What is the most likely organism found in a bloody stool sample with certain biochemical results indicating non-lactose-fermenting colonies?

The organism most likely found in a bloody stool sample with specified biochemical results indicating non-lactose-fermenting colonies is Shigella spp. This is due to Shigella being a well-known pathogenic bacterium that causes dysentery, which is characterized by bloody diarrhea. In the laboratory, Shigella typically does not ferment lactose, which leads to the formation of non-lactose-fermenting colonies on selective media like MacConkey agar.

Shigella has a low infectious dose and is primarily transmitted via the fecal-oral route. Its biochemical profile often aligns with that of lactose non-fermenters and allows for its identification in stool samples, especially in cases of bloody diarrhea that suggest a bacterial infection.

In summary, the consistent association of Shigella spp. with bloody stool and its non-lactose-fermenting characteristic aligns well with the conditions outlined in the question, making it the most likely organism in such scenarios.