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A bacteriophage or phage is any virus that can infect bacteria.

A typical bacteriophage has a double-stranded genome enclosed in a protein coat, and lacks a viral envelope.

They are between $$25$$ and $$200\text{ nm}$$ (nanometres) long.

The protein coat of a phage is made up of two distinct structures:

  • A polyhedral head which encloses the genetic material.
  • A helical tail which the genetic material travels down in order to be injected into the host cell.

Phages may also have tail fibres that allow them to bind to the host cell membrane.

One of the best-characterised phages is the T4 phage, which infects E. coli.

T4 bacteriophage
T4 bacteriophage