Bacteriophage tail architecture: In which of the following classic phages is a contractile tail sheath present as part of the virion structure?

Introduction / Context: Contractile tail sheaths are hallmark features of many Myoviridae bacteriophages (for example, T2, T4, and T6). When the sheath contracts, it drives a rigid tail tube through the bacterial envelope to inject the phage genome. Recognizing which phages possess this mechanism is a frequent exam checkpoint in basic virology and microbiology.

Given Data / Assumptions:

• T-even phages (such as T2) are classic contractile-tailed phages.
• T3 and P22 are short-tailed or non-contractile-tailed Podoviridae representatives.
• Only one option should be correct.

Concept / Approach: Phage tails come in three broad morphotypes: Myoviridae (long, contractile), Siphoviridae (long, flexible, non-contractile), and Podoviridae (short, stubby, non-contractile). T2 is Myoviridae; T3 and P22 are Podoviridae. Therefore, the presence of a contractile sheath correctly applies to T2 but not to T3 or P22.

Step-by-Step Solution: Classify each phage by family/morphotype. T2 → Myoviridae → contractile tail sheath present. T3 → Podoviridae → short, non-contractile tail. P22 → Podoviridae → short, non-contractile tail. Select T2 as the only correct option.

Verification / Alternative check: Electron micrographs of T-even phages clearly show a sheath that shortens during infection, while Podoviridae display compact tails without a sheath capable of contraction.

Why Other Options Are Wrong: T3 lacks a contractile sheath; P22 lacks a contractile sheath; “All of these” is incorrect because not all listed phages have contractile tails; “None of these” is wrong because T2 does.

Common Pitfalls: Confusing general “tail” with “contractile sheath.” Many phages have tails, but only Myoviridae have the dedicated contractile sheath apparatus.

Final Answer: T2.