Growth of the M. catarrhalis WT isolate O35E and tat mutant strains in liquid medium. Plate-grown bacteria were used to inoculate sidearm flasks containing 20-mL of broth to an optical density (OD) of ~50 Klett units. The cultures were then incubated with shaking at a temperature of 37°C for seven hours. The OD of each culture was determined every 60-min using a Klett Colorimeter. Results are expressed as the mean OD ± standard error (Panel A). Aliquots (1-mL) were taken out of each culture after recording the OD, diluted, and spread onto agar plates to determine the number of viable colony forming units (CFU). Results are expressed as the mean CFU ± standard error (Panel B). Growth of the wild-type (WT) isolate O35E is compared to that of its tatA (O35E.TA), tatB (O35E.TB), and tatC (O35E.TC) isogenic mutant strains carrying the control plasmid pWW115. Asterisks indicate a statistically significant difference in the growth rates of mutant strains compared to that of the WT isolate O35E.