The Effect on Virulence of Converting the O Antigen of Salmonella cholerae-suis from 627 to 617 by Phage

SUMMARY: Of 86 strains of Salmonella cholerae-suis isolated in the U.K., 83 possessed the O antigen 617 and three the O antigen 627. All the O 617 strains were latently infected with the same phage which was active on the O 627 strains; during lyso-genization this phage converted O 627 strains to O 617. None of 900 strains of Escherichia coli was latently infected with this phage; nor were they susceptible to it.

Two O 627 strains converted to O 617 by lysogenization were significantly more virulent for chicks than were their O 627 parent strains; lysogenization of one of these O 627 strains by two different non-converting phages was not accompanied by an increase in virulence. One of the converted strains tested in mice was more virulent for these animals than was the O 627 strain from which it was derived.

The O 617 organisms were found at higher concentrations in the blood and liver of chickens into which they had been injected intravenously than were organisms of the O 627 strain from which they were derived. No difference was detected between the survival rates of the two kinds of organisms in normal chicken serum.