Which organism cannot grow on MacConkey agar?

• A. Bordetella bronchiseptica
• B. Burkholderia cepacia
• C. Kingella denitrificans
• D. Plesiomonas shigelloides

Answer: (C)

MacConkey agar combines selective and differential properties: it inhibits many Gram-positive bacteria with bile salts and crystal violet, allowing Gram-negative organisms to grow, and it differentiates lactose fermenters from nonfermenters using a pH indicator. Some fastidious organisms, however, cannot grow on this medium because they require enriched conditions not provided by MacConkey. Kingella denitrificans is one of those organisms; it tends to be strict about its growth requirements and does not tolerate the bile salts/crystal violet in MacConkey, so it does not grow there. The other organisms listed are able to grow on MacConkey agar, though they may appear as nonfermenters or produce colorless colonies if they do not ferment lactose.