CLED Agar, which stands for Cystine Lactose Electrolyte Deficient Agar, is a differential culture medium specifically designed for the isolation, enumeration, and differentiation of urinary tract pathogens. It is widely used in clinical microbiology laboratories for routine urine culture because it supports the growth of common urinary pathogens while preventing the swarming of Proteus species. The medium also allows differentiation of organisms based on their ability to ferment lactose, making it particularly useful for the diagnosis of urinary tract infections.
Principle
The principle of CLED Agar is based on differential lactose fermentation and controlled electrolyte content. The medium contains lactose as a fermentable carbohydrate and bromothymol blue as a pH indicator. Organisms that ferment lactose produce acid, leading to a color change in the medium. The electrolyte-deficient formulation inhibits the swarming growth of Proteus species, ensuring well-isolated colonies and accurate colony counts during urine culture analysis.
Composition
CLED Agar typically contains enzymatic digest of casein and gelatin as sources of nitrogen and growth factors, lactose as the fermentable carbohydrate, L-cystine to support the growth of cystine-dependent organisms, bromothymol blue as the pH indicator, agar as the solidifying agent, and a reduced concentration of electrolytes to prevent swarming of Proteus species. The formulation is optimized for urinary bacteriology.
Colony Morphology on CLED Agar
- Escherichia coli: Typically produces smooth, opaque yellow colonies indicating active lactose fermentation with yellowing of the surrounding medium.
- Klebsiella & Enterobacter species: Form large, mucoid, yellow to yellow-green colonies.
- Proteus species: Produce small, translucent blue to green colonies without swarming waves.
- Pseudomonas aeruginosa: Produces flat, characteristic blue-green colonies with a rough periphery.
- Enterococcus species: Form small, compact yellow colonies (approx. 0.5 mm).
Differential & Growth Characteristics
Lactose fermenting organisms produce yellow colonies due to acid production and corresponding color change of the indicator. Non-lactose fermenting organisms produce blue to green colonies without significant acidification of the medium. The background color of the medium may also change depending on the degree of fermentation. CLED Agar supports the growth of most Gram-negative and Gram-positive bacteria commonly associated with urinary tract infections, facilitating accurate colony count determination for diagnosing significant bacteriuria.
Applications
The medium is routinely used in hospital diagnostic laboratories, reference laboratories, and clinical microbiology settings for quantitative urine culture and antimicrobial surveillance of urinary pathogens.
Key Features and Benefits
- Reliable differentiation of lactose fermenters and non-fermenters.
- Prevents swarming of Proteus species for accurate CFU quantification.
- Supports both Gram-positive and Gram-negative urinary pathogens.
- Enriched with L-cystine for dwarf colony mutant detection.
- Ready-to-use pre-poured sterile 90mm plates save preparation time and eliminate contamination risks.
Quality Control & Standards
Each batch of CLED Agar is manufactured under strict ISO 13485 quality control systems and evaluated using reference ATCC strains to verify sterility, growth promotion, lactose fermentation differentiation, and inhibition of Proteus swarming.
Storage & Handling Precautions
Prepared CLED Agar plates should be stored refrigerated at 2–8°C in sealed packaging and protected from direct light, dehydration, and freezing. Shelf life is 60 days under recommended conditions. Plates should be brought to room temperature before inoculation to prevent moisture condensation. Standard aseptic inoculation techniques must be followed.