The document discusses two genera within the Enterobacteriaceae family: Edwardsiella and Citrobacter. It provides details on their characteristics, habitats, pathogenic species, and methods of identification. Edwardsiella is commonly found in reptile and fish guts, with E. tarda causing rare human infections related to trauma or aquatic environments. Citrobacter species are environmental and occasionally cause infections in humans, differing from E. coli in being citrate positive and lysine decarboxylase negative. Identification of specific Citrobacter species involves testing for indole production and H2S.

1. Edwardsiella &
Citrobacter

2. Enterobacteriaceae-1
FAMILY CHARACTERS (GENERAL PROPERTIES)
Members of the family En1erobacteriaceae should have !he
following properties:
• They are gram-negative bacilli
• Aerobes and facultative anaerobes
• Non-fastidious. can grow in ordinary media like nutrient agar
• Ferment glucose to produce acid with or without gas.
• Reduce nitrate to nitrite.
• They produce catalase (except Shigella dysenterie type-l).
• They do not produce oxidase.

3. Cont…..
• They are generally motile with peritrichous flagella, except
some members which are non motile, such as Shigella and
Klebsiella.
• Natural habitat: Most of them are commensals in human
intestine, called coliform bacilli, e.g. Escherichia, Klebsiella,
Proteus, Morganella, Providencia and Citrobacter, etc. The
exceptions are Shigella, Salmonella which are enteric pathogens,
not commensal.

4. Classification
1) The oldest method of classification of family
Enterobacteriaceae was based on fermentation of lactose on
MacConkey agar.
1) Newer methods of classification- After the availability of
molecular methods, the taxonomy is greatly changed.

6. Edwardsielleae
Edwardsiella is a commensal in the gut of reptiles and fishes.
Human infection is rare.
• E. tarda is the most frequently isolated species of Edwardsiella in
clinical specimens. It is associated with septic shock, liver abscess
and infections rela1ed to trauma and aquatic environment.
• E. tarda is so named because it is biochemically slow. It is motile
and biochemical properties are similar to E. coli with some
exceptions:
• Ferments fewer sugars (only glucose and maltose)
than E. coli
• Non-lactose fermenter
• Produces H2S.

7. TRIBE IV: CITROBACTEREAE
Citrobacter species are mostly environmental contaminants
isolated from water, soil, food and faeces of man and animals.
They occasionally cause urinary tract, gallbladder and middle ear
infections and neonatal meningitis (C. koseri).

8. CONT…
Identification: Citrobacter species are motile, lactose fermenters
like E. coli, bur differ from the latter in being citrate positive and
lysine decarboxylase negative. Various species can be
differentiated by:
• C. freundii- indole negative and H2S positive
• C. koseri (previously, C. diversus)- indole positive and H2S
negative
• C. amalonaticus- indole positive, H2S negative and grows in
KCN medium.

9. Cont…
• Some strains of Citrobacter freundii (formerly called Ballerup-
Bethesda group) possess Vi antigen, which is antigenically
similar to that of salmonellae and may lead to confusion in
identification.

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