Which test is most appropriate for presumptive identification of Clostridioides (Clostridium) difficile isolates?

Which test is most appropriate for presumptive identification of Clostridioides (Clostridium) difficile isolates?

• A. SPS sensitivity test
• B. Nagler test
• C. Lecithinase positive
• D. Fluorescence on cycloserine-cefoxitin-fructose agar

Answer: (D)

The key idea is using a medium that both selects for Clostridioides difficile and reveals a distinctive colony appearance, giving a reliable presumptive identification. Cycloserine-cefoxitin-fructose agar fits this role because it suppresses many other bacteria while allowing C. difficile to grow; the colonies show a characteristic yellow color and often fluoresce under UV light, a pattern labs recognize as typical for this organism. This combination of selective growth and visible colony phenotype makes it the most practical presumptive test for identifying C. difficile isolates. In contrast, the Nagler test detects lecithinase production associated with Clostridium perfringens, not C. difficile, so it wouldn’t be useful for presumptive ID of this organism. The lecithinase-positive result points to C. perfringens rather than C. difficile. The SPS sensitivity approach isn’t a standard method used for identifying C. difficile. Final confirmation should come from toxin detection or molecular methods.

The key idea is using a medium that both selects for Clostridioides difficile and reveals a distinctive colony appearance, giving a reliable presumptive identification. Cycloserine-cefoxitin-fructose agar fits this role because it suppresses many other bacteria while allowing C. difficile to grow; the colonies show a characteristic yellow color and often fluoresce under UV light, a pattern labs recognize as typical for this organism. This combination of selective growth and visible colony phenotype makes it the most practical presumptive test for identifying C. difficile isolates.

In contrast, the Nagler test detects lecithinase production associated with Clostridium perfringens, not C. difficile, so it wouldn’t be useful for presumptive ID of this organism. The lecithinase-positive result points to C. perfringens rather than C. difficile. The SPS sensitivity approach isn’t a standard method used for identifying C. difficile. Final confirmation should come from toxin detection or molecular methods.