Which of the following organisms is unable to grow on MacConkey agar?

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

Answer: (C)

MacConkey agar is a selective medium that favors Gram-negative bacteria and inhibits most Gram-positive organisms. It also differentiates lactose fermenters from nonfermenters by colony color. Some fastidious Gram-negative bacteria cannot grow on MacConkey because the medium lacks certain nutrients or growth factors they require in addition to the inhibitory bile salts. Kingella denitrificans is such a fastidious Gram-negative coccobacillus that does not grow well, if at all, on MacConkey agar and needs enriched media like blood or chocolate agar for recovery. The other organisms listed are capable of growth on MacConkey under standard conditions (though many are non-lactose fermenters, they still form colonies on this medium). Thus, the organism unable to grow on MacConkey agar is Kingella denitrificans.