Construction of delta aroA his delta pur strains of Salmonella typhi

Salmonella typhi strains with two deletion mutations, each causing an attenuating auxotrophy, have been constructed from strains Ty2 and CDC 10-80 for possible use as oral-route live vaccines. An aroA(serC)::Tn10 transposon insertion was first transduced from a Salmonella typhimurium donor into each wild-type S. typhi strain. Transductants of the Aro- SerC- phenotype were treated with transducing phage grown on an S. typhimurium strain with an extensive deletion at aroA; selection for SerC+ yielded transductants, some of which were delta aroA. A his mutation was next inserted into a delta aroA strain in each line by two steps of transduction. Two deletions affecting de novo purine biosynthesis were used as second attenuating mutations: delta purHD343, causing a requirement for hypoxanthine (or any other purine) and thiamine, and delta purA155, causing an adenine requirement. The purHD343 deletion was introduced into the delta aroA his derivatives of each strain by cotransduction with purH::Tn10, and the purA155 deletion was introduced into the CDC 10-80 delta aroA his derivative by cotransduction with an adjacent silent Tn10 insertion by selection for tetracycline resistance. Tetracycline-sensitive mutants of each of the three delta aroA his delta pur strains were isolated by selection for resistance to fusaric acid. The tetracycline-sensitive derivative of the CDC 10-80 delta aroA his delta purA155 strain, designated 541Ty, and its Vi-negative mutant, 543Ty, constitute the candidate oral-route live-vaccine strains used in a recent volunteer trail (M. M. Levine, D. Herrington, J. R. Murphy, J. G. Morris, G. Losonsky, B. Tall, A. A. Lindberg, S. Stevenson, S. Baqar, M. F. Edwards, and B. A. D. Stocker, J. Clin. Invest. 79:885-902, 1987). Tetracycline-sensitive mutants of the delta aroA his delta purHD derivative of strains Ty2 and CDC 10-80 may also be appropriate as live vaccines but have not been tested as such.