Which organism is most likely indicated by the culture results showing pink to dark pink colonies on MacConkey agar?

Which organism is most likely indicated by the culture results showing pink to dark pink colonies on MacConkey agar?

• A. Serratia marcescens
• B. Proteus vulgaris
• C. Enterobacter cloacae
• D. Klebsiella pneumoniae

Answer: (D)

Pink to dark pink colonies on MacConkey agar indicate the presence of lactose-fermenting bacteria. MacConkey agar is a selective medium designed to differentiate between lactose fermenters and non-fermenters, and it also inhibits the growth of Gram-positive organisms. Klebsiella pneumoniae is a well-known lactose fermenter, producing colonies that appear pink due to the acid produced from lactose fermentation, which then causes a color change in the pH indicator in the medium. This trait aligns with the results observed in the culture. While other organisms listed may also exhibit some form of growth on MacConkey agar, they typically do not produce the same strong lactose fermentation reaction that leads to the pink to dark pink coloration. For instance, Serratia marcescens can sometimes be non-lactose fermenting under certain conditions, leading to colorless colonies. Proteus vulgaris is primarily non-lactose fermenting as well, often resulting in clear colonies. Enterobacter cloacae can ferment lactose, but it tends to produce colonies that may not have the same distinct dark pink color as Klebsiella pneumoniae. Thus, the distinct pink to dark pink colonies strongly point to Klebsiella pneumoniae as the most likely organism

Pink to dark pink colonies on MacConkey agar indicate the presence of lactose-fermenting bacteria. MacConkey agar is a selective medium designed to differentiate between lactose fermenters and non-fermenters, and it also inhibits the growth of Gram-positive organisms.

Klebsiella pneumoniae is a well-known lactose fermenter, producing colonies that appear pink due to the acid produced from lactose fermentation, which then causes a color change in the pH indicator in the medium. This trait aligns with the results observed in the culture.

While other organisms listed may also exhibit some form of growth on MacConkey agar, they typically do not produce the same strong lactose fermentation reaction that leads to the pink to dark pink coloration. For instance, Serratia marcescens can sometimes be non-lactose fermenting under certain conditions, leading to colorless colonies. Proteus vulgaris is primarily non-lactose fermenting as well, often resulting in clear colonies. Enterobacter cloacae can ferment lactose, but it tends to produce colonies that may not have the same distinct dark pink color as Klebsiella pneumoniae.

Thus, the distinct pink to dark pink colonies strongly point to Klebsiella pneumoniae as the most likely organism