The virulence of trimethoprim-resistant thymine-requiring strains of Salmonella.

A thymine-requiring (thy-), trimethoprim-resistant (tmpr) mutant isolated from the faeces of chickens experimentally infected with salmonella typhimurium and treated with 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. Relatiely 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 briliant green agar because of its deficiency in thymine; their colonial and appearance on other kinds of media used for isolating salmonellae from clinical material was often 'un-salmonella-like'.