Bacteriophages are viruses of bacteria i.e. they attack members of a particular bacterial species, or strains within a species.
Bacteriophage (phage) typing is based on the specificity of phage surface receptors for cell surface receptors. Only those bacteriophage that can attach to these surface receptors can infect bacteria and cause lysis.
In bacteriophage typing the clinical microbiologist inoculates the bacterium to be tested onto a petri plate. The plate is heavily and uniformly inoculates so that the bacteria will grow to form a solid sheet or lawn of cells. The plate is then marked off into squares (15 to 20 mm per side), and each square is inoculated with a drop of suspension from the different phages available for typing. After the plate is incubated for 24 h, it is observed for plaques.
The phage type is reported as a specific genus and species followed by the types that can infect the bacterium. For example, the series 10/16/24 indicates that this bacterium is sensitive to phages 10, 16 and 24, and belongs to a collection of strains, called a phagovar, that have this particular phage sensitivity. Bacteriophage typing remains a tool of the research and reference laboratory.