Cystine Lactose Electrolyte Deficient (CLED) agar is a type of differential medium generally used for isolation and enumeration of bacteria from urine specimen form patients suspected of urinary tract infection.  CLED agar supports the growth of all potential uropathogens such as gram negative bacilli belonging to Enterobacteriaceae and gram positive cocci such as Enterococcus spp, Staphylococcus aureus and Staphylococcus saprophyticus. It also supports the growth of yeasts like Candida albicans.

Blood agar and MacConkey agar are commonly used medium for routine culture of urine specimen. CLED can be used as a substitute for these medium where Proteus spp is encountered as a big nuisance as it swarms all over the blood agar plate. Moreover, CLED agar can be used as a sole medium for urine culture reducing the cost without compromising the quality.Many researches have proven the productivity of CLED as similar to that of Blood agar and MacConkey agar. Its economy and convenience make it a medium worth using for routine culture of urine specimens.

Merits of CLED

  1. Good differentiation between lactose fermenter and non lactose fermenter
  2. Inhibits swarming of Proteus spp as electrolytes are restricted in CLED
  3. Relatively low cost as a use of single medium is sufficient as compared to Blood Agar and MacConkey agar use for urine culture.

Demerits: Poor growth of some gram-positive bacteria

 

Composition of CLED

  1. Lactose (10.0gm)
  2. Pancreatic Digest of Gelatin (4.0gm)
  3. Pancreatic Digest of Casein (4.0gm)
  4. Beef Extract (3.0gm)
  5. L-Cystine (0.128gm)
  6. Bromothymol Blue (0.02gm)
  7. Agar (15gm )

Final pH 7.1 – 7.5

Lactose is a source of energy for organisms capable of utilizing it. Peptic digest of animal tissue, beef extract, casein enzymic hydrolysate provide essential growth nutrients. L-cystine supports the growth of dwarf coliform colony. Bromo thymol blue is the pH indicator which turns yellow at acidic pH.

 

Typical colony morphology on CLED Agar:

  1. Escherichia coli: Opaque yellow colonies with a slightly deeper yellow center
  2. Klebsiella spp: Yellow to whitish-blue colonies, extremely mucoid
  3. Proteus spp: Translucent blue colonies
  4. Pseudomonas aeruginosa: Green colonies with typical matted surface and rough periphery
  5. Enterococci: Small yellow colonies, about 0.5mm in diameter
  6. Staphylococcus aureus: Deep yellow colonies, uniform in color
  7. Coagulase Negative Staphylocci (CONS): Pale yellow colonies, more opaque than Enterococcus faecalis

 

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