Diverse strains of typable and NTHi lineages including the biogroup aegyptius also bind to the CFG face of the N-domain of CEACAM1 (Virji et al., 2000) . In our previous analysis of receptor binding, THi isolates were shown to behave similarly: each strain tested having a primary requirement for Ile-91 and in addition, was affected by Y34A and Q44A substitutions. In the current study also, the THi strain Rd and Eagan demonstrate similar overall binding patterns. However, in all cases, intraspecies dif- Fig. 9 . Binding of N. meningitidis isolate C751OpaB to cell-expressed receptors: competition between cell-expressed and soluble receptors. Bacterial binding to cell-expressed CC1 or Q89N construct was detected using anti-Nm antisera and TRITC-conjugated secondary antibodies. The interactions of bacteria with the cell-expressed receptors were investigated in the absence (A, D) or presence of competing soluble receptors. Although binding to the soluble receptor Q89N is much lower than the native CC1 in dot-blots (Fig. 3B) , it is relatively high on certain cells (presumably, on those expressing high levels of the receptor) (D). In competitive experiments, the native CC1-Fc (B, E) and the CC1(Q89N)-Fc (C, F) were preincubated at 10 ug ml -1 with bacteria for 15 min, prior to infection of target cells. CC1-Fc inhibited bacterial binding to the homologous receptor significantly (B), and almost abrogated binding to cell-expressed Q89N (E); whereas the soluble Q89N was inefficient at inhibiting bacterial binding to CHO-CC1 (C). However, a level of homologous inhibition was apparent when examining the adhesion of bacteria to cells expressing low levels of the receptor (e.g. 'peppered' areas shown in D are less evident in F).