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

Legionnaires disease is caused by Legionella pneumophila, an organism with unusual nutritional needs that will not grow on the routine media used for most respiratory pathogens. Let's go through the choices.

1. Thayer Martin: This chocolate agar based medium with added antibiotics is meant for Neisseria gonorrhoeae from genital or other mixed flora sites, and it has no cysteine or iron supplement for Legionella.
2. Buffered charcoal with yeast extract: This medium, called BCYE, is buffered for a steady pH, has charcoal to absorb toxic substances, and is supplemented with L cysteine and iron salts, exactly the nutrients Legionella needs, so it is the correct choice.
3. Chocolate agar: This supports fastidious organisms such as Haemophilus and Neisseria that need heat lysed red cells for nutrients, but it does not carry the cysteine and iron Legionella specifically requires.
4. Bordet Gengou: This potato based glycerol medium is built for Bordetella pertussis, the whooping cough organism, and is unrelated to Legionella.

The correct option is buffered charcoal yeast extract agar, since it is the only one supplying the cysteine and iron that Legionella pneumophila needs to grow.

Let's summarize:

• Legionella is fastidious and needs L cysteine plus iron salts, supplied only by BCYE agar.
• Thayer Martin, chocolate agar and Bordet Gengou each serve a different fastidious organism, not Legionella.

So BCYE agar is the medium of choice when legionnaires disease is suspected.