid medium.
Lysine + (except E. cloacae).
The most clinically important species of this genus is Klebsiellapneumoniae.
This is probably due to the bacterium's antibiotic resistance properties.
Klebsiellaspecies may contain resistance plasmids(R-plasmids) which confer resistance tosuch antibiotics as ampicillin and carbenicillin.
To make matters worse, the Rplasmids can be transferred to other enteric bacteria not necessarily of the same species.
Morphology and Staining:
- Gram-negative Bacillus
- Encapsulated (the capsule is greater in size than the cell itself).
- Non-spore former.
Medium used for cultivation: EMB, MacConkey Agar.
The organism develops large mucoid colonies, semifluid, with slimy appearance.
Colonies exhibit positive string test.
Urease producer (Slower and less intense than Proteus strains)
Indole is usually (-) for K. pneumonia.
Citrate is usually positive.
IMVIC reaction - - + +
Does not liquefies Gelatin.
Klebsiella species possess O and K antigens
= The capsule enables the organism to resist phagocytosis.
= Endotoxin and enterotoxin are also produced.
Primary community-acquired pneumonia, nosocomial pneumonia, urinary tract and wound infection, bacteremia and meningitis.DifferentiationCITROBACTER
Citrobacter is not considered to be an enteric pathogen because it is normal gut flora.
Morphologically appear similar to Escherichia coli.
Biochemically resembles Salmonella.
IMVIC - + - +
Hydrogen sulfide + (C. freundii)