Culture media used for diagnosis of EHEC O157:H7
**Core Concept**
The diagnosis of EHEC O157:H7, a serotype of Shiga toxin-producing Escherichia coli (STEC), involves the use of specific culture media that selectively inhibit the growth of non-target bacteria while allowing the target pathogen to grow. This is crucial for identifying the organism responsible for the infection.
**Why the Correct Answer is Right**
The correct culture medium for the diagnosis of EHEC O157:H7 is Cefsulodin-Irgasan-Novobiocin (CIN) agar, which is a selective and differential medium. This medium contains cefsulodin, an antibiotic that inhibits the growth of many Gram-negative bacteria, and novobiocin, which differentiates E. coli from other Enterobacteriaceae. The CIN agar medium is particularly useful for isolating EHEC O157:H7 from stool samples, as it allows for the growth of the target organism while suppressing the growth of other bacteria.
**Why Each Wrong Option is Incorrect**
* **Option A:** MacConkey agar is a selective and differential medium for the isolation of Gram-negative bacteria, but it is not specific for EHEC O157:H7. While it can be used for the initial isolation of E. coli, it does not provide the selective pressure required to inhibit the growth of non-target bacteria.
* **Option B:** Sorbitol MacConkey agar is a variant of MacConkey agar that is used to differentiate E. coli that ferment sorbitol from those that do not. However, this medium is not specific for EHEC O157:H7, as it can also be used to isolate other E. coli strains that do not ferment sorbitol.
* **Option D:** Blood agar is a non-selective medium that is used to grow a wide range of bacteria, including E. coli. However, it is not specific for EHEC O157:H7, as it allows the growth of many other bacteria that can contaminate stool samples.
**Clinical Pearl / High-Yield Fact**
The use of specific culture media, such as CIN agar, is crucial for the accurate diagnosis of EHEC O157:H7. This is because the organism is a common cause of foodborne outbreaks, and accurate diagnosis is essential for implementing control measures and preventing further transmission.
**Correct Answer:** C. Cefsulodin-Irgasan-Novobiocin (CIN) agar