Citrobacter koseri is a Gram-negative, rod-shaped bacterium belonging to the Enterobacteriaceae family. It is known for causing various infections, particularly in the urinary tract, central nervous system, and other body parts, especially in immunocompromised individuals and neonates. Proper identification and understanding of its colony morphology are crucial for diagnosing and managing infections caused by this organism.

CLED Agar: Composition and Purpose

Cystine-Lactose-Electrolyte-Deficient (CLED) agar is a differential culture medium used primarily for isolating and differentiating urinary pathogens. Its unique formulation includes:

Cystine: Supports the growth of cystine-dependent coliforms.
Lactose: Differentiates lactose fermenters from non-fermenters.
Bromothymol Blue: A pH indicator that changes color based on lactose fermentation.
Electrolyte-Deficient Medium: Prevents the swarming of Proteus species, ensuring clear, isolated colonies.
Growth Characteristics of Citrobacter koseri on CLED Agar

When cultured on CLED agar, Citrobacter koseri exhibits distinct colony morphology that aids in its identification:

Colony Morphology: Colonies of C. koseri on CLED agar are typically medium-sized, smooth, round, and slightly raised.

Color: C. koseri is a lactose fermenter, resulting in yellow colonies on CLED agar. The fermentation of lactose lowers the pH, leading to a color change in the bromothymol blue indicator from green to yellow.

Growth Rate: C. koseri grows well on CLED agar, with visible colonies forming within 24-48 hours of incubation at 35-37°C.

Differentiation from Other Bacteria

CLED agar helps differentiate Citrobacter koseri from other urinary pathogens:

Escherichia coli: E. coli also produces yellow colonies due to lactose fermentation but tends to form more abundant and slightly larger colonies.
Klebsiella spp.: Klebsiella species produce larger, more mucoid yellow colonies.
Proteus spp.: Unlike C. koseri, Proteus species typically do not swarm on CLED agar and do not ferment lactose, resulting in blue or green colonies.
Clinical Significance

Identifying Citrobacter koseri in urine cultures is clinically significant for diagnosing urinary tract infections (UTIs) and other related infections. Accurate identification guides appropriate antibiotic therapy, especially given the increasing prevalence of antibiotic-resistant strains of C. koseri. This bacterium can exhibit multiple resistance mechanisms, making susceptibility testing essential for effective treatment.

Conclusion

Citrobacter koseri, when cultured on CLED agar, displays distinctive colony morphology that aids in its identification and differentiation from other urinary pathogens. Understanding these characteristics is essential for microbiologists and clinicians to accurately diagnose infections and determine appropriate therapeutic strategies. CLED agar remains a valuable tool in the clinical microbiology laboratory for isolating and differentiating urinary pathogens, including C. koseri.

свернуть