Occurrence of secondary attenuating mutations in avirulent Salmonella typhimurium vaccine strains

The attenuating delta aroA554 mutation in Salmonella typhimurium strain SL3261 was complemented in vitro by selecting for AroA+ recombinant DNA clones. SL3261 containing cloned aroA+ genes did not require exogenous phenylalanine, tryptophan, tryosine, p-aminobenzoic acid, or dihydroxybenzoic acid for growth in defined media. Cloned aroA+ genes did not restore wild-type virulence to SL3261, however, in a murine typhoid model. The delta aroA554 mutation was transduced into S. typhimurium strain SR-11, a mouse-virulent strain recently passaged in mice. The SR-11 delta aroA554 mutant was highly attenuated for mice challenged parenterally. The same cloned aroA+ genes isolated in SL3261 restored the virulence of the SR-11 delta aroA554 mutant to that of wild-type SR-11. These results suggest that while the delta aroA554 allele remains effective in reducing S. typhimurium virulence, laboratory passage of attenuated vaccine strains may lead to the accumulation of additional attenuating defects.