Applications of phage typing: Which of the following bacterial pathogens have established phage-typing schemes used in epidemiology and strain differentiation?

Introduction / Context:
Phage typing exploits the specificity of bacteriophages for particular bacterial surface receptors to differentiate strains. Historically, it has been valuable in outbreak investigations and surveillance of several pathogens including Staphylococcus aureus, Salmonella typhi, and Vibrio cholerae.

Given Data / Assumptions:

• Phage panels are used to determine lytic patterns of bacterial isolates.
• Multiple human pathogens have well-described phage-typing systems.
• The question asks which listed pathogens can be typed this way.

Concept / Approach:
A strain’s susceptibility or resistance to standardized phage sets yields a lysis pattern code. S. aureus, S. typhi, and V. cholerae have each had phage sets curated for public health use, allowing fine-grained epidemiologic tracking before widespread genomic typing.

Step-by-Step Solution:
Identify pathogens historically typed by phages. Confirm S. aureus, S. typhi, and V. cholerae as classic examples. Choose the inclusive option “All of these.” Note that genomic methods now complement or replace phage typing in many labs.

Verification / Alternative check:
Public health manuals list standard phage panels and codes for these organisms, confirming their applicability.

Why Other Options Are Wrong:
Selecting a single organism ignores the broader historical usage; “None of these” is incorrect because phage typing has been widely applied.

Common Pitfalls:
Assuming modern genotyping eliminated the historical relevance; older questions still test these classic methods.

Final Answer:
All of these.