Which genus is a lactose fermenter among Enterobacteriaceae?

Which genus is a lactose fermenter among Enterobacteriaceae?

• A. Shigella
• B. Enterobacter
• C. Salmonella
• D. Proteus

Answer: (B)

Lactose fermentation is a key test to separate members of the Enterobacteriaceae into lactose fermenters versus non-fermenters. Shigella and Salmonella do not ferment lactose, so they appear colorless on lactose-containing media. Proteus also typically does not ferment lactose and shows other distinguishing traits like swarming. Enterobacter, on the other hand, readily ferments lactose, producing acid (and often gas) that changes the media color (for example, pink colonies on MacConkey) and gives characteristic colony appearances. That makes Enterobacter the lactose fermenter among these options.

Lactose fermentation is a key test to separate members of the Enterobacteriaceae into lactose fermenters versus non-fermenters. Shigella and Salmonella do not ferment lactose, so they appear colorless on lactose-containing media. Proteus also typically does not ferment lactose and shows other distinguishing traits like swarming. Enterobacter, on the other hand, readily ferments lactose, producing acid (and often gas) that changes the media color (for example, pink colonies on MacConkey) and gives characteristic colony appearances. That makes Enterobacter the lactose fermenter among these options.