Blocking effect of homologous and cross-reactive antibodies on adhesion of M. catarrhalis clinical strains to A549 epithelial cell monolayer was determined by biotin–avidin-HRP ELISA. Bacterial adhesion in the absence of serum was used to established 100% binding ( dashed line). Relative adhesion was determined by calculating the absorbance at λ = 490 nm, reflecting the adhered bacteria as a proportion of the control adhesion. The error bars indicate SEM. Histogram (a) refers to inhibition of M. catarrhalis adherence by homologous or cross-reactive antibodies. The bacteria (∼ 2 × 10⁸ CFU ml⁻¹) were incubated with post-immunized sera at a 1:50 dilution. Histogram (b) refers to the lack of inhibition of the representative M. catarrhalis Mc1 adhesion in the presence of various non-specific sera and 5% (w/v) human albumin. The human serum contains no specific antibodies against M. catarrhalis as determined by whole-cell ELISA (titer < 1:100). The data are averages of the results from two independent experiments performed at least in triplicate. The differences between control adhesion and treatment condition adhesion were significant if P < 0.05 (one-way ANOVA followed by Tukey's post-hock test) and are given in the text.