Different R factors were transferred from R to two strains of in the alimentary tract of chicks. The virulence of the R strains was then compared with the R strains from which they were derived by giving them orally to groups of 1-day-old chicks.

The mortality rates from infection with the R strains were similar to, slightly lower than, or much lower than those from infection with the corresponding R strains.

Virulence tests on strains from which R factors had been eliminated, and on strains to which R factors had been transferred at a high rate , suggested that the very possession of R factors was responsible for the smaller reductions in mortality rate observed with some of the R strains; but the other strains with which greatly reduced mortality rates were observed may have been low-virulence variants of the R parent strain that had subsequently acquired R factors from the R .


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