To assess whether CEACAM mutations that either significantly increase or decrease bacterial binding to the soluble constructs also affect bacterial binding to cellexpressed receptor constructs, we analysed COS cells transiently transfected with CEACAM1-4L containing Q89A, Q89N and I91A substitutions. Initially, binding of the anti-CEACAM antibodies to transfected CHO cells was examined and their binding was as observed for the soluble receptors (Fig. 7) . However, bacterially expressed ligands may overcome decreased affinity for specific mutant proteins due to multiple ligand-receptor engagement at the target cell surface. Examination of adherent bacteria and levels of receptor expression by microscopy revealed that bacteria bound to all transfectants other than sham-transfected cells (Fig. 8) . However, receptor expression levels had to be considerably high in the case of I91A for significant bacterial numbers to bind the transfectants. Whereas for Q89A and Q89N, bacteria could bind to cells with barely detectable levels of receptors (Fig. 8) .