Moraxella catarrhalis is a respiratory pathogen responsible for acute bacterial otitis media in children and exacerbation of chronic bronchitis in adults. M. catarrhalis strains are frequently resistant to the bactericidal activity of normal human serum. In order to determine if the lipooligosaccharide (LOS) of M. catarrhalis has a role in serum resistance, the UDP-glucose-4-epimerase (galE) gene was identified, cloned, and sequenced and a deletion/insertion mutation was introduced into M. catarrhalis strain 2951. GalE enzymatic activity, measured in whole-cell lysates, was ablated in M. catarrhalis 2951 galE. Mass spectrometric analysis of LOS isolated with hot phenol-water confirmed that strain 2951 produced a type A LOS. These studies showed that the LOS from 2951 galE had lost two hexose residues due to the galE mutation and that the resultant LOS structure lacked the (Galalpha1-4Galbeta1-4Glc) P(k) epitope found on M. catarrhalis 2951. Wild-type M. catarrhalis 2951 is resistant to complement-mediated serum bactericidal activity. In contrast, a greater than 2-log(10)-unit reduction in CFU occurred after incubation of 2951 galE in either 50 or 25% pooled human serum (PNHS), and CFU in 10% PNHS decreased by about 1 log(10) unit. These studies suggest that the P(k) epitope of the LOS may be an important factor in the resistance of M. catarrhalis to the complement-mediated bactericidal effect of normal human serum.