A thymine-requiring (thy−), trimethoprim-resistant (tmpr) mutant isolated from the faeces of chickens experimentally infected with Salmonella typhimurium and treated with a mixture of trimethoprim and sulphadiazine was less virulent for chickens than the parent strain and a thy+tmps revertant prepared in vitro from the mutant. The difference in chicken-virulence was more noticeable when the strains were administered orally than when they were administered subcutaneously. All tmpr mutants prepared in vitro from four other salmonella strains were also thy−; those tested were less virulent for chickens and mice than their parent strains. After oral infection, thy− salmonella organisms were found much less commonly in the alimentary tract of chickens then were thy+ organisms. This was especially so in the caeca, the principal site of colonization of both the thy+ and thy− organisms. Relatively high concentrations of thymine or related compounds were found in the contents of all regions of the alimentary tract of chickens except the caeca; the caeca usually contained low or undetectable concentrations.

The thy− salmonella strains would not grow on one brand of brilliant green agar because of its deficiency in thymine; their colonial appearance on other kinds of media used for isolating salmonellae from clinical material was often ‘un-salmonella-like’.