The virulence of trimethoprim-resistant thymine-requiring strains ofSalmonella

Abstract

SUMMARYA thymine-requiring (thy−), trimethoprim-resistant (tmpr) mutant isolated from the faeces of chickens experimentally infected withSalmonella typhimuriumand treated with a mixture of trimethoprim and sulphadiazine was less virulent for chickens than the parent strain and athy+tmpsrevertant preparedin vitrofrom the mutant. The difference in chicken-virulence was more noticeable when the strains were administered orally than when they were administered subcutaneously. Alltmprmutants preparedin vitrofrom four other salmonella strains were alsothy−; 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 werethy+organisms. This was especially so in the caeca, the principal site of colonization of both thethy+andthy−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.Thethy−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’.