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

CLED, cystine lactose electrolyte deficient medium, is the standard plate for urine culture. This question asks exactly why it beats MacConkey for that job, so let's compare the two media property by property.

1. It is a differential medium: CLED does show colony color differences based on lactose fermentation, but this is not the specific advantage the question is pointing to when comparing it against MacConkey, which is also differential in the same way.
2. Prevents swarming of Proteus: This is true and happens because CLED lacks electrolytes, but it is a side benefit, not the main reason CLED is chosen over MacConkey for detecting the full range of urinary organisms.
3. Supports the growth of Pseudomonas: Pseudomonas is not picky, it grows on many media including MacConkey itself, so this is not a special advantage unique to CLED.
4. Supports growth of Candida and Staphylococcus: MacConkey contains bile salts and crystal violet specifically to block gram positive organisms and fungi. CLED has neither, so gram positive cocci like Staphylococcus and yeasts like Candida grow normally on it, letting the lab catch mixed urinary infections that MacConkey would hide.

The correct option is that CLED supports growth of Candida and Staphylococcus, since this is the property that actually lets it pick up organisms MacConkey's inhibitory ingredients would suppress.

Let's summarize:

• MacConkey deliberately blocks gram positive bacteria and fungi with bile salts and crystal violet.
• CLED has no such inhibitors, so it grows gram positive cocci and Candida alongside gram negative rods, which is why it is favored for urine culture.

So CLED is preferred to MacConkey mainly because it supports the growth of Candida and Staphylococcus.