Taxonomy
of the Enterobacteriaceae
The taxanomy
of the Enterobacteriaceae is complex
and is rapidly changing as further DNA homology studies are performed. A
comprehensive approach to the classification of this family is presented by Kelly,
Brenner and farmer in manual of clinical microbiology.
The family Enterobacteriaceae in characterized biochemically by the ability to
reduce nitrates to nitrites and to ferment glucose with the production of acid
or acid and gas. The Enterobacteriaceae
do not require increased amounts of sodium chloride for growth and are
oxidase-nagative.
Different species of Enterobacteriaceae are classified or
grouped using series of Biochemical tests.
Table 1.1 shows Biochemical reaction
pattern in primary tests for the common clinically significant Enterobacteriaceae.
Typically organism of Enterobacteriaceae is seen in growth on
solid media as short gram-nagative rods often capsulated in some species.
1.
Eschherichia:-
E. coli typically produces positive
tests for indole, lysine decarboxylase, and mannitol fermentation and produces
gas from glucose. An isolate from urine can quickly be classified as E.coli by its heamolysis on blood agar
with an iridescent sheen on differential media like EMB.
2. Klebsiella- Enterobacter
–Serratia group:-
Klebsiella species
exhibit mucoid growth, large polysaccharide capsules, lack motility, gives
positive tests for lysine decarboxylase and citrate.
Enterobacter
species give positive tests for motility, citrate, and produces gas from
glucose.
3. Proteus- Morganella-
providencia group:
Members of this group on potassium cyanide medium
(KCM), and ferment xylose proteus
spp. Move very actively by means of peritrichous flagella resulting in swarming
on solid media. Proteus and morganella are urease positive while
providencia are urease negative.
4. Citrobacter:
They are citrate positive and differ from the salmonellae in that they do
not decarboxylate lysine ferment lactose very slowly.
5. Samonellae:
They are motile rods, ferments glucose and mannose without producing gas. Do
not ferment lactose or sucrose. They produce H2S.
6. Shigellae:
These are non-motile and usually ferment lactose (but do not ferment other carbohydrates
and producing acid but not gas they do not produces H2S.
Antigenic
classification
A part
from biochemical characteristics, Enterobacteriaceae
are classified by more than 150 different heat stable somatic O (Oligopolysaccharide)
antigen, more than 100 heat labile K (Capsular) antigen, and more than 50 H (flagella
antigens. Antigenic classification of Enterobacteriaceae
often indicate the presence of each specific antigen for organisms to share a
common environment, they must have something in common (characteristic)
G + C content
–
haemolysis
Sterner
The Enterobacteriaceae are a large,
heterogeneous group of gram negetive rods whose natural habitat is the
intestinal tract of humans and animals. Some enteric organisms, e.g E. coli are part of the normal flora and
incidentally cause disease, while others, the salmonellae and shigellae, are
regularly pathogenic for humans. The enterobacteriaeae are facultative
anaerobes or aerobes, ferment a wide range of carbohydrates, posses a complex
antigenic structure and produce a variety of toxics and other virulent factors.
These enteric gram-negative rods or enteric bacteria may also be called
coliforms.
Carbohydrate
fermentation patterns and the activity of amino acid decarboxylase and other
enzymes are commonly used in biochemical differentiation and classification. Culture
on differential media that contain special dyes and carbohydrates e.g EMB, MacConkey,
etc distinguishes lactose fermenters from non-lactose fermenters and may allow
rapid presumptive identification/classification of enteric bacteria together
with production of indole for tryptophan.
One
complex medium have been devised to differentiate between salmonella and
shigella from other coliforms. The medium is called triple sugar iron agar.
Rapid
presumptive identification/classification of gram-negative enteric bacteria
into 3 groups
Lactose
fermented rapidly
|
Lactose
fermented slowly
|
Lactose
not fermented
|
E. coli:
metallic sheen on differential media; motile, flat; non-viscous colonies.
|
Edward
siella, Serratia, citrobacter
|
Shigella Spp:
Non Motile; No Gas From Dextrose
|
Enterobacter aerogenes:
Raised colonies, no metallic sheen, often motile, more viscous growth.
|
Arizona,
Providencia
Erwinia.
|
Proteus
spp: ‘swarming’ on agar; urea rapidly hydrolyzed (smell of ammonia).
Pseudomonas spp:
Soluble
pigments, blue-green and fluorescing; Sweetish smell
|
TABLE: BIOCHEMICAL REACTION PATTERN IN PRIMARY
TEST FOR COMMON CLINICALLY SIGNIFICANT ENTEROBACTERIACEA
Indole
|
Lysine
decarboxylase
|
Mannitol
|
Glucose
|
Citrate
|
xylose
|
Urease
|
lactose
|
Mannose
|
||
1
|
E. coli
|
+ve
|
+ve
|
+ve
|
Pdtn of Gas
|
+ve
|
||||
2
|
Klebsiella spp
|
+ve
|
Pdtn of Gas
|
-
|
||||||
3
|
Proteus
morganella
poovidencia
|
-
|
-
|
-
|
-
|
+ve
|
+ve
|
+ve
+ve
-ve
|
-
|
-
|
4
|
Citrobacter
|
-
|
Pdtn of Gas
|
slowly
|
||||||
5
|
Salmonellae
Pattern of H2S
|
+ve
|
+ve
|
+ve
|
||||||
6
|
NO H2O
pattern
Shigellae
|
+ve
|