Pasteurised milk is most commonly tested by
**Core Concept**
Pasteurization is a process that kills pathogenic microorganisms in milk by applying heat for a specified period. This process is crucial in ensuring the safety of milk for human consumption by reducing the risk of milk-borne diseases such as brucellosis, tuberculosis, and listeriosis.
**Why the Correct Answer is Right**
The correct answer is **Mannitol Salt Agar (MSA)**. MSA is a selective medium that contains 7.5% NaCl, which inhibits the growth of most bacteria except for staphylococci. Staphylococcus aureus, a common contaminant in unpasteurized milk, can be isolated using MSA. The high salt concentration in MSA suppresses the growth of other bacteria, allowing S. aureus to grow, making it an ideal medium for testing pasteurized milk.
**Why Each Wrong Option is Incorrect**
**Option A:** MacConkey agar is a selective medium for gram-negative bacteria, particularly enteric bacteria, and is not suitable for testing pasteurized milk.
**Option B:** Blood agar is a general-purpose medium that supports the growth of a wide range of bacteria, including pathogens, but is not specific for testing pasteurized milk.
**Option C:** Deoxycholate agar is a selective medium for enteric bacteria, but it is not used for testing pasteurized milk.
**Clinical Pearl / High-Yield Fact**
When testing pasteurized milk, it is essential to use a selective medium that can distinguish between pathogens and contaminants. MSA is a reliable choice for this purpose, and its use can help ensure the quality and safety of milk for human consumption.
**Correct Answer: MSA**