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Abstract
Different R factors were transferred from R+ Escherichia coli to two strains of Salmonella typhimurium in the alimentary tractof chicks. The virulence of the R+ S. typhimurium 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 S. typhimurium strains from which R factors had been eliminated, and on strains to which R factors had been transferred at a high rate in vitro, 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+ E. coli.
- Received:
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