Immunoglobulin-binding structure on bovine group G streptococci different from type III Fc receptors on human group G streptococci

Binding of murine myeloma proteins of different Ig classes and subclasses to Fc-reactive surface structures in gram-positive cocci

Twelve different murine myeloma proteins were tested for binding to seventy Gram-positive strains belonging to group A, C and G streptococci and to Staphylococcus aureus. Group A streptococci, known to bind human IgG, were incapable of binding any of eight murine IgG immunoglobulins tested except for one strain that bound an IgG2b myeloma protein. In contrast, group C and G streptococci interacted with murine immunoglobulins of subclasses IgG2a, IgG2b and IgG3, and G strains also to a lesser extent with IgG1. Bovine and equine-group C streptococci did not differ from human group C streptococci in their IgG reactivity. Staphylococcal strains showed a high reactivity with murine myeloma components of IgG subclasses 2a, 2b and 3 and a low but definite binding of an IgG1 myeloma protein. IgA myeloma protein S-122 interacted with nine of fifteen group A streptococci. This binding could not be inhibited by human IgG and the reactivity is thus different from Fc-mediated binding of immunoglobulins. One of three IgA myeloma proteins tested, TEPC 15, bound to staphylococci. The Fc specificity of this interaction was confirmed by chromatography on protein A-Sepharose and by inhibition studies using polyclonal human IgG. The protein A reactivity of this monoclonal protein was detected in IgA aggregates and absent in the monomeric form of IgA.

Nucleotide sequence of the gene for peptostreptococcal protein L

A gene bank of Peptostreptococcus magnus DNA was established using an E. coli host-vector system. Western blot analysis identified a clone expressing protein L which bound to the light chain of human immunoglobulins. DNA sequence determination and analysis revealed an open reading frame of 992 amino acids, giving a theoretical secreted protein of 106 kD with a pl of 4.67.

Two types of receptors for human plasminogen on group G streptococci

To investigate the nature of plasminogen binding to streptococci, strains selected for high reactivity with human plasminogen were examined for binding pattern against a panel of plasminogen fragments. The strains included human isolates of groups A, C and G as well as bovine isolates of group G. All strains reacted substantially with the plasminogen fragment kringle 1-3. Using the miniplasminogen fragment (kringle 5 and the B chain) a small but reproducible uptake was detected for human group G strains but not for group A or C strains. The group G strains of bovine origin on the other hand demonstrated high uptake of miniplasminogen, suggesting the possibility of an alternative plasminogen receptor for this species. This interpretation was supported by blocking experiments with the lysine analogue EACA where low concentrations (1 mM) completely blocked plasminogen binding to human streptococci, whereas a 100-fold higher concentration was needed for bovine group G strains. Scatchard plots with human isolates resulted in straight lines and Kd values were generally in the range of 20-80 nM. The number of receptors was estimated to be 45,000 for a selected group A strain and about 10,000 for the selected group C and G strains. Scatchard analysis with bovine group G isolates on the other hand revealed a two phase interaction, supporting the assumption of two different receptor structures on these strains. Kd for the first phase was estimated to be about 20 nM (10,000-20,000 receptors per bacterium), which was similar to the human strains, whereas the second phase was in the range of 400-500 nM (50,000 and 150,000 receptors per bacterium with two selected strains). Scatchard plots with the miniplasminogen fragment as ligand mimicked the phase two reaction with plasminogen, supporting the concept that this reaction represents a new and not previously described receptor. Both the receptor reacting with the kringle 1-3 portion and the one reacting with the miniplasminogen portion bound plasmin and plasminogen with similar affinity.

Comparison of albumin receptors expressed on bovine and human group G streptococci

The albumin receptor expressed by bovine group G streptococci was extracted and affinity purified. The protein was characterized for species reactivity, and monospecific antibodies were prepared to the purified receptor. The bovine group G albumin receptor was compared functionally, antigenically, and for DNA homology with the albumin-binding protein expressed by human group G streptococci. In agreement with previous reports, the albumin-binding activity of human strains was mediated by a unique domain of the type III immunoglobulin G-Fc-binding molecule, protein G. The albumin receptor expressed by bovine group G strains was found to lack any immunoglobulin G-binding potential but displayed a wider profile of species albumin reactivity than protein G. Both albumin receptors could inhibit the binding of the other to immobilized human serum albumin, and each displayed similar binding properties. Antigenic comparison of the two albumin receptors demonstrated a low level of cross-reactivity; however comparison at the DNA level, using an oligonucleotide probe specific for the albumin-binding region of protein G, demonstrated that the two albumin receptors expressed by human and bovine group G streptococcal strains do not display significant homology.

New receptor for human plasminogen on gram positive cocci

180 bacterial strains representing 17 different species of gram positive cocci were tested for the ability to interact with human plasminogen. Receptors for plasminogen could be detected on 23/24 strains of S. pyogenes, 15/15 strains of S. equisimilis, 14/16 strains of human group G streptococci and 14/14 strains of S. pneumoniae. Eight of nineteen strains representing five species of alpha-hemolytic streptococci were also positive. S. equisimilis demonstrated the highest uptake with a median value of 58 per cent (20%-67%). On the other hand, all strains of S. agalactiae, the majority of S. faecalis and all S. aureus, S. epidermidis and S. saprophyticus strains tested were negative. The concentration of unlabelled plasminogen causing a 50 per cent reduction of bound tracer was between 50 and 150 mM. These estimates of the dissociation constant confirmed the specific nature of the interaction. Binding of plasminogen could be blocked by addition of plasmin-aprotinin complex, suggesting that plasminogen and plasmin bind to the same receptor. Binding was also blocked by the plasminogen fragment kringle 1-3, but not by miniplasminogen, a fragment containing kringle 5 and the B-chain region. As streptokinase interacts mainly with the B-chain of plasmin it is clear that the bacterial receptor for plasminogen is not identical to streptokinase.

Comparison of the mitogenic activities of streptococcal protein-G and staphylococcal protein-A on human mononuclear cells

In the present work we report data from experiments comparing the proliferative stimuli demonstrated by Streptococcal Protein-G and Staphylococcal Protein A on human peripheral blood mononuclear cells. Protein-G and Protein-A are presented to the cells in solution as well as linked to plastic or Sepharose beads, or incorporated within the cell wall of whole bacteria. The cellular response is measured by incorporation of 3H-thymidine in 72 hr cultures. The soluble and the immobilized forms of Protein-A, but not those of Protein-G, displayed high mitogenicity. Possible explanations for the absence of Protein-G induced mitogenicity are discussed.

Non-immune Fab- and Fc- mediated interactions of avian Ig with S. aureus and group C and G streptococci

Serum samples from 19 avian species representing 8 orders were tested for their capacity to inhibit the Fab- and Fc-mediated immunoglobulin binding to protein A-carrying S. aureus and protein G-carrying group C and G streptococci. Four species (mallard, dunlin, starling and blackbird) belonging to three different orders showed a high degree of Fc-mediated protein A- and protein G-reactivity. Five species demonstrated a high level and nine species exhibited a low level of Fab-mediated protein A-reactivity. The four species identified as Fc-reactive were capable of Fab-mediated immunoglobulin binding with streptococcal surface proteins but incapable of Fab-mediated protein A binding. SDS-PAGE analysis confirmed that the protein A-Sepharose affinity purified material contained proteins corresponding to immunoglobulin chains. Inhibition results by avian sera were confirmed by direct binding of protein A-reactive proteins to bacteria, by precipitation in gel and by Western blot analysis of binding to protein A and protein G, respectively.

Isolation and properties of a novel IgG-binding protein from streptococci of serological group U

A nonimmune binding of immunoglobulin (Ig) G has been detected in streptococci of group U. The group U Fc-binding site differed from the five previously known types of staphylococcal and streptococcal Fc-binding sites by its strong affinity for murine IgG, with dissociation constants in nanomolar range for rat and mouse IgG, as well as for mouse IgG subclasses 1, 2a, 2b and 3. It also differed from other binding sites by the high sensitivity towards trypsin. The Fc-binding protein could be solubilized from the streptococci of group U with papain and purified by gel filtration on sephacryl S-200 and by subsequent affinity chromatography on human IgG-Sepharose. The purified binding protein was homogeneous on SDS-polyacrylamide gel electrophoresis and had a molecular weight of approximately 58,000 daltons. It retained its binding activities for murine IgG subclasses as revealed by western blotting. Coupled to CNBr-activated sepharose, the purified Fc-binding protein could be effectively used for the isolation of murine IgG subclasses by affinity chromatography.

Non-immune F(ab')2- and Fc-mediated interactions of mammalian immunoglobulins with S. aureus and group C and G streptococci

The distribution among mammalian species of non-immune F(ab')2- and Fc-mediated immunoglobulin interactions with surface proteins of S. aureus (protein A) and of group C and G streptococci was studied. Serum samples from 48 mammalian species representing 15 orders were first tested for their capacity to inhibit streptococcal F(ab')2-mediated binding; 26 of these sera were also tested for streptococcal IgG Fc-mediated binding. Analogous inhibition experiments were then carried out with staphylococci. All mammalian species studied inhibited both types of immunoglobulin binding to streptococci, viz the serum samples contained both F(ab')2- and Fc-reactive immunoglobulins. The reactivity was equal to that of human serum in 26 out of 47 mammalian sera. Seven sera showed a low degree of inhibition compared to human serum. The inhibiting capacities of the two streptococcal non-immune interactions showed a direct correlation (r = 0.91, p less than 0.0001 for the r-value) for individual species. The inhibition patterns observed with S. aureus differed from the profiles recorded with the streptococcal strains, suggesting that these organisms interact with separate sites on the immunoglobulin molecules. Isolated F(ab')2-binding was recorded in 5 out of 24 sera, and Fc-binding alone was noted in 7 sera. Taken together, the present studies demonstrate that mammalian immunoglobulins possess F(ab')2- and Fc-binding sites for protein A and for receptors on group C and G streptococci. The F(ab')2-mediated binding to streptococci is associated with Fc-reactivity, in contrast to protein A which may interact exclusively with a complementary structure in either the F(ab')2- or the Fc-portion of the immunoglobulins.

Specific attachment of Staphylococcus aureus to immobilized fibronectin

Staphylococcus aureus cells have been shown to possess surface-associated proteins with affinity for soluble fibronectin. We have investigated the ability of these surface proteins to mediate attachment to immobilized fibronectin and collagen. Attachment was quantified by determination of bacterial ATP in a bioluminescence assay. The ability to attach to fibronectin- or collagen-coated plastic surfaces was investigated for four S. aureus strains: Cowan 1, Newman, SA113(83A), and Wood 46. Cells from the different strains varied in their attachment properties, but all cells except those of strain Wood 46 attached readily to substrates coated with fibronectin. Only cells from strain Cowan 1 attached reproducibly to collagen-coated substrates in the absence of fibronectin. The attachment of cells from strain SA113(83A) to fibronectin-coated surfaces was shown to be dependent on time, fibronectin concentration, and bacterial growth phase. Soluble fibronectin or NH2-terminal fibronectin fragment (Mr, 29,000) disturbed the attachment to surfaces coated with fibronectin bound to denatured collagen type I. The attachment process to such substrates was also effectively inhibited by preincubating the substrate with fibronectin-binding proteins isolated from S. aureus Newman and SA113 (83A) and purified with affinity chromatography.

Binding of chicken, bovine, and rabbit immunoglobulins by avian, bovine, and human strains of Staphylococcus aureus

Sixty-six strains of Staphylococcus aureus of avian, bovine, and human origin were tested for their ability to bind chicken, bovine, and rabbit immunoglobulin G (IgG). A microtitration plate hemagglutination assay and a direct-tube enzyme immunoassay were used to determine qualitative differences. Twice as many chicken and bovine S. aureus isolates than human strains reacted positively to chicken IgG. Mean binding values of chicken IgG were also twice as high for chicken and bovine S. aureus isolates when compared with human-derived strains. Isolates of bovine and human origin displayed a high affinity for bovine IgG and rabbit IgG, whereas few isolates from chickens bound substantial quantities of the nonavian IgG species. These results demonstrate that preparations of staphylococcal protein A with affinities for immunoglobulins from poultry and other animals can be obtained by screening large numbers of isolates, especially those obtained from an animal species, such as chickens, that is the same as the source of the immunoglobulin one wishes to bind.

A non-immune interaction between the light chain of human immunoglobulin and a surface component of a Peptococcus magnus strain

The immunoglobulin-binding capacity of a Peptococcus magnus strain was studied in a sensitive binding assay using purified human immunoglobulin preparations. The P. magnus strain 312 was capable of binding 48% of polyclonal IgG. Twenty-four of 40 purified myeloma proteins (60%) representing immunoglobulin classes A, G and M showed definite reactivity with an uptake level ranging from 45 to 90%. The remaining 16 monoclonal proteins were non-reactive, binding less than 15%. One myeloma protein with antistaphylolysin and two with antistreptolysin O specificity, i.e. monoclonal proteins with defined antigen specificity, were highly reactive. Binding capacity was observed in all four IgG subclasses and in Ig classes A and M. Twenty-three of 27 myeloma proteins of kappa type were reactive but only one of 13 myeloma proteins of lambda type interacted with the P. magnus strain. Isotope-labelled Fab gamma, F(ab')2 gamma and F(ab')2 alpha fragments were effectively bound by the strain. IgG Fc fragments were completely non-reactive. Isolated light immunoglobulin chains inhibited in a dose-dependent way the uptake of intact IgG to bacteria. Purified heavy chains were non-inhibitory. Isotope-labelled antistaphylolysin IgG F(ab')2 fragments preincubated with staphylolysin were as reactive as free antibody fragments, suggesting that the bacterial binding structure is located outside the antibody-combining site. The immunoglobulin reactivity of P. magnus was not affected by heating the bacteria to 80 degrees C for 5 min nor by treatment with trypsin or sodium metaperiodate. Digestion of 2 X 10(9) organisms with 100 micrograms of pepsin and papain reduced the binding by 58 and 90%, respectively. These data indicate that the binding of immunoglobulin to P. magnus is a non-immune reactivity mediated by a heat-stable surface protein interacting with specific sites on the light chain of the immunoglobulin molecule.

Variations in binding of mammalian fibrinogens to streptococci groups A, B, C, E, G and to Staphylococcus aureus

Twenty-eight beta-hemolytic streptococci of groups A, B, E, G and Streptococcus equisimilis as well as four Staphylococcus aureus strains were tested for their ability to bind fibrinogen preparations from different animal species: homo, baboon, rabbit, rat, guinea-pig, dog, horse, pig, cow and sheep. The patterns of binding indicated differences in the structures of the bacterial fibrinogen receptors. There were higher binding levels in streptococci groups A, G, and S. equisimilis than in representative group B and E strains. Considerable differences in the binding capacity were found within streptococci groups A and E. Group C and group G strains showed rather similar patterns and could be further divided into high-level and low-level binding strains. There is no correlation between binding levels of different animal fibrinogen preparations and the strains isolated from corresponding animals. Recent studies by others have shown that resistance to phagocytosis is mediated by fibrinogen-binding in streptococci group A. The existence of similar fibrinogen-binding structures in several streptococcal species indicates an important role with a definite survival value. It also suggests that M or T protein analogues are present in streptococci groups C, G and E.

Non-immune IgG F(ab')2 binding to group C and G streptococci is mediated by structures on gamma chains

The present investigation was designed to determine whether the heavy or the light immunoglobulin chain is involved in the non-immune binding of IgG F(ab')2 fragments to specific surface receptors on human group C and G streptococci. Purified human polyclonal IgG was mildly reduced with dithiothreitol and alkylated with iodoacetamide. Light (L) and heavy (H) chains were separated. Intact IgG and purified L and H chains of polyclonal immunoglobulin G were tested in an inhibition assay for non-immune IgG F(ab')2-mediated binding to group C and G streptococci. H chains inhibited the uptake of isotope-labelled IgG F(ab')2 fragments. Isolated L chains were non-reactive. Intact IgG molecules were more potent inhibitors than isolated H chains tested in equimolar concentrations. These results indicate that the non-immune interaction between human group C and G streptococci and F(ab')2 fragments of human IgG is mediated by reactive sites exposed on the immunoglobulin G H chains. The observation that intact IgG on a molar basis was more inhibitory than purified gamma chains suggests that the L chains may contribute to the reactivity, presumably by passive stabilization of the immunoglobulin molecule.

Interaction of thermally aggregated human IgG with bacteria

Ninety-three bacterial strains, representing 16 Gram-positive and Gram-negative species, were tested for quantitative binding of 125I-labelled monomeric and thermally aggregated human IgG. Aggregated IgG bound to all bacterial species studied, in contrast to monomeric IgG, which interacted only with S. aureus, group A, C and G streptococci, viz. bacteria possessing previously described IgG-Fc receptors. A positive correlation was observed between binding of monomeric IgG and the uptake of thermally aggregated IgG (r = 0.92). Monomeric IgG inhibited effectively the binding of monomeric IgG but only partially the uptake of aggregates. Absorption with bacteria revealed that only a fraction of aggregated IgG could interact with bacteria lacking specific IgG-Fc receptors. A human group G streptococcus strain (G-148), tested with increasing amounts of immunoglobulin, was capable of binding at least ten times as much aggregates as monomeric IgG, implying binding to separate binding sites. These data indicate that polymeric IgG produced by thermal aggregation of human polyclonal IgG can interact with bacterial surface components found in most pathogenic microorganisms. This interaction seems to be less specific than the binding to previously described IgG-Fc receptors.

Demonstration of a receptor for mouse and human serum albumin in Streptococcus pyogenes

A new type of surface receptor for serum albumin was detected in strains of Streptococcus pyogenes (group A). This receptor, called type e, was different from albumin receptors in other streptococcal species. Only mouse and human serum albumin was bound to the receptor. The albumin-binding capacity was high: 2 X 10(8) bacterial organisms bound 11 micrograms of mouse albumin. The receptor was stable even when treated at 100 degrees C for 5 min. Binding of albumin was not mediated by lipoteichoic acid (LTA) because of lack of correlation to surface LTA, restricted albumin reactivity, and positive binding in presence of 2% Tween 20. Presence of albumin receptor type e correlated to presence of M-protein as measured by growth in the bactericidal test. All 51 M-protein positive group A streptococcal strains tested could bind mouse albumin whereas only 3 out of 8 M-protein negative strains showed positive binding (P less than 0.001). The sensitivity to trypsin digestion suggests that the albumin receptor is of protein nature or mediated by a protein.

Binding of human fibronectin to group A, C, and G streptococci

A total of 387 bacterial strains belonging to 35 species were tested in direct binding experiments for the uptake of purified radiolabeled human fibronectin. Positive binding was found in group A, C, and G streptococci and in Staphylococcus aureus. The group C streptococcal species Streptococcus equisimilis, Streptococcus zooepidemicus, Streptococcus equi, and Streptococcus dysgalactiae were uniformly reactive with fibronectin. Beta-hemolytic bovine group G streptococci showed the same degree of reactivity as human group G strains. In contrast, only 4 out of 15 alpha-hemolytic bovine group G strains were able to bind fibronectin. The uptake of fibronectin measured at room temperature with a human group G streptococcus was a slow, time-dependent process with maximum binding after approximately 1 h. Human polyclonal immunoglobulin G and serum albumin tested in inhibition experiments did not affect the fibronectin binding. Fibronectin seems, therefore, to interact with a surface component that is different from the specific binding sites previously described for human immunoglobulin G and serum albumin.

Binding of fibronectin to the surface of group A, C, and G streptococci isolated from human infections

Sixty-nine haemolytic and non-haemolytic streptococcal strains were isolated from various human infections and serogrouped with the coagglutination test. The amount of 125I-fibronectin bound to bacterial cells in a standard assay was determined. Most of the group A, C, and G strains were able to bind fibronectin. None of the group B or D strains bound significant amounts of fibronectin. Group A, C, and G streptococci appear to preferentially bind the N-terminal region of the fibronectin molecule because the 25K N-terminal fragment of the protein could effectively inhibit the binding of 125I-fibronectin to cells. Furthermore, the ability of representative strains of group A, C, and G to bind fibronectin was markedly reduced after trypsin treatment of the cells. Fibronectin binding components released from one strain by trypsin treatment inhibited the binding of 125I-fibronectin to all group A, C, and G streptococci strains. These findings indicate similarities among fibronectin binding proteins of the three groups of streptococci tested. However, the relative susceptibility to trypsin of fibronectin receptors of the three strains differed as did the relative potency of the inhibitory activity of receptors solubilized from different strains. Binding of fibronectin to the cell surface of group A, C, and G streptococci may contribute to virulence, for instance by promoting specific attachment to exposed fibronectin in open wounds and tissue lesions.

Receptor in group C and G streptococci detects albumin structures present in mammalian species

The presence of albumin structures with the capacity to bind to a surface receptor in group C and G streptococci was studied in serum samples from 45 mammalian species representing 15 different orders, using an inhibition assay. The ability of animal sera to inhibit the uptake of radiolabeled human serum albumin by the streptococci indicated the presence of such albumin structures. Positive reactions were found in species of most orders tested, with Marsupialia as a notable exception. All Carnivora sera tested were strongly positive. In some orders such as Artiodactyla both positive and negative species were identified. Serum samples from 62 bird species representing 15 orders and from 5 fish species were also tested in the inhibition assay. None of these serum samples was capable of inhibiting the uptake of human serum albumin by streptococci. Some differences were also noted in the results obtained with group C and G streptococci from human and bovine sources, respectively, indicating the presence of two types of receptors. The present studies suggest a phylogenetic origin of albumin structures with affinity for the streptococcal receptor to a period after the divergence of Marsupialia from the other mammalian orders.

Lymphocyte cell subpopulations during acute post-streptococcal glomerulonephritis: cell surface antigens and binding of streptococcal membrane antigens and C-reactive protein

T lymphocyte surface markers were examined in 23 patients with acute post-streptococcal glomerulonephritis (AGN) in parallel with normal controls and individuals without nephritis who showed evidence of pharyngeal or skin-sore beta-haemolytic streptococcal infection. Numbers of T gamma cells were similar in AGN and normal controls but were significantly lower (P less than 0.05) than those in skin-sore culture-positive streptococcal infection controls. Numbers of T mu cells were similar in AGN and normal controls but were lower (P less than 0.05) than those observed in streptococcal controls. Percentages of T mu cells were similar in AGN and normal controls but were lower (P less than 0.05) than those recorded in streptococcal infection control groups. Proportions of T cells were reduced during AGN (P less than 0.05). Lymphocytes capable of binding type 12 group A streptococcal membranes were increased (30.4%) in patients with AGN as compared to normal controls (4.1%). Subjects with streptococcal infection alone showed elevated but intermediate relative numbers (10.5%) of lymphocytes binding group A membranes. Increased relative numbers of both B and T lymphocytes binding group A streptococcal membranes were present in both AGN and non-nephritogenic streptococcal infection controls.

Binding of fibrinogen degradation products to S. aureus and to beta-hemolytic streptococci group A, C and G

Binding of fibrinogen degradation products was measured to Gram-positive cocci known to carry receptors for human fibrinogen. Forty-one strains of S. aureus and group A, C and G streptococci were studied. The largest plasmin produced fragment (X-fragment, HMWDP) showed higher binding levels to the bacterial receptors in all four species as compared to intact fibrinogens. The enzymatically produced C-terminal fragment D was strongly reactive, whereas the N-terminal fragment E was non-reactive. Inhibition experiments showed that reactive fragments were bound to the same receptors as fibrinogen. Three chemically-produced and well-characterized fragments from the terminal and the middle portions of the fibrinogen molecule were all negative in binding assays. The binding patterns for fibrinogen and the fragments tested were identical in the four bacterial species tested, S. aureus and group A, C and G streptococci. The results confirm and extend earlier data on similarities between fibrinogen receptors on these Gram-positive bacterial species.

Analysis of bacterial cell wall proteins and human serum proteins bound to bacterial cell surfaces

A method was developed for the characterization of proteins non-covalently bound to the cell wall of Gram-positive cocci. The method employs radioactive labelling of cell wall proteins followed by solubilisation and analysis on polyacrylamide gel electrophoresis in sodium dodecyl sulphate. Using this experimental procedure, protein patterns obtained from group A, C and G streptococcal strains showed marked similarities within each group. Protein peaks were also found to be shared between group C and G strains. Two major peaks with molecular weights of about 30 000 and 70 000 characterized group A strains, whereas group C and G strains showed one consistent peak of about 45 000, thus reflecting the closer relationship between these two groups as compared to group A streptococci. By incubating bacteria with human serum proteins before labelling, solubilisation and electrophoretic analysis, it was also possible to study external proteins specifically bound to the bacterial surface. A group G streptococcus, strain G 148, showed protein peaks corresponding to its known specific binding of human albumin and immunoglobulin G, but also additional protein peaks. When Staphylococcus aureus, strain Cowan I, was pre-incubated with human serum in excess, protein peaks corresponding to heavy and light chains of immunoglobulins were seen. Three more protein peaks of serum origin were also detected, indicating binding of proteins other than Ig to S. aureus. Experiments with protein A-coated Sepharose beads resulted in the same protein pattern, suggesting that binding of these different polypeptides is indeed mediated by protein A.

Demonstration of a new type of immunoglobulin G receptor in Streptococcus zooepidemicus strains

Forty-seven bacterial strains representing four different group C streptococcal species were tested for binding of human and bovine immunoglobulin G (IgG) subclasses. Specific binding sites for IgG were found in all bacterial species studied. The four species included differed, however, in their capacities to interact with various IgG subclasses, indicating the existence of different types of IgG receptors. Streptococcus equisimilis and Streptococcus dysgalactiae were found to carry the same type of IgG receptor, one that is identical to the previously described Fc-binding structure type III. A new type of bacterial IgG receptor was detected in Streptococcus zooepidemicus strains. This receptor exhibits a protein A-like subclass specificity, with binding of human IgG1, IgG2, and IgG4 and of bovine IgG2. However, differences in their capacities to interact with other non-human immunoglobulins indicated that these two immunoglobulin-reactive structures were different. All types of IgG receptors in group C streptococci were found to be heat stable but susceptible to proteolytic enzymes. The inability of human serum albumin or fibrinogen to inhibit the uptake of radiolabeled IgG shows that the IgG receptor is separate from binding sites for these two other proteins on the bacterial cell surface. The existence of similar IgG receptors in closely related streptococcal species suggests that these structures have a common origin.

Demonstration of specific binding sites for human serum albumin in group C and G streptococci

A total of 297 bacterial strains belonging to 27 species was tested for quantitative uptake of radiolabeled human serum albumin. Specific binding sites with high affinity for human serum albumin were found exclusively in group C and G streptococci. The albumin binding was found to be a time-dependent, saturable, and displaceable process which obeyed simple kinetic equations. Scatchard analysis revealed that human serum albumin bound to a homogeneous population of receptors with an affinity in the order ot 10(7) liters/mol and that the average bacterial cell carried more than 80,000 binding sites. The albumin receptor is a heat-stable component susceptible to proteolytic digestion. It has a surface localization separate from the receptors for immunolgobulin G, fibrinogen, aggregated beta 2-microglobulin, and haptoglobin. In individual strains, albumin reactivity was also detected independently of these other types of interactions with human proteins.

Specific absorption of human serum albumin, immunoglobulin A, and immunoglobulin G with selected strains of group A and G streptococci

Five gram-positive bacterial strains were selected for absorption studies of human serum samples. Strain AR1 (group A, M-type 1) and G148 (group G), with strong immunoglobulin G (IgG) binding capacities, and strain AW43 (group A, M-type 60), binding both IgA1 and IgA2, were compared with Staphylococcus aureus Cowan I and with Staphylococcus epidermidis L603. Both AR1 and G148 were capable of completely absorbing out serum IgG. In contrast, S. aureus Cowan I left a fraction unabsorbed, as expected from its known lack of IgG3 binding. Strain AW43 absorbed out all serum IgA, using a 10-microliter bacterial pellet for 20 microliter of serum. Serum IgM levels were slightly reduced by S. aureus Cowan I absorption. On the basis of the experiments, a bacterial mixture was designed consisting of S. aureus Cowan I and group A streptococcus strains AR1 and AW43, with absorption characteristics suitable for use in discriminating between early IgM and late IgG and IgA immune responses in routine serological work. A new type of bacteria-mammalian protein binding was discovered. Human serum albumin was completely absorbed out by strain G148 and to a lesser extent by strain AR1 and AW43. S. aureus Cowan I and S. epidermidis were negative. The binding capacity of G148 for albumin equalled that of Cowan I for IgG. The binding pattern of albumin to the strains was different from those of IgG, IgA, IgM, fibrinogen, haptoglobin, or aggregated beta 2-microglobulin and therefore seems to represent another type of bacterial-mammalian interaction with a specific albumin receptor on the surface of streptococci.