Question

What medium is used for culture in a suspected case of legionnaires disease?

• A. Thayer Martin
• B. Buffered charcoal with yeast extract
• C. Chocolate agar
• D. Bordet Gengou

Hint:

Legionella needs cysteine and iron to grow, which is why buffered charcoal yeast extract (BCYE) agar is the standard culture medium for it.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
Legionnaires disease is caused by Legionella pneumophila, and the question asks which culture medium is used to grow this organism.

Step 2: Key Concept:
Legionella is a fastidious gram negative bacillus, meaning it will not grow on ordinary media because it has special nutrient needs, in particular it needs L cysteine and iron salts to grow.

Step 3: Detailed Explanation:
Buffered charcoal yeast extract agar, usually written as BCYE agar, is prepared with yeast extract as a nutrient source, activated charcoal to soak up toxic compounds that would otherwise stop the bacteria from growing, and it is buffered to keep the pH steady, plus it is supplemented with L cysteine and ferric salts that Legionella specifically needs.
Antibiotics can also be added to BCYE to make a selective version, which helps recover Legionella from samples like sputum or bronchoalveolar lavage that also contain other faster growing bacteria.
This is why BCYE is the standard recommended medium whenever legionnaires disease is suspected.

Step 4: Why the other options are wrong:
Thayer Martin medium is a selective chocolate agar based medium used to culture Neisseria gonorrhoeae from sites with mixed flora, and does not support Legionella growth.
Plain chocolate agar is used for fastidious organisms like Haemophilus influenzae and Neisseria species, but it lacks the cysteine and iron supplementation Legionella needs, so Legionella will not grow well on it.
Bordet Gengou medium is the classic medium for Bordetella pertussis, the whooping cough organism, and has nothing to do with Legionella.

Step 4: Final Answer:
Buffered charcoal yeast extract agar is the medium used to culture Legionella in a suspected case of legionnaires disease.