For phage typing, how many phages of staphylococcus aureus are used –

## **Core Concept**
Phage typing is a method used to classify bacteria, particularly *Staphylococcus aureus*, based on their susceptibility to a panel of bacteriophages. This technique exploits the specificity of bacteriophages for certain bacterial strains, allowing for epidemiological typing and outbreak investigations.

## **Why the Correct Answer is Right**
The correct answer involves the use of a specific number of phages for typing *Staphylococcus aureus*. The method commonly used is the **phage typing scheme** that involves a set of 23 bacteriophages. These phages are selected for their ability to lyse specific strains of *S. aureus*, and the pattern of phage susceptibility is used to classify the bacteria into different types. This scheme helps in tracing the source of infections and understanding the spread of specific strains.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option suggests a much lower number of phages than what is standardly used, which would not provide sufficient discriminatory power for epidemiological investigations.
- **Option B:** Similarly, this option suggests a lower number of phages, which might not adequately represent the diversity of *S. aureus* strains.
- **Option D:** This option suggests a higher number of phages than typically used in the standard scheme.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the **standard phage typing scheme for *Staphylococcus aureus* uses 23 phages**. This scheme is a valuable tool in infection control, particularly for tracing outbreaks of methicillin-resistant *S. aureus* (MRSA).

## **Correct Answer:** . 23 phages.