CITROBACTER

10 plus species including:

SPECIES

Citrobacter amalonaticus
Citrobacter braakii
Citrobacter diversus
Citrobacter farmeri
Citrobacter freundii
Citrobacter koserii
Citrobacter sedlakii
Citrobacter werkmanii
Citrobacter youngae

MICROSCOPIC APPEARANCE

Gram Stains: Gram-negative.
Morphology: Straight rods, occurring singly and in pairs.
Size: Approximately 1 micrometer in diameter by 2.0-6.0 micrometers in length.
Motility: Usually motile by peritrichous flagella.
Capsules: None.
Spores: None.

MACROSCOPIC APPEARANCE

Citrobacter spp. grow readily on ordinary media. At 24 hours, colonies on nutrient agar are generally 2-4 millimeters in diameter, smooth, low, convex and moist. They usually appear translucent or opaque and gray with a shiny surface and an entire edge. Mucoid or rough strains may occur occasionally. Colonies which slowly ferment lactose can resemble Salmonella colonies on enteric media.

METABOLIC PROPERTIES

Facultatively anaerobic. Chemoorganotrophic, having both a respiratory and a fermentative type of metabolism.

KEY BIOCHEMICAL REACTIONS

METABOLIC PROPERTIES

Citrobacter spp. often occur in water, sewage, soil and food and may be present in clinical specimens where they can be opportunistic pathogens.

PATHOGENICITY

Citrobacter is found in clinical bacteriology, as an opportunistic pathogen, not only in stools but also in urine, sputum, and specimens from bacteremia, meningitis, otitis media, wounds, abscesses, the throat and autopsies.

RECOMMENDED MEDIA

For culture: Nutrient Agar, Blood Agar, Tryptic Soy Agar (TSA).
For selective isolation: MacConkey, Eosin Methylene Blue agar (EMB), Xylose Lysine Deoxycholate Agar (XLD). Most strains can also grow in Selenite Broth, Tetrathionate Broth and on SS Agar, Deoxycholate Agar, Brilliant Green Agar and Bismuth Sulfite Agar.
For maintenance: Nutrient Agar, TSA. Brucella with 20% Glycerol, or Skim Milk for storage at -70 degrees C. Lyophilization may be used for long-term preservation.
Optimum pH: 6.8-7.2.

INCUBATION

Temperature: 35 degrees C.
Time: 16-18 hours.
Atmosphere: Aerobic.

REFERENCES

1. Holt, J.G., et al. 1994. Bergey's Manual of Determinative Bacteriology , 9th ed. Williams & Wilkins, Baltimore, MD.

2. Holt, J.G., et al. 1986. Bergey's Manual of Systemic Bacteriology, Vol. I & II. Williams & Wilkins, Baltimore, MD.

3. The Oxoid Vade-Mecum of Microbiology . 1993. Unipath Ltd., Basingstoke, UK.


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