Download The Gram Negative Bacilli

Diagnostic microbiology
lecture: 1
Enterobacteriaceae
Abed ElKader Elottol
MSc. microbiology
The Gram Negative Bacilli
Family Enterobacteriaceae
•Organisms in this group form one of the largest
and well defined groups amongst Gram (-)
•non-photosynthetic bacteria.
•All have small, rod shaped cells, either straight or
curved, not more than 1.5 μm in width.
•Facultative aerobes fermenting sugars into a
variety of end products.
• Produce acid from glucose.
•Ability to ferment glucose, separates them from
obligate aerobes.
Enterobacteriaceae
• Some immotile and some motile with
peritrichous flagella.
• Catalase (+) and oxidase (-).
• Enterobacteriacae are always oxidase (-)
hence separates them from oxidase
(+)bacteria such as Pseudomonas, species
which have similar morphology.
•Best known is Escherichia coli, characteristic member of
normal intestinal flora of mammals and also a important
pathogen causing intestinal and urinary tract infections.
•Closely related are other Enterobacteriacae such as
Salmonella and Shigella species, pathogens which cause
intestinal infections such as dysentery, typhoid fever and
food Poisonings.
•from different ecology are the genera Serratia and
Proteus which primarily occur in soil and water, and the
plant pathogen Erwina.
•Another member is Yersinia species which includes
Yersinia pestis, the agent of bubonic plague.
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Family Enterobacteriacae
Lactose Fermentation
ANTIGENIC PROPERTIES OF ENTEROBACTERIACAE
1. O Antigen: Somatic antigen. Heat stable antigen.
O antigens are lipopolysaccharides and are found in
the cell wall of most of gram-negative bacilli.
With sera containing anti-O antibodies, such antigens
agglutinate slowly in granular masses.
Antibodies to O antigens are predominantly IgM.
ANTIGENIC PROPERTIES OF ENTEROBACTERIACAE
2. H Antigen: Flagellar Antigen: This antigen is heat
labile and could be inactivated by heating over 60 oC.
With sera containing anti-H antibodies, such antigens
agglutinate rapidly.
Within a single Salmonella species, flagella antigens
may occur in either or both of 2 forms, called phase I
and phase II.
The organism tends to change from one phase to
other; this is called phase variation.
Antibodies to H antigen are predominantly IgG.
ANTIGENIC PROPERTIES OF ENTEROBACTERIACAE
3. The “Vi” Antigen: Capsular (K) antigens that are
present at the extreme periphery of the bacteria. Often
interfere with agglutination of freshly isolated strains by
antisera containing mainly anti-O agglutinins.
Vi antigens are destroyed by heating for 1 hour at 60 oC.
Example of antigenic designation of Salmonella:
Salmonella typhi O 1,2,(Vi): a: 1
= 1,2 are O antigens
= (Vi) if present
= a phase one H antigen.
= 1 phase two H antigen.
General Characteristics
1. Gram negative bacilli
2. Facultative anaerobes (grow with or without
oxygen).
3. Glucose fermenters.
4. Oxidase negative
5. Nitrate positive
General Characteristics
Shape and structure
gram-negative rods
motile or nonmotile
pili
capsule (some)
non-spore-forming
The Cultivation, Isolation and Differentiation of
Enterobacteriacae
Media of Choice:
1. Eosin Methylene Blue (EMB)
2. MacConkey Agar (the most commonly used media)
3. Hektoen enteric
4. Endo agar
5. Kligler`s Iron Agar (KIA) For initial differentiation
6. Triple Sugar Iron Agar (TSIA) For initial differentiation