Question

CLED is preferred to McConkeys medium because?

• A. It is a differential medium
• B. Prevents swarming of Proteus
• C. Supports the growth of Pseudomonas
• D. Supports growth of Candida and Staphylococcus

Hint:

CLED lacks the bile salts and crystal violet found in MacConkey, so it does not inhibit gram positive cocci or Candida, unlike MacConkey.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
CLED stands for cystine lactose electrolyte deficient medium, and it is commonly used for culturing urine samples. The question asks for the specific reason it is preferred over MacConkey medium.

Step 2: Key Concept:
MacConkey medium contains bile salts and crystal violet, which are added on purpose to inhibit gram positive bacteria and fungi, since MacConkey is designed to be selective for gram negative organisms only. CLED medium does not contain these inhibitory substances.

Step 3: Detailed Explanation:
Because a urine sample can contain a mix of gram negative rods, gram positive cocci like Staphylococcus saprophyticus, and yeasts like Candida, a lab wants one plate that lets all of these grow so none are missed.
CLED is nutritionally rich with peptone, lactose, and cystine, and lacks bile salts, so gram positive organisms and Candida grow on it just as well as gram negative organisms do, letting the lab detect the true range of urinary pathogens.
MacConkey, in contrast, would suppress the growth of Staphylococcus and Candida because of its bile salts and crystal violet content, so a mixed infection could be missed if MacConkey were used alone.
CLED also happens to lack electrolytes, which as a side effect stops Proteus from swarming across the plate, but the main and most commonly quoted reason it is chosen over MacConkey specifically is its ability to support the wider range of urinary organisms, including gram positive cocci and Candida.

Step 4: Why the other options are wrong:
CLED is not really described as a differential medium in the way lactose fermenting media like MacConkey are, since CLED uses a bromothymol blue lactose system mainly to show colony color, not primarily to separate organism groups.
Preventing Proteus swarming is a real property of CLED but it is a secondary benefit tied to the missing electrolytes, not the chief reason it is preferred over MacConkey in this comparison.
CLED does not specifically favor Pseudomonas growth over other organisms, Pseudomonas can grow on many non selective and selective media alike, so this is not the distinguishing feature being asked about.

Step 4: Final Answer:
CLED is preferred to MacConkey because it supports the growth of Candida and Staphylococcus, organisms that MacConkey's bile salts and crystal violet would otherwise inhibit.