Biofilm Formation by Moraxella Catarrhalis
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This is the first detailed study of biofilm formation in vitro by the Gram-negative bacterial pathogen Moraxella catarrhalis. Growth of M. catarrhalis in a continuous-culture biofilm system resulted in little detectable change in outer membrane protein production compared to broth-grown M. catarrhalis. Biofilm-grown M. catarrhalis may produce an extracellular polysaccharide and different colony phenotypes when grown in this continuous culture biofilm system. Transmission electron microscopy of biofilm-grown M. catarrhalis cells revealed abundant projections extending from the bacterial cell surface that were identified as the proteins UspA1, UspA2, and Hag, which are all putative members of the autotransporter protein family. The Hag protein of strain O35E was shown to be necessary for hemagglutination, autoagglutination, and binding of human IgD. A crystal violet-based assay utilizing 24-well tissue culture plates was also used to evaluate biofilm formation by M. catarrhalis. The ability of M. catarrhalis strains to form biofilms in this crystal violet-based assay varied considerably, but most strains form little or no biofilm in this system. Screening of M. catarrhalis transposon insertion mutants using the crystal violet-based assay revealed that the UspA1 or the related UspA2H proteins play a strain-dependent, positive role in biofilm formation. Expression of the Hag protein prevented biofilm formation in tissue culture plates by several M. catarrhalis strains. In contrast, there appears to be a positive selection for Hag expression by strain O46E in the continuous-culture biofilm system. Proteins involved in cell wall recycling may also be involved in biofilm formation by M. catarrhalis. Nucleotide sequence analysis, site-directed mutagenesis, and domain swapping experiments indicated that the N-terminal region of UspA1 or UspA2H is likely involved in biofilm formation by M. catarrhalis.