Culture media used for O157 : H7 Entero- hemorrhagic E coli –
**Core Concept**
The question requires knowledge of specific culture media used to isolate and identify certain bacterial pathogens, specifically the Sorbitol Nonfermenting (SN) agar for the detection of Shiga toxin-producing E. coli, including the O157:H7 strain.
**Why the Correct Answer is Right**
Sorbitol Nonfermenting (SN) agar is a selective and differential culture medium used to isolate and identify E. coli O157:H7. This medium contains sorbitol, which the O157:H7 strain of E. coli is unable to ferment, resulting in a pink or colorless colony after incubation at 37°C. The inability to ferment sorbitol is a key characteristic of this pathogenic strain, allowing for its identification on this specific agar. This medium is designed to inhibit the growth of other enteric bacteria, making it easier to isolate and identify the O157:H7 strain.
**Why Each Wrong Option is Incorrect**
**Option B:** Mannitol containing agar is used to isolate and identify other bacteria, such as E. coli that can ferment mannitol, which is not the case for E. coli O157:H7.
**Option C:** Sucrose containing agar is used to isolate and identify bacteria that can ferment sucrose, such as certain species of Enterobacteriaceae, but not specifically E. coli O157:H7.
**Option D:** Dextrose containing agar is a general-purpose medium used for the growth of many types of bacteria, but it is not specific for the isolation and identification of E. coli O157:H7.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that Sorbitol Nonfermenting (SN) agar is a selective medium that allows for the isolation of E. coli O157:H7 based on its inability to ferment sorbitol, which is a key characteristic of this pathogen.
✓ Correct Answer: A. Sorbitol containing agar