Question

how bacteriophage are specific?

Answer 1

A phage is much smaller than a bacterial cell and consists of its hereditary material (nucleic acid,  DNA) that is embedded in a protein envelope.

This envelope is the “head” of the phage and has a crystalline shape that is only visible in an electron microscope. Additionally, a phage has a protein “tail” with a morphologically delicate fine structure at the end, for adsorption to the bacterial cell surface, the receptor.

This receptor structure is so specific that a phage can only attack bacteria having a cell surface that exactly “matches”.