Specific human anti-MID IgG antibodies are bactericidal. (A) Flow cytometry profile of human anti-MID IgG against the M. catarrhalis RH4 wild type (dark arrow) and its corresponding MID-deficient mutant (light arrow). (B) Profile of anti-UspA1/A2 IgG against M. catarrhalis RH4 wild type (dark arrow) and its UspA1/A2-deficient mutant (light arrow). M. catarrhalis RH4 (C) and BBH18 (D) wild-types or MID-deficient mutants were incubated with nonspecific IgG (1/20), human anti-MID IgG (1/20), anti-UspA1/A2 IgG (1/50), or a combination of anti-MID/anti-UspA1/A2 IgG in PBS supplemented with 1 mM CaCl2 and 0.2 mM MgCl2 at 4°C for 30 min. Complement (30%) was added, and the suspensions were incubated at 37°C. Bacteria were collected at the indicated time points. After overnight incubation, CFU were counted. CFU at the initiation of the experiments was defined as 100%. Mean values of three to five separate experiments are shown. Error bars indicate standard errors of the means.