Question

What are the likely conclusions about an organism's gram stain reaction and lactose fermentation ability if it grows on MacConkey agar, but the growth is colorless?

Answer 1

Answer: The organism will be a gram-negative organism and will not be able to ferment the lactose(non-lactose fermenter).

Explanation: The MacConkey agar medium is selective for growth of the gram-negative organism and it excludes the growth of Gram-positive organism as it contains bile salts which inhibit the growth of the gram-positive organism.
hence it is confirmed from the ability of the organism to grow on the MacConkey agar medium, that its a gram-negative organism.
MacConkey agar medium also differentiates the gram-negative organism based on the colony characteristics. The organisms which can ferment lactose appear as pink colonies and those who cannot ferment lactose produce colorless colonies.
So, this organism mentioned in the question is a non-lactose fermenter as it produces a colorless colony.