How This Exercise Demonstrates the Principle of Phage Typing

Phage typing is a classic microbiological technique used to identify bacterial strains based on their susceptibility to specific bacteriophages—viruses that infect bacteria. This method has been instrumental in epidemiology and bacterial classification, helping scientists track sources of bacterial infections and understand bacterial diversity.

In this article, we will describe how a typical phage typing exercise demonstrates the fundamental principles behind this technique, why it works, and its practical applications.

What is Phage Typing?

Phage typing relies on the specific interactions between bacteriophages (or phages) and their host bacteria. Each phage infects only certain strains or species of bacteria because they recognize unique receptors on the bacterial surface.

• When a phage infects a susceptible bacterium, it attaches, injects its genetic material, and hijacks the bacterial machinery to replicate.
• This infection usually results in the lysis (destruction) of the bacterial cell.
• The presence or absence of bacterial lysis after exposure to different phages allows microbiologists to identify bacterial strains.

The Principle Behind the Exercise

Step 1: Preparing a Bacterial Lawn

In a phage typing exercise, the first step is to create a bacterial lawn by spreading a uniform layer of the bacterial strain to be tested on an agar plate.

This lawn represents a dense, continuous growth of bacteria, providing a surface on which phages can act.

Step 2: Applying Different Phages

Next, small drops or streaks containing different known phages are applied onto specific areas of the bacterial lawn.

Each phage has a known host range—meaning it can infect only certain strains.

Step 3: Incubation and Observation

The plate is incubated to allow phages to interact with the bacteria.

After incubation, microbiologists observe the plate for clear zones called plaques where the bacteria have been lysed.

What Does This Show?

• Clear plaques indicate that the phage successfully infected and destroyed the bacteria in that area.
• No plaques mean that the bacteria were resistant or not susceptible to that phage.

By recording which phages cause lysis and which do not, a phage typing pattern or profile is created.

How This Demonstrates the Principle of Phage Typing

This exercise effectively demonstrates the core principle of phage typing: bacteriophage specificity and bacterial susceptibility.

1. Specificity of Phages:
   Each phage used in the exercise infects only specific bacterial strains due to unique surface receptors. The exercise highlights this specificity as some phages will produce plaques while others will not.
2. Differentiation of Bacterial Strains:
   Bacterial strains with different surface receptors respond differently to phage infection. This creates a unique pattern of susceptibility and resistance, allowing scientists to distinguish one strain from another.
3. Use in Epidemiology:
   By comparing the phage typing pattern of an unknown bacterial isolate to known patterns, microbiologists can trace the source of infections and track outbreaks.

Practical Applications of Phage Typing

• Tracking disease outbreaks:
  For example, during an outbreak of Salmonella or Staphylococcus aureus, phage typing helps identify if isolates come from the same source.
• Bacterial strain identification:
  Clinics use phage typing to classify bacterial isolates quickly and accurately.
• Research on bacterial evolution:
  Phage susceptibility patterns can give insights into bacterial mutations and adaptations.

Limitations to Keep in Mind

While phage typing is useful, it has some limitations:

• Not all bacteria have corresponding phages available for typing.
• Some bacteria may develop resistance, changing their phage typing profile.
• It requires well-characterized phage libraries and skilled interpretation.

This method remains important in microbiology for outbreak investigations, bacterial classification, and research.