Which statement correctly describes a non-lactose-fermenting enteric bacterium?

Which statement correctly describes a non-lactose-fermenting enteric bacterium?

• A. Salmonella enterica causes non-lactose fermentation and typically presents with watery diarrhea
• B. Escherichia coli is a non-lactose fermenter and forms colorless colonies on MacConkey
• C. Shigella species are non-lactose fermenters and commonly cause inflammatory diarrhea
• D. Pseudomonas aeruginosa is a lactose fermenter

Answer: (C)

Non-lactose fermentation on MacConkey agar is shown by colorless (or pale) colonies because these organisms don’t acidify lactose. Among enteric bacteria, Shigella and Salmonella are classic non-lactose fermenters, but Shigella is particularly known for causing inflammatory diarrhea (dysentery) due to invasion of the colonic mucosa, leading to fever, abdominal cramps, and blood or mucus in stool. That combination—non-lactose fermenting and inflammatory diarrhea—is why this statement fits best. In contrast, an organism like E. coli easily ferments lactose, so it forms pink colonies on MacConkey, not colorless ones. Pseudomonas is not considered an enteric lactose fermenter and isn’t typically described in this enteric context, so its claim is inaccurate. Salmonella also doesn’t ferment lactose, but the hallmark inflammatory process isn’t as specifically tied to it in typical testing as it is to Shigella.

Non-lactose fermentation on MacConkey agar is shown by colorless (or pale) colonies because these organisms don’t acidify lactose. Among enteric bacteria, Shigella and Salmonella are classic non-lactose fermenters, but Shigella is particularly known for causing inflammatory diarrhea (dysentery) due to invasion of the colonic mucosa, leading to fever, abdominal cramps, and blood or mucus in stool. That combination—non-lactose fermenting and inflammatory diarrhea—is why this statement fits best.

In contrast, an organism like E. coli easily ferments lactose, so it forms pink colonies on MacConkey, not colorless ones. Pseudomonas is not considered an enteric lactose fermenter and isn’t typically described in this enteric context, so its claim is inaccurate. Salmonella also doesn’t ferment lactose, but the hallmark inflammatory process isn’t as specifically tied to it in typical testing as it is to Shigella.