Phage typing

Purpose of examination

Phage typing of Salmonella is a useful typing tool for subcategorising the more common Salmonella enterica serotypes, i.e. S.Typhimurium and S.Enteritidis. This can be useful to determine whether isolates, which come from different places at different times are similar or different in their reactions with specific sets of phages used for typing. Phage typing can show possible linkages between seemingly sporadic cases which could help in recognising and tracing a common source outbreak.

Principle of examination

A bacteriophage is a virus which specifically attacks bacteria. Phage typing is based on specific bacteriophage lysing bacteria. Banks of phage have been developed for a number of serotypes including S.Typhimurium and S.Enteritidis. Very dry agar plates containing a rich nutrient source, are flooded with a liquid culture of the bacterial isolate. The liquid culture is removed, the culture film allowed to dry, and a set of phage suspensions are innoculated onto the surface of the plate by means of a multipoint innoculator. Phage spots are left to dry, and plates are incubated inverted overnight at 37oC. The next day, phage lysis reactions are recorded, and compared to a set of standards developed by PHLS, Colindale, England. Pictures of S.Enteritidis Phage types

Specimen requirements

A pure culture of an overnight growth of the isolate on nutrient agar.

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