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The UspA surface antigen ofMoraxella catarrhalis was recently shown to be comprised of two different proteins (UspA1 and UspA2) which share an internal region containing 140 amino acids with 93% identity (C. Aebi, I. Maciver, J. L. Latimer, L. D. Cope, M. K. Stevens, S. E. Thomas, G. H. McCracken, Jr., and E. J. Hansen, Infect. Immun. 65:4367–4377, 1997). IsogenicuspA1 ,uspA2 , anduspA1 uspA2 mutants were tested in a number of in vitro systems to determine what effect these mutations, either individually or together, might exert on the phenotype ofM. catarrhalis 035E. Monoclonal antibodies specific for UspA1 or UspA2 were used in an indirect antibody accessibility assay to prove that both of these proteins were expressed on the surface ofM. catarrhalis . All three mutants grew in vitro at the same rate and did not exhibit autoagglutination or hemagglutination properties that were detectably different from those of the wild-type parent strain. When tested for the ability to adhere to human epithelial cells, the wild-type parent strain and theuspA2 mutant readily attached to Chang conjunctival cells. In contrast, theuspA1 mutant and theuspA1 uspA2 double mutant both attached to these epithelial cells at a level nearly 2 orders of magnitude lower than that obtained with the wild-type parent strain, a result which suggested that expression of UspA1 byM. catarrhalis is essential for attachment to these epithelial cells. Both the wild-type parent strain and theuspA1 mutant were resistant to the bactericidal activity of normal human serum, whereas theuspA2 mutant and theuspA1 uspA2 double mutant were readily killed by this serum. This latter result indicated that the presence of UspA2 is essential for expression of serum resistance byM. catarrhalis .

Phenotypic Effect of IsogenicuspA1anduspA2Mutations onMoraxella catarrhalis035E