A bloody stool cultured after severe diarrhea showed many non-lactose-fermenting colonies on MacConkey agar. What is the most likely identification?

• A. Shigella spp.
• B. Salmonella spp.
• C. Proteus spp.
• D. E. coli

Answer: (A)

The presence of many non-lactose-fermenting colonies on MacConkey agar, along with symptoms of severe diarrhea and bloody stool, suggests the presence of a pathogenic organism. Shigella spp. is well known for causing bloody diarrhea and does not ferment lactose, which results in clear colonies on MacConkey agar. This aligns perfectly with the provided clinical scenario.

In contrast, while Salmonella spp. also does not ferment lactose, it is less commonly associated with bloody diarrhea when compared to Shigella. Proteus spp. typically does not cause severe diarrhea and would be an uncommon cause of such symptoms, despite being non-lactose fermenters. E. coli, particularly the type associated with diarrhea (like Enterohemorrhagic E. coli), usually ferments lactose—resulting in pink colonies on MacConkey agar—so it would not fit the criteria of being a non-lactose fermenter with bloody diarrhea.

Thus, the combination of clinical presentation and culture results strongly indicates Shigella spp. as the most likely identification.