Which organism is a non-lactose fermenter that commonly causes bacillary dysentery?

• A. Escherichia coli
• B. Campylobacter jejuni
• C. Shigella dysenteriae
• D. Vibrio cholerae

Answer: (C)

Non-lactose fermentation combined with the ability to cause invasive, inflammatory diarrhea that presents as bacillary dysentery points to Shigella dysenteriae. This organism doesn’t ferment lactose, so on culture it forms colorless colonies on MacConkey agar, and its key pathogenic feature is invasion of the colonic mucosa leading to blood and pus in the stool. Infections by Shigella dysenteriae are a classic cause of bacillary dysentery (shigellosis), often with a low infectious dose and toxin involvement in some species.

Other options either ferment lactose (like many strains of E. coli) or cause diarrhea with different characteristics: Campylobacter jejuni can cause dysentery-like symptoms but is not the classic non-lactose fermenter associated with bacillary dysentery, and Vibrio cholerae produces severe watery diarrhea rather than the dysentery pattern.