A phage typing system for Salmonella infantis.

A phage typing method applying 9 type phages was elaborated to subdivide Salmonella infantis. Results are reported by the use of Farmer's mnemonic. Out of 4847 S. infantis strains, 4602 were of human and 245 of non-human origin. The strains were typable in 98.9%. Two phage types occurred more frequently than 20%, four phage types between 5 and 10%, seven phage types less than 5%, and twenty-eight phage types less than 1%. The strains originated from outbreaks in 28.7% and from sporadic cases in 71.3%. A total of 1320 strains examined for phage type was isolated from 4 field epidemics, 39 community outbreaks and 370 family infections. In the second version of the method two phages were substituted by two more effective ones. The phage typing method was suitable for epidemiological purposes. Inducing in vitro changes in phage types by lysogenization and plasmid acquisition, phage types 111, 113, 311, 313 and 343 changed to phage types 213, 243, 513 and 543 after lysogenization and phage types 311 and 543 to phage types 548, 565 and 885 due to plasmid acquisition.