The full-length elaD gene (NCBI protein sequence identifier GI: 16130204) was amplified by PCR from the K12 strain BL21 (Novagen) and cloned into pET28a (Novagen The putative proteases are separated by groups (as shown in Figure 1 ), by species, and by subdivision of Proteobacteria. This list is not complete in terms of orthologs/paralogs or bacterial species.  Mutational analysis of human CEACAM1: the potential of receptor polymorphism in increasing host susceptibility to bacterial infection A common overlapping site on the N-terminal IgV-like domain of human carcinoembryonic antigen (CEA)-related cell adhesion molecules (CEACAMs) is targeted by several important human respiratory pathogens. These include Neisseria meningitidis (Nm) and Haemophilus influenzae (Hi) that can cause disseminated or persistent localized infections. To define the precise structural features that determine the binding of distinct pathogens with CEACAMs, we have undertaken molecular modelling and mutation of the receptor molecules at previously implicated key target residues required for bacterial binding. These include Ser-32, Tyr-34, Val-39, Gln-44 and Gln-89, in addition to Ile-91, the primary docking site for the pathogens. Most, but not all, of these residues located adjacent to each other in a previous N-domain model of human CEACAM1, which was based on REI, CD2 and CD4. In the current studies, we have refined this model based on the mouse CEACAM1 crystal structure, and observe that all of the above residues form an exposed continuous binding region on the N-domain. Examination of the model also suggested that substitution of two of these residues 34 and 89 could affect the accessibility of Ile-91 for ligand binding. By introducing selected mutations at the positions 91, 34 and 89, we confirmed the primary importance of Ile-91 in all bacterial binding to CEACAM1 despite the inter- and intraspecies structural differences between the bacterial CEACAM-binding ligands. The studies further indicated that the efficiency of binding was significantly enhanced for specific strains by mutations such as Y34F and Q89N, which also altered the hierarchy of Nm versus Hi strain binding. These studies imply that distinct polymorphisms in human epithelial CEACAMs have the potential to decrease or increase the risk of infection by the receptor-targeting pathogens. The bacterial pathogens Neisseria meningitidis (Nm) and Haemophilus influenzae (Hi) are frequently found in the nasopharynx of a substantial proportion of the healthy population but are capable of causing serious infections in susceptible individuals (Turk, 1984; Foxwell et al., 1998) . Nm and typable Hi (THi) can invade the nasopharyngeal epithelial barrier to cause septicaemia and meningitis, which in the case of Nm, may rapidly become life threatening (van Deuren et al., 2000) . Non-typable Hi (NTHi), which lack a polysaccharide capsule, are associated with localized respiratory tract and conjunctival infections (Foxwell et al., 1998) . Strains belonging to Hi-biogroup aegyptius (Hi-aeg) are also associated with Brazilian purpuric fever (Foxwell et al., 1998) . The factors that determine susceptibility to infection by these frequent colonizers are not entirely clear.