Outer membrane vesicles shield Moraxella catarrhalis ß-lactamase from neutralization by serum IgG.

ABSTRACT

OBJECTIVES: The aim of this study was to detect the presence of IgG against Moraxella catarrhalis ß-lactamase in healthy adults, and to determine whether outer membrane vesicles (OMVs) could protect the enzyme from inhibition by anti-ß-lactamase IgG. METHODS: Transmission electron microscopy was used to detect the presence of ß-lactamase in OMVs. Sera were examined by ELISA for specific IgG directed against recombinant M. catarrhalis ß-lactamase in addition to the outer membrane adhesins MID/Hag, UspA1 and A2. Binding of anti-ß-lactamase IgG from serum to OMVs was analysed by flow cytometry. The chromogenic substrate nitrocefin was used to quantify ß-lactamase enzyme activity. RESULTS: The presence of ß-lactamase was determined in OMVs from a 9-year-old child suffering from M. catarrhalis sinusitis. Furthermore, anti-ß-lactamase IgG was detected in sera obtained from healthy adults. Out of 40 adult blood donors (aged 18-65 years) tested, 6 (15.0%) carried anti-ß-lactamase IgG. No correlation between IgG titres against ß-lactamase and the adhesins was found. Flow cytometry analyses revealed that anti-ß-lactamase IgG from serum bound to ß-lactamase-positive OMVs. By comparing the ß-lactamase activity of intact OMV with OMV that were permeabilized with saponin we found that OMVs shielded active ß-lactamase from the anti-ß-lactamase IgG. CONCLUSIONS: Moraxella catarrhalis ß-lactamase is found in, or associated with, OMVs, providing clinical relevance for the vesicles in the spread of antibiotic resistance. Furthermore, OMVs protect ß-lactamase from specific IgG.