Abstract
BackgroundThe outer membrane protein M35 is a conserved porin of type 1 strains of the respiratory pathogen Moraxella catarrhalis. It was previously shown that M35 is involved in the uptake of essential nutrients required for bacterial growth and for nasal colonization in mice. The aim of this study was (i) to characterize the potential roles of M35 in the host-pathogen interactions considering the known multifunctionality of porins and (ii) to characterize the degree of conservation in the phylogenetic older subpopulation (type 2) of M. catarrhalis.ResultsIsogenic m35 mutants of the type 1 strains O35E, 300 and 415 were tested for their antimicrobial susceptibility against 15 different agents. Differences in the MIC (Minimum Inhibitory Concentration) between wild-type and mutant strains were found for eight antibiotics. For ampicillin and amoxicillin, we observed a statistically significant 2.5 to 2.9-fold MIC increase (p < 0.03) in the m35 mutants. Immunoblot analysis demonstrated that human saliva contains anti-M35 IgA. Wild-type strains and their respective m35 mutants were indistinguishable with respect to the phenotypes of autoagglutination, serum resistance, iron acquisition from human lactoferrin, adherence to and invasion of respiratory tract epithelial cells, and proinflammatory stimulation of human monocytes. DNA sequencing of m35 from the phylogenetic subpopulation type 2 strain 287 revealed 94.2% and 92.8% identity on the DNA and amino acid levels, respectively, in comparison with type 1 strains.ConclusionThe increase in MIC for ampicillin and amoxicillin, respectively, in the M35-deficient mutants indicates that this porin affects the outer membrane permeability for aminopenicillins in a clinically relevant manner. The presence of IgA antibodies in healthy human donors indicates that M35 is expressed in vivo and recognized as a mucosal antigen by the human host. However, immunoblot analysis of human saliva suggests the possibility of antigenic variation of immunoreactive epitopes, which warrants further analysis before M35 can be considered a potential vaccine candidate.
Highlights
The outer membrane protein M35 is a conserved porin of type 1 strains of the respiratory pathogen Moraxella catarrhalis
We found no evidence of cold shock regulation, but the construction of an isogenic mutant lacking the expression of a currently incompletely described outer membrane proteins (OMP) of M. catarrhalis provided us with the opportunity to conduct a phenotypic analysis of the function of M35
In vitro growth of m35 mutants Standard growth curves of the three wild-type/mutant pairs in brain heart infusion (BHI) broth revealed no difference in growth velocity measured as broth density at OD600
Summary
The outer membrane protein M35 is a conserved porin of type 1 strains of the respiratory pathogen Moraxella catarrhalis. The recent introduction of routine infant immunization with pneumococcal conjugate vaccines has - in some studies [3] - led to a substantial increase in otitis media caused by M. catarrhalis [3]. It is a major cause of the most common bacterial infection in children requiring medical attention. In our attempts to identify cold shock regulated outer membrane proteins (OMP) of M. catarrhalis [5] we investigated a recently described OMP called M35. M35 is currently the only well characterized porin of M. catarrhalis [6,28]
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