An isolate of E. coli recovered from the stool of a patient with severe bloody diarrhea should be tested for which sugar before sending it to a reference laboratory for serotyping?

An isolate of E. coli recovered from the stool of a patient with severe bloody diarrhea should be tested for which sugar before sending it to a reference laboratory for serotyping?

• A. Sorbitol (fermentation)
• B. Mannitol (oxidation)
• C. Raffinose (fermentation)
• D. Sucrose (fermentation)

Answer: (A)

The correct answer is testing for sorbitol fermentation. In cases of severe bloody diarrhea, particularly in a patient suspected of having an infection caused by Shiga toxin-producing E. coli (STEC), it is vital to identify the sorbitol fermentation capability of the isolate. Typically, E. coli strains that can ferment sorbitol are non-pathogenic, while the pathogenic strain O157:H7, which is commonly associated with hemolytic uremic syndrome (HUS) and bloody diarrhea, does not ferment sorbitol on MacConkey agar. Thus, if the isolate shows no fermentation of sorbitol, it raises suspicion for a pathogenic strain, indicating a need for further testing and possible serotyping. Determining the fermentation of other sugars like mannitol, raffinose, or sucrose is less critical for identifying pathogenic E. coli in cases of bloody diarrhea, as these sugars do not specifically distinguish between pathogenic and non-pathogenic strains as effectively as sorbitol does in this context. Therefore, testing for sorbitol fermentation is a standard diagnostic step for rapid identification of potentially serious E. coli infections.

The correct answer is testing for sorbitol fermentation. In cases of severe bloody diarrhea, particularly in a patient suspected of having an infection caused by Shiga toxin-producing E. coli (STEC), it is vital to identify the sorbitol fermentation capability of the isolate.

Typically, E. coli strains that can ferment sorbitol are non-pathogenic, while the pathogenic strain O157:H7, which is commonly associated with hemolytic uremic syndrome (HUS) and bloody diarrhea, does not ferment sorbitol on MacConkey agar. Thus, if the isolate shows no fermentation of sorbitol, it raises suspicion for a pathogenic strain, indicating a need for further testing and possible serotyping.

Determining the fermentation of other sugars like mannitol, raffinose, or sucrose is less critical for identifying pathogenic E. coli in cases of bloody diarrhea, as these sugars do not specifically distinguish between pathogenic and non-pathogenic strains as effectively as sorbitol does in this context. Therefore, testing for sorbitol fermentation is a standard diagnostic step for rapid identification of potentially serious E. coli infections.