Different strains of a serologically or otherwise identical species of bacteria are susceptible to one or more different bacterio-phages. When a suspension of phages is deposited on the lawn culture of a susceptible bacterium, an area of clearing occurs after incubation due to lysis of the susceptible bacteria by the phages. These zones of lysis are called plaques. The shape, size, and nature of plaques are characteristic for different phages. Since a single phage particle is capable of producing one plaque, plaque assay can be used for titrating the number of viable phages in preparation. On the basis of this phenomenon, many bacterial species can be divided into various phage types. Phage typing has been used in epidemiological study of infec-tions or outbreaks caused by Staphylococcus aureus, Salmonella spp., Vibrio cholerae, and many other bacteria.
Different phages are available, which show difference in their specificity for genus, species, or strains. Examples are (i) genus-specific bacteriophages for Salmonella, (ii) specific bacteriophages for all members or strains of
Bacillus anthracis, and (iii) for all members of V. cholerae bio-type classical (e.g., Mukherjee’s phage IV). Mukherjee’s phage IV lyses all strains of V. cholerae biotype classical, but not V. cholerae biotype Eltor.
Phage typing of S. aureus is a pattern method in which a set of standard phages is employed for intraspecies typing of staphylococci. A strain of Staphylococcus may be lysed by a num-ber of phages. Hence, the phage type of a strain is designated by the number of the different phages that lyse it. Phage typing of Salmonella Typhi is carried out by using prophage, which is active against only fresh isolates of S. Typhi possessing the Vi antigen.