In the stool culture of a patient with severe diarrhea, what does the presence of clear colonies on MacConkey agar indicate?

The presence of clear colonies on MacConkey agar indicates the presence of non-lactose fermenting bacteria. Typically, MacConkey agar is used to isolate Gram-negative bacteria and differentiate them based on their ability to ferment lactose. Lactose fermenters, such as most E. coli strains, produce acid and lead to the formation of pink colonies due to the pH indicator in the agar, while non-lactose fermenters produce clear colonies.

In this case, the identification of clear colonies suggests that the bacteria present do not ferment lactose, which aligns with the characteristics of E. coli O157:H7. This particular strain is known for causing severe diarrhea and is a Shiga toxin-producing variant. While E. coli O157:H7 does not ferment lactose, it is significant in clinical practice due to its association with foodborne outbreaks and severe gastrointestinal illness.

Other Gram-negative bacteria such as Salmonella and Shigella can also appear as clear colonies on MacConkey agar. However, the characteristic of non-lactose fermentation specifically points to pathogens like E. coli O157:H7 in the context of severe diarrhea in a patient. Thus, recognizing the implications of clear colonies aids in the identification of potential pathogens, particularly in the scenario presented