Organisms in this group form one of the largest and well defined groups amongst Gram (-)
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
Best known is Escherichia coli, characteristic member of normal intestinal flora of mammals and also a important pathogen causing intestinal and urinary tract infections.
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.
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.
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.
1. Gram negative bacilli
2. Facultative anaerobes (grow with or without oxygen).
3. Glucose fermenters.
4. Oxidase negative
5. Nitrate positiveGeneral Characteristics
Shape and structure
motile or nonmotile
non-spore-formingThe 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