Growth of M. catarrhalis O35E and its isogenic m35 mutant under acidic (A) and osmotic (B) stress. (A) The effect of exposure to different acidic conditions was measured by growing bacteria in BHI adjusted to pH 4.0, pH 5.0, pH 6.0, or pH 7.0, respectively. Suspensions were incubated for 2 h and 4 h, respectively, and the number of viable bacteria was quantified by plating on BHI agar plates. (B) Different osmotic conditions were investigated by culturing bacteria in BHI supplemented with 0.25 M, 0.5 M or 1.0 M NaCl. During cultivation to the stationary phase bacterial density was measured at OD600.