AT LEAST 4 PERCENT OF THE SALMONELLA-TYPHIMURIUM GENOME IS REQUIRED FOR FATAL INFECTION OF MICE

Salmonella typhimurium infection of mice is an established model syste m for studying typhoid fever in humans. Using this model, we identifie d S. typhimurium genes which are absolutely required to cause fatal mu rine infection by testing independently derived transposon insertion m utants for loss of virulence in vivo. Of the 330 mutants tested intrap eritoneally and the 197 mutants tested intragastrically, 12 mutants wi th 50% lethal doses greater than 1,000 times that of the parental stra in were identified. These attenuated mutants were characterized by in vitro assays which correlate with known virulence functions.;In additi on, the corresponding transposon insertions were mapped within the S. typhimurium genome and the nucleotide sequence of the transposon-flank ing DNA was obtained. Salmonella spp. and related bacteria were probed with flanking DNA for the presence of these genes. All 12 attenuated mutants had insertions in known genes, although the attenuating effect s of only two of these were previously described. Furthermore, the pro portion of attenuated mutants obtained in this study suggests that mut ations in about 4% of the Salmonella genome lead to 1,000-fold or grea ter attenuation in the mouse typhoid model of infection. Most of these genes appear to be required during the early stages of a natural infe ction.