Probing the surface of bacteria: use of gold sphere immunological probes

Isolation of a 30 kDa immunoglobulin binding protein from Pseudomonas maltophilia

We have demonstrated that Pseudomonas maltophilia (ATCC No. 13637) possesses an exposed, immunologically accessible protein which binds to the Fc region of several species of immunoglobulins. Whole bacteria suspensions were incubated for 18 h with purified 125I-labelled antibodies with and without added non-labelled immunoglobulins. The suspensions were centrifuged for 30 min and the pellet containing bacteria was assessed for radioactivity. Using this crude assay, the whole organism bound 125I-labelled rabbit and mouse immunoglobulins and the purified Fc portion of human IgG. All of these labelled preparations were competitively displaced by unlabelled rabbit and mouse immunoglobulins, and Fc of human IgG, as well as human immunoglobulin subclasses. The organism was sonicated to solubilize this immunoglobulin binding protein. Using this sonicated preparation, it was shown that unlabelled Fc of IgG, unlabelled mouse and rabbit immunoglobulins, all competitively displaced 125I-labelled human Fc of IgG in a dose-response manner. A partially purified protein was prepared by Sephacryl S-300 followed by Sephadex G-100 column chromatography. This preparation was incubated with 125I-Fc gamma and with the following purified unlabelled preparations: F(ab')2 of IgG, Fc of IgG, murine monoclonal IgA, IgG1, IgG2, IgG3, and IgG4. All except F(ab')2 of IgG produced dose response competitive displacement. The molecular weight, as estimated by SDS-PAGE and Western blot, was 30,000 daltons. In Western blots, Fc gamma, murine monoclonal IgA, and human immunoglobulin subclasses, all showed affinity for the immobilized protein. Human F(ab')2 fragments did not show affinity for the protein. Radioiodinated pseudomonal Ig-binding protein showed affinity for human IgG coupled to Sepharose, and was displaced by unlabelled pseudomonal Ig-binding protein. Scatchard analysis of binding showed two binding affinities: two distinct types of Ig-binding proteins were obtained, a high affinity with Kd = 1.54 x 10(-10) and a lower affinity with Kd = 2.36 x 10(-8). This immunoglobulin binding protein may be useful in immunoglobulin purification or identification.

Antimicrobial interference with bacterial mechanisms of pathogenicity: effect of sub-MIC azithromycin on gonococcal piliation and attachment to human epithelial cells

The effects of subinhibitory concentrations of azithromycin (CP-62,993) on the piliation and attachment properties of Neisseria gonorrhoeae were examined. Subinhibitory concentrations of azithromycin significantly reduced the percentage of gonococci that expressed assembled pili on their surfaces by decreasing pilin subunit synthesis and substantially decreased gonococcal adherence to human mucosal cells.

Accessibility of gonococcal and meningococcal surface antigens: immunogold labeling for quantitative electron microscopy

The parallel application of two electron microscopic immunogold labeling procedures was used to assess the surface exposure and accessibility of gonococcal and meningococcal surface antigens. Monoclonal antibodies were used as markers for the surface antigens, i.e., outer membrane proteins and lipooligosaccharides. To evaluate the labeling densities obtained after incubation of whole bacteria in suspension or ultrathin cryosections of bacteria, a method of electron microscopic quantitation was developed. Incubation of whole bacterial suspensions with monoclonal antibodies and protein A-gold resulted in specific labeling of the bacterial surfaces. However, the labeling densities varied largely in each cell. By contrast, cryosections showed uniform heavy labeling densities at the surface of the outer membranes of all cells. Apparently, by sectioning the cells the antigen-masking barrier could be evaded, and steric hindrance was no longer restrictive. Thus, a better estimate of both the presence and the surface exposure, i.e., the accessibility of antigens, could be made. Such information is essential for us to better understand host-bacterial interactions and to develop new vaccines.

Ultrastructural localization of gonococcal antigens in infected epithelial cells as visualized by post-embedding immuno-electronmicroscopy

Since ultrastructural localization of bacterial antigens during infection might considerably contribute to the understanding of the infectious process, we investigated the usefulness of post-embedding immuno-electronmicroscopy in the visualization of neisserial antigens. With the use of proper fixation conditions in combination with ultracryomicrotomy, we have been able to specifically mark gonococcal antigens in ultrathin sections incubated with a polyclonal antiserum and subsequently with gold-labelled protein A. Electron microscopy of cryosections from infected Chang conjunctiva epithelial cells showed that gonococcal antigens could be marked during various stages of the infectious process. With the polyclonal antiserum used there appeared to be no difference in labelling between extracellular and intracellular bacteria, suggesting that the probed antigens were not altered during the internalization process. Our results indicate post-embedding immuno-electronmicroscopy as a useful technique to localize bacterial antigens during the adherence, the internalization and the intracellular degradation of bacteria in host cells.