In microbiology, phage typing of bacteria is based on which fundamental fact about bacteriophages and their host cells?

Introduction / Context:
Phage typing is an important laboratory technique used to differentiate bacterial strains based on their susceptibility to certain bacteriophages. Bacteriophages are viruses that infect bacteria, and they often show a high degree of specificity for particular bacterial types. Microbiology exams frequently test whether you understand this specificity and how it is used in typing. This question asks which fundamental fact about bacteriophages underlies the method of phage typing.

Given Data / Assumptions:
The stem mentions phage typing and asks what it is based on. The options talk about viruses causing disease, relationships between bacteria and viruses, and the idea that bacteria are destroyed by viruses. We assume the learner knows that phage typing uses patterns of susceptibility to different phages to identify bacterial strains. The key is that not every phage infects every bacterium; instead, each phage usually has a narrow host range.

Concept / Approach:
The central concept is specificity of bacteriophages. A particular phage may infect only certain strains of a bacterial species and will not infect others. In phage typing, a panel of known phages is applied to a bacterial isolate, and the pattern of lysis (clear zones) is observed. This pattern acts like a fingerprint that helps identify the bacterial strain. Therefore, the correct option must state that bacterial viruses attack specific bacterial cells or strains, not that they infect all bacteria equally or that they affect all cells without specificity.

Step-by-Step Solution:
Step 1: Recall the definition of a bacteriophage as a virus that infects bacteria.Step 2: Remember that different bacteriophages have different host ranges and often infect only particular bacterial strains.Step 3: Connect this fact with phage typing, where known phages are used to see which strains they can lyse.Step 4: Read the options and look for the statement that mentions attack on specific bacterial strains or species.Step 5: Choose the option that correctly emphasizes host specificity of bacteriophages.

Verification / Alternative check:
You can verify this concept by thinking of how phage typing is used in epidemiology. When an outbreak of bacterial infection occurs, laboratories may use phage typing to see whether patient isolates belong to the same strain. They do this by exposing the bacteria to a standard set of phages and recording which phages cause lysis. Only if phages have specific host ranges can these patterns reliably distinguish strains. If all phages infected all bacteria equally, phage typing would not provide useful information, which confirms that specificity is the key principle.

Why Other Options Are Wrong:
Viruses cause disease in all living cells without specificity is incorrect because many viruses are species specific or even strain specific. Bacteria and viruses are closely related and share the same genetic material is wrong because viruses are not closely related cells; they are distinct infectious particles with their own types of nucleic acid. Bacteria are destroyed equally by any virus that infects them is incorrect because host range varies and not every virus can infect every bacterium. Bacterial growth always stops completely in the presence of any type of virus is an exaggeration and does not account for resistance and specificity. Only the option mentioning specific bacterial strains or species correctly captures the basis of phage typing.

Common Pitfalls:
Students sometimes think that any virus can infect any cell, which leads them to overlook the importance of host specificity. Others may focus on the general idea that viruses cause disease and pick an option that sounds broadly true but does not address the key property used in typing. To avoid such mistakes, remember that phage typing works only because each bacteriophage recognizes and binds to specific receptors on certain bacteria. This specific interaction is what generates the informative lysis patterns used for identification.

Final Answer:
Phage typing is based on the fact that bacterial viruses, called bacteriophages, attack only specific bacterial strains or species.