Transcript Escherichia coli is a bacterium, which inhabits the intestinal tract of

Diagnostic microbiology
lecture: 2
Enterobacteriaceae
Abed ElKader Elottol
MSc. microbiology
1
•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.
•E. coli is a normal inhabitant of the intestinal tract and may
cause a wide variety of diseases. Infants diarrhea, meningitis,
wound infection, urinary tract infection......etc.
•The presence of this organism in water is used as an indicator
of recent fecal contamination because this organism does not
survive in water for long. Strains of E.coli which are capable of
causing disease, possess one or more virulence factor:
•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
8
9
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 +
• Lactose fermenter
SEROLOGY:
Since not all strains are enteropathogenic, agglutination with
specific antisera must be performed before issuing a report of
isolating EPEC.
TREATMENT:
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
13
1. Enterobnacter aerogenes (Type species)
2. E. cloacae
3. E. liquefaciens
4. E. sakazakii (cause of diarrhea; associated with infant
formula) new genus Cronobacter.
•Formerly known as aerobacter.
• Often confused with Klebsiella.
• Motile .
• ornithine decaroboxylase (ODC) positive .
• These tests are used to differentiate it from the similar genus
of Klebsiella.
A. Morphology:
•
•
•
Short and plump rods occurring singly in pairs or in
short Chains.
non-spore forming.
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).
Ornithine +.
KLEBSIELLA
19
•The most clinically important species of this genus is
Klebsiella pneumoniae.
• K.pneumoniae infections are common in hospitals
where they cause pneumonia (characterized by emission
of bloody sputum) and urinary tract infections in
catheterized patients.
•K. pneumoniae is second only to E. coli as a urinary tract
pathogen. Klebsiella infections are encountered far more
often now than in the past.
20
This is probably due to the bacterium's antibiotic resistance
properties.
Klebsiella species may contain resistance plasmids(R-plasmids)
which confer resistance to such antibiotics as ampicillin and
carbenicillin.
To make matters worse, the Rplasmids can be transferred to
other enteric bacteria not necessarily of the same species.
21
Morphology and Staining:
- Gram-negative Bacillus
- Encapsulated (the capsule is greater in size than the cell
itself).
- Non-motile
- 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.
•TSIA A/AG
•IMVIC reaction - - + +
• Does not liquefies Gelatin.
•Lysine +
• Non-motile
•Ornithine -
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.
Differentiation
Reaction
K. pneumonia
E. aerogenes
IMVIC
++--
++--
Motility
Non-motile
motile
String Test
Positive
Negative
Gelatine liquefaction
Negative
Negative
Ornithine decaroboxylase Negative
Positive
CITROBACTER
28
•Citrobacter is not considered to be an enteric pathogen
because it is normal gut flora.
• When plated, Citrobacter colonies bare a strong
resemblance to E. coli colonies.
•This group of bacteria is of small clinical interest. C.
freundii is suspected to cause diarrhea and possibly
extra-intestinal infections.
•C. diversus has been linked to a few cases of meningitis
in newborns.
29
•Morphologically appear similar to Escherichia coli.
• Biochemically resembles Salmonella.
•TSI: A/A+
• IMVIC - + - +
• Urease ±
• Lysine –
•Hydrogen sulfide + (C. freundii)
•Citrate +