Figure 6. Effect of arginine decarboxylase on the gastric survival of S. Typhi.
Pairs of attenuated S. Typhi strains differing only in their arginine decarboxylase locus were grown to stationary phase in EGA medium under aerobic conditions. Cells were combined in a 1:1 ratio in PBS containing 1 mM arginine. Low gastric pH was induced by histamine injection in mice fasted for 6 h. Mice were inoculated with 109 CFU of each strain. Sixty min after inoculation, mice were euthanized and the entire small intestine removed and homogenized. Strain survival was assayed by plating onto LB agar containing kanamycin or streptomycin. Data shown are the competitive index of the two strains in each mouse with the geometric mean of two independent experiments (10 mice total) indicated as a solid line.