Of 86 strains of isolated in the U.K., 83 possessed the O antigen 67 and three the O antigen 67. All the O 67 strains were latently infected with the same phage which was active on the O 67 strains; during lyso-genization this phage converted O 627 strains to O 67. None of 900 strains of was latently infected with this phage; nor were they susceptible to it.

Two O 67 strains converted to O 67 by lysogenization were significantly more virulent for chicks than were their O 67 parent strains; lysogenization of one of these O 67 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 67 strain from which it was derived.

The O 67 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 67 strain from which they were derived. No difference was detected between the survival rates of the two kinds of organisms in normal chicken serum.


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