Isolation of a 30 kDa immunoglobulin binding protein from Pseudomonas maltophilia

Establishment of a method to detect human immunoglobulin G1 binding microorganisms

In this study, we developed an original and rapid anti-idiotypic antibody-based ELISA method, different from the techniques defined in the literature for this purpose so far, to detect immunoglobulin binding proteins (IBP) on the surface of bacteria. The test antibody used in our study to detect IBP is a recombinant human immunoglobulin G1 Kappa molecule, and has been used as a drug, Tocilizumab (Actemra®), in humans for therapeutic purposes. As a result, the test antibody in the supernatant after centrifugation is reduced compared to the initial moment due to antibody binding. Staphylococcus aureus cowan 1 strain used as positive control causes at least a 50% decrease in OD value in this respect. A similar observation at this level indicated that among a total of 189 microorganisms tested, 3 Staphylococcus aureus and 1 MRSA carrying high-affinity IgBP showed greater than 50% inhibition. This level of inhibition was not detected in the remaining microorganisms.

The Retropepsin-Type Protease APRc as a Novel Ig-Binding Protein and Moonlighting Immune Evasion Factor of Rickettsia

Rickettsiae are obligate intracellular Gram-negative bacteria transmitted by arthropod vectors. Despite their reduced genomes, the function(s) of the majority of rickettsial proteins remains to be uncovered. APRc is a highly conserved retropepsin-type protease, suggested to act as a modulator of other rickettsial surface proteins with a role in adhesion/invasion. However, APRc’s function(s) in bacterial pathogenesis and virulence remains unknown. This study demonstrates that APRc targets host serum components, combining nonimmune immunoglobulin (Ig)-binding activity with resistance to complement-mediated killing. We confirmed nonimmune human IgG binding in extracts of different rickettsial species and intact bacteria. Our results revealed that the soluble domain of APRc is capable of binding to human (h), mouse, and rabbit IgG and different classes of human Ig (IgG, IgM, and IgA) in a concentration-dependent manner. APRc-hIgG interaction was confirmed with total hIgG and normal human serum. APRc-hIgG displayed a binding affinity in the micromolar range. We provided evidence of interaction preferentially through the Fab region and confirmed that binding is independent of catalytic activity. Mapping the APRc region responsible for binding revealed the segment between amino acids 157 and 166 as one of the interacting regions. Furthermore, we demonstrated that expression of the full-length protease in Escherichia coli is sufficient to promote resistance to complement-mediated killing and that interaction with IgG contributes to serum resistance. Our findings position APRc as a novel Ig-binding protein and a novel moonlighting immune evasion factor of Rickettsia, contributing to the arsenal of virulence factors utilized by these intracellular pathogens to aid in host colonization.

Agitation down-regulates immunoglobulin binding protein EibG expression in Shiga toxin-producing Escherichia coli (STEC)

Shiga toxin (Stx)-producing Escherichia coli (STEC) carrying eibG synthesize Escherichia coli immunoglobulin binding protein (EibG). EibG nonspecifically binds to immunoglobulins and tends to aggregate in multimers but is poorly expressed in wild-type strains. To study synthesis of the proteins and their regulation in the pathogens, we identified natural growth conditions that increased EibG synthesis. EibG proteins as well as corresponding mRNA were highly expressed under static growth conditions while shearing stress created by agitation during growth repressed protein synthesis. Further regulation effects were driven by reduced oxygen tension, and pH up-regulated EibG expression, but to a lesser extent than growth conditions while decreased temperature down-regulated EibG. Bacteria with increased EibG expression during static growth conditions showed a distinct phenotype with chain formation and biofilm generation, which disappeared with motion. High and low EibG expression was reversible indicating a process with up- and down-regulation of the protein expression. Our findings indicate that shear stress represses EibG expression and might reduce bacterial attachments to cells and surfaces.

Antibody orientation at bacterial surfaces is related to invasive infection

Several of the most significant bacterial pathogens in humans, including Streptococcus pyogenes, express surface proteins that bind IgG antibodies via their fragment crystallizable (Fc) region, and the dogma is that this protects the bacteria against phagocytic killing in blood. However, analysis of samples from a patient with invasive S. pyogenes infection revealed dramatic differences in the presence and orientation of IgG antibodies at the surface of bacteria from different sites. In the throat, IgG was mostly bound to the bacterial surface via Fc, whereas in the blood IgG was mostly bound via fragment antigen-binding (Fab). In infected and necrotic tissue, the Fc-binding proteins were removed from the bacterial surface. Further investigation showed that efficient bacterial IgGFc-binding occurs only in IgG-poor environments, such as saliva. As a consequence, the bacteria are protected against phagocytic killing, whereas in blood plasma where the concentration of IgG is high, the antibodies preferentially bind via Fab, facilitating opsonization and bacterial killing. IgG-poor environments represent the natural habitat for IgGFc-binding bacteria, and IgGFc-binding proteins may have evolved to execute their function in such environments. The lack of protection in plasma also helps to explain why cases of severe invasive infections with IgGFc-binding bacteria are so rare compared with superficial and uncomplicated infections.

IgG-binding proteins of bacteria

Proteins capable of non-immune binding of immunoglobulins G (IgG) of various mammalian species, i.e. without the involvement of the antigen-binding sites of the immunoglobulins, are widespread in bacteria. These proteins are located on the surface of bacterial cells and help them to evade the host's immune response due to protection against the action of complement and to decrease in phagocytosis. This review summarizes data on the structure of immunoglobulin-binding proteins (IBP) and their complexes with IgG. Common and distinctive structural features of IBPs of gram-positive bacteria (staphylococci, streptococci, peptostreptococci) are discussed. Conditions for IBP expression by bacteria and their functional heterogeneity are considered. Data on IBPs of gram-negative bacteria are presented.

Antibiotic susceptibility of Stenotrophomonas (Xanthomonas) maltophilia: comparative (NCCLS criteria) evaluation of antimicrobial drugs with the agar dilution and the agar disk diffusion (Bauer-Kirby) tests

Ninety-six clinical isolates of Stenotrophomonas maltophilia were examined with the agar dilution method for susceptibility to 19 antimicrobial drugs. Doxycycline, cotrimoxazole, timentin, ofloxacin, fosfomycin, and piperacillin + tazobactam, in that order, inhibited the majority of strains. All isolates were resistant to nitrofurantoin. Concurrent disk susceptibility (Bauer-Kirby method) testing, using currently valid NCCLS interpretative criteria for Pseudomonas aeruginosa, uncovered a significant incidence of very major (category I), major (category II), and minor (categories III and IV) discrepancies for aminoglycosides, cephalosporins, chloramphenicol, and piperacillin + tazobactam and ticarcillin + clavulanic acid. Therefore, new interpretative criteria indicative of intermediate (I) susceptibility of S. maltophilia to these various antibiotics were proposed. In addition, new intermediate susceptibility criteria were proposed for the two beta-lactam-beta-lactamase inhibitor combinations. It was recommended to exclude ciprofloxacin from test batteries against this microorganism due to the wide scatter of minimal inhibitory concentration values and diameters of inhibition zones; the same was true for polymyxin B. It is hoped that the proposed modified, species-specific criteria will improve the clinical utility of laboratory-generated disk antibiograms with respect to the inherently multiple antibiotic-resistant, opportunistic pathogen S. maltophilia.

Escherichia coli strains with nonimmune immunoglobulin-binding activity

We have identified several strains of Escherichia coli which contain immunoglobulin-binding activity on the cell surface. Affinity-purified antibodies ordinarily used as secondary antibodies in immunodetection protocols were bound by 6 of 72 strains of the ECOR reference collection of E. coli. The Fc fragments of both human and sheep immunoglobulin G (IgG) were also bound, demonstrating the nonimmune nature of the phenomenon. Binding of conjugated IgG Fc directly to unfixed cells was observed by fluorescence microscopy. Western blots showed that the immunoglobulin-binding material occurs in the form of multiple bands, with the apparent molecular masses of the most prominent bands exceeding 100 kDa. No two of the strains have the same pattern of bands. The binding activity in extracts was sensitive to proteinase K. The binding activity of intact cells was reduced preferentially by trypsin digestion, demonstrating exposure at the cell surface. Expression of binding activity in Luria-Bertani broth cultures was favored by a temperature of 37 degrees C and entry into stationary phase of growth.

Complete sequence of the gene encoding a chorionic gonadotropin-like protein from Xanthomonas maltophilia

Our laboratory has previously reported that: (i) Xanthomonas maltophilia (Xm) produces a protein which has immunological resemblance to the beta-subunit of human chorionic gonadotropin (hCG) and (ii) possesses a high-affinity receptor which binds holo hCG, and the endogenous ligand, Xm chorionic gonadotropin (xCG), but does not bind human luteinizing hormone (hLH). We have also previously published a 492-bp partial nucleotide sequence of the gene (xcg) coding for xCG. We report herein the entire xcg sequence of 1362 bp, which codes for a 48-kDa protein. This sequence confirmed the 492-bp sequence, as well as two partial amino acid (aa) sequences which we have previously reported. The sequence has a region which is homologous to aa 56-139 of the beta-subunit of hCG, and a second region homologous to the C-terminal tail of hCG. This is the first report of a prokaryotic gene homologous to the hCG beta-subunit-encoding gene.

Partial characterization of the 30 kD Ig-binding protein from Pseudomonas maltophilia

We have previously demonstrated that Pseudomonas maltophilia (ATCC 13637) possess a 30 kDa cell wall protein which binds various subclasses of IgG's and IgA by their Fc region. The protein was solubilized by papain and purified by affinity chromatography on cyanogen bromide activated sepharose beads conjugated with human IgG. The eluent was electrophoresed on a 12% polyacrylamide gel under denaturing conditions, and the immunoactive bands identified by Western blot analysis, a second gel was stained with Coomassie blue. The affinity purified eluent was electrophoresed on a one-dimensional 15% polyacrylamide gel and stained with Coomassie blue. The protein band of interest was cut. The protein band was then digested in situ with Staphylococcus aureus V-8 protease. The peptide bands were separated by electrophoresis on a second one dimensional 15% polyacrylamide gel and then electroblotted into a polyvinylidine difluoride membrane. The bands were visualized by staining with Coomassie blue, cut out, and sequenced using an automated gas phase sequencer. Minimal amino acid composition was determined in a similar fashion. We have thus obtained partial N-terminal amino acid sequence data from the above method.