Phage typing is widely used for the intraspecies classification of one of the following bacteria:
## Core Concept
Phage typing is a method used for the intraspecies classification of bacteria based on their susceptibility to specific bacteriophages. This technique exploits the unique interaction between bacteriophages and bacterial strains, allowing for the differentiation of strains within a species. It is particularly useful for epidemiological studies and tracing the source of outbreaks.
## Why the Correct Answer is Right
The correct answer, *Salmonella Typhi*, is a bacterium for which phage typing has been widely used. This method was notably used in the investigation of outbreaks and in tracing the epidemiology of typhoid fever. The phage typing system for *Salmonella Typhi* was developed by Craigie and Yen, and it has been instrumental in controlling typhoid fever outbreaks. The method works by identifying specific phages that can infect certain strains of *Salmonella Typhi*, thereby classifying them.
## Why Each Wrong Option is Incorrect
* **Option A:** *Escherichia coli* - While phage typing can theoretically be applied to *E. coli*, it is not as commonly associated with this method for intraspecies classification as *Salmonella Typhi*. Other methods like serotyping are more commonly used for *E. coli*.
* **Option B:** *Staphylococcus aureus* - Phage typing has been used for *S. aureus*, but the emergence of methicillin-resistant *S. aureus* (MRSA) and the development of molecular typing methods have somewhat limited its use for this species.
* **Option D:** *Pseudomonas aeruginosa* - Phage typing can be used for *P. aeruginosa*, but it is not as historically significant or widely recognized for this species as it is for *Salmonella Typhi*.
## Clinical Pearl / High-Yield Fact
A key point to remember is that phage typing was historically crucial in the control of typhoid fever outbreaks, particularly in areas where the disease was endemic. The method's specificity and the unique phage patterns observed in different strains made it a valuable epidemiological tool.
## Correct Answer Line
**Correct Answer: C. Salmonella Typhi.**