Citrobacter koseri, formerly known as Citrobacter diversus, is a Gram-negative bacillus that can be found in the environment, water, soil, and the intestines of humans and animals. It is an opportunistic pathogen and can occasionally be implicated in urinary tract infections among other infections.
When Citrobacter koseri is grown on Cystine-Lactose-Electrolyte-Deficient (CLED) Agar, especially from urine culture samples, the colonies typically exhibit the following morphology:
Size: Medium to large-sized colonies after 24 hours of incubation.
Color: They are usually pale yellow or colorless because Citrobacter koseri is typically lactose-negative or slow lactose fermenting on CLED. However, some strains might ferment lactose, producing a yellow colony.
Appearance: Colonies are usually smooth, moist, and translucent or opaque.
Elevation: Slightly raised or flat.
Margin: Entire (smooth) edges.
Other Features: On some differential media, not CLED, they may produce a characteristic metallic sheen, which is often associated with E. coli on EMB agar. However, it's essential to employ a battery of biochemical tests to correctly identify and differentiate Citrobacter koseri from other closely related bacteria.
It's also worth noting that the appearance of bacteria can vary depending on the specific strain, conditions of the culture, and incubation time. Therefore, colony morphology is just one piece of the puzzle when identifying bacteria. A combination of colony morphology, gram stain, and biochemical tests is often used for precise identification.
Citrobacter koseri
CLED Agar
Urine culture
Gram-negative bacillus
Opportunistic pathogen
Medium to large-sized colonies
Pale yellow or colorless
Lactose-negative or slow lactose fermenting
Smooth colonies
Moist
Translucent or opaque
Slightly raised or flat elevation
Entire (smooth) edges
Colony morphology
Biochemical tests
Identification
Differential media
Environmental isolate
Pathogenicity
Incubation
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