Which organism is unable to grow on MacConkey agar?

Which organism is unable to grow on MacConkey agar?

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

Answer: (A)

MacConkey agar is selective for Gram-negative bacteria and differential for lactose fermentation, using bile salts and crystal violet to inhibit many Gram-positive organisms and a pH indicator to show lactose fermentation. Some organisms are fastidious and require enriched or nonselective media; they won’t grow well, or at all, on MacConkey. Kingella denitrificans is a fastidious Gram-negative coccobacillus from the oropharynx that has specific nutrient and growth requirements. It does not tolerate the bile salts and other components of MacConkey agar and fails to form colonies, so it cannot grow on this medium. The other organisms listed can grow on MacConkey agar, even if they do not ferment lactose. They are capable of growing under the conditions MacConkey provides, yielding colonies (often colorless if they are non-lactose fermenters).

MacConkey agar is selective for Gram-negative bacteria and differential for lactose fermentation, using bile salts and crystal violet to inhibit many Gram-positive organisms and a pH indicator to show lactose fermentation. Some organisms are fastidious and require enriched or nonselective media; they won’t grow well, or at all, on MacConkey.

Kingella denitrificans is a fastidious Gram-negative coccobacillus from the oropharynx that has specific nutrient and growth requirements. It does not tolerate the bile salts and other components of MacConkey agar and fails to form colonies, so it cannot grow on this medium.

The other organisms listed can grow on MacConkey agar, even if they do not ferment lactose. They are capable of growing under the conditions MacConkey provides, yielding colonies (often colorless if they are non-lactose fermenters).