Virulence of Salmonella typhimurium mutants for White Leghorn chicks.

A strain of Salmonella typhimurium that is highly virulent for 1-day-old white leghorn chicks was genetically modified by deletion (delta) of the adenylate cyclase (cya) and cyclic AMP receptor protein (crp) genes or by removal (curing) of the 91-kilobase virulence plasmid. These mutants were then compared with the wild-type S. typhimurium strain for virulence in 1-day-old chicks and for their ability to colonize chicks of various ages. The plasmid-cured mutant showed a slight reduction in virulence, whereas the delta cya delta crp mutant was completely avirulent. The wild-type strain and both mutant strains were capable of colonizing various organs within the chicks. At all time points, the delta cya delta crp strain colonized chicks at lower levels than the wild-type strain. Titers of the plasmid-cured strain increased more slowly in visceral organs than did those of the wild type.