Urease mediates survival at acid pH. Survival of H. influenzae strain 11P6H and urease mutants at pH 4. Bacteria were suspended in buffer at pH 4 and incubated for 30 minutes at 37°C. Urea concentrations are as follows: white bars: no urea; gray bars: 50 mM urea; black bars: 100 mM. Bars indicate %survival calculated from colony counts performed at time 0 and 30 minutes. Values represent the mean of 3 independent experiments and error bars indicate standard deviation.