Diagnostic microbiology lecture: 1 Enterobacteriaceae Abed ElKader Elottol MSc . microbiology - PowerPoint PPT Presentation

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Diagnostic microbiologylecture: 1EnterobacteriaceaeAbed ElKaderElottolMSc. microbiology

The Gram Negative BacilliFamily 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 Yersiniapestis, the agent of bubonic plague.
Family EnterobacteriacaeLactose 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

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