Phagocytosis of opsonized Treponema pallidum subsp. pallidum proceeds slowly

Immunodominant extracellular loops of Treponema pallidum FadL outer membrane proteins elicit antibodies with opsonic and growth-inhibitory activities

The global resurgence of syphilis has created a potent stimulus for vaccine development. To identify potentially protective antibodies (Abs) against Treponema pallidum (TPA), we used Pyrococcus furiosus thioredoxin (PfTrx) to display extracellular loops (ECLs) from three TPA outer membrane protein families (outer membrane factors for efflux pumps, eight-stranded β-barrels, and FadLs) to assess their reactivity with immune rabbit serum (IRS). Five ECLs from the FadL orthologs TP0856, TP0858 and TP0865 were immunodominant. Rabbits and mice immunized with these five PfTrx constructs produced ECL-specific Abs that promoted opsonophagocytosis of TPA by rabbit peritoneal and murine bone marrow-derived macrophages at levels comparable to IRS and mouse syphilitic serum. ECL-specific rabbit and mouse Abs also impaired viability, motility, and cellular attachment of spirochetes during in vitro cultivation. The results support the use of ECL-based vaccines and suggest that ECL-specific Abs promote spirochete clearance via Fc receptor-independent as well as Fc receptor-dependent mechanisms.

IFNγ Enhances CD64-Potentiated Phagocytosis of Treponema pallidum Opsonized with Human Syphilitic Serum by Human Macrophages

Syphilis is a multi-stage, sexually transmitted disease caused by the spirochete Treponema pallidum (Tp). Considered broadly, syphilis can be conceptualized as a dualistic process in which spirochete-driven inflammation, the cause of clinical manifestations, coexists to varying extents with bacterial persistence. Inflammation is elicited in the tissues, along with the persistence of spirochetes to keep driving a robust immune response while evading host defenses; this duality is best exemplified during the florid, disseminated stage called secondary syphilis (SS). SS lesions typically contain copious amounts of spirochetes along with a mixed cellular infiltrate consisting of CD4⁺ T cells, CD8⁺ T cells, NK cells, plasma cells, and macrophages. In the rabbit model, Tp are cleared by macrophages via antibody-mediated opsonophagocytosis. Previously, we demonstrated that human syphilitic serum (HSS) promotes efficient uptake of Tp by human monocytes and that opsonophagocytosis of Tp markedly enhances cytokine production. Herein, we used monocyte-derived macrophages to study Tp–macrophage interactions ex vivo. In the absence of HSS, monocyte-derived macrophages internalized low numbers of Tp and secreted little cytokine (e.g., TNF). By contrast, these same macrophages internalized large numbers of unopsonized Borrelia burgdorferi and secreted robust levels of cytokines. Maturation of macrophages with M-CSF and IFNγ resulted in a macrophage phenotype with increased expression of HLA-DR, CD14, inducible nitric oxide synthase, TLR2, TLR8, and the Fcγ receptors (FcγR) CD64 and CD16, even in the absence of LPS. Importantly, IFNγ-polarized macrophages resulted in a statistically significant increase in opsonophagocytosis of Tp accompanied by enhanced production of cytokines, macrophage activation markers (CD40, CD80), TLRs (TLR2, TLR7, TLR8), chemokines (CCL19, CXCL10, CXCL11), and T_H1-promoting cytokines (IL-12, IL-15). Finally, the blockade of FcγRs, primarily CD64, significantly diminished spirochetal uptake and proinflammatory cytokine secretion by IFNγ-stimulated macrophages. Our ex vivo studies demonstrate the importance of CD64-potentiated uptake of opsonized Tp and suggest that IFNγ-activated macrophages have an important role in the context of early syphilis. Our study results also provide an ex vivo surrogate system for use in future syphilis vaccine studies.

Vaccine development for syphilis

INTRODUCTION

Syphilis, caused by the spirochete Treponema pallidum subspecies pallidum, continues to be a globally prevalent disease despite remaining susceptible to penicillin treatment. Syphilis vaccine development is a viable preventative approach that will serve to complement public health-oriented syphilis prevention, screening and treatment initiatives to deliver a two-pronged approach to stemming disease spread worldwide. Areas covered: This article provides an overview of the need for development of a syphilis vaccine, summarizes significant information that has been garnered from prior syphilis vaccine studies, discusses the critical aspects of infection that would have to be targeted by a syphilis vaccine, and presents the current understanding within the field of the correlates of protection needed to be achieved through vaccination. Expert commentary: Syphilis vaccine development should be considered a priority by industry, regulatory and funding agencies, and should be appropriately promoted and supported.

Progress towards an effective syphilis vaccine: the past, present and future

Syphilis is a disease caused by infection with the spirochetal pathogen Treponema pallidum subspp. pallidum. Despite intensive efforts, the unusual biology of T. pallidum has hindered progress towards the development of a vaccine to prevent infection. This review describes previous endeavors to develop a syphilis vaccine, outlines the key issues in the field and proposes new directions in the design of a T. pallidum vaccine. Following a brief overview of the disease symptoms, epidemiology, diagnosis and treatment, a case is put forward for the benefit of pursuing a syphilis vaccine. Relevant material concerning immunity to T. pallidum infection is summarized and evaluated, and pilot experiments describing the use of whole-cell bacterin vaccines and similar preparations are included. A detailed section concerning subunit vaccines is provided, incorporating discussions pertaining to relevant antigen selection, the identification of putative T. pallidum surface-exposed outer membrane proteins, factors hindering previous attempts to vaccinate with recombinant outer membrane proteins, problems and pitfalls of syphilis outer membrane protein-based vaccines, anti-attachment vaccines and the potential use of nonprotein subunit preparations as vaccinogens. Subsequently, critical aspects concerning vaccine antigen preparation and delivery are noted, including protein conformation, synergy, post-translational modifications, live attenuated organisms as vaccine vectors, prime-boost methodologies, adjuvant selection and immunization routes. Finally, animal models are discussed with particular reference to immunoprotection studies. A more thorough understanding of immunity to syphilis, a comprehensive assessment of the immunoprotective capacity of the putative surface-accessible antigens of T. pallidum and utilization of the latest advances in vaccine science should set the scene for future development of a syphilis vaccine.

Surface immunolabeling and consensus computational framework to identify candidate rare outer membrane proteins of Treponema pallidum

Treponema pallidum reacts poorly with the antibodies present in rabbit and human syphilitic sera, a property attributed to the paucity of proteins in its outer membrane. To better understand the basis for the syphilis spirochete's "stealth pathogenicity," we used a dual-label, 3-step amplified assay in which treponemes encapsulated in gel microdroplets were probed with syphilitic sera in parallel with anti-FlaA antibodies. A small (approximately 5 to 10%) but reproducible fraction of intact treponemes bound IgG and/or IgM antibodies. Three lines of evidence supported the notion that the surface antigens were likely β-barrel-forming outer membrane proteins (OMPs): (i) surface labeling with anti-lipoidal (VDRL) antibodies was not observed, (ii) immunoblot analysis confirmed prior results showing that T. pallidum glycolipids are not immunoreactive, and (iii) labeling of intact organisms was not appreciably affected by proteinase K (PK) treatment. With this method, we also demonstrate that TprK (TP0897), an extensively studied candidate OMP, and TP0136, a lipoprotein recently reported to be surface exposed, are both periplasmic. Consistent with the immunolabeling studies, TprK was also found to lack amphiphilicity, a characteristic property of β-barrel-forming proteins. Using a consensus computational framework that combined subcellular localization and β-barrel structural prediction tools, we generated ranked groups of candidate rare OMPs, the predicted T. pallidum outer membrane proteome (OMPeome), which we postulate includes the surface-exposed molecules detected by our enhanced gel microdroplet assay. In addition to underscoring the syphilis spirochete's remarkably poor surface antigenicity, our findings help to explain the complex and shifting balance between pathogen and host defenses that characterizes syphilitic infection.

Killing of Treponema denticola by mouse peritoneal macrophages

Treponema denticola has been identified as an important cause of periodontal disease and hypothesized to be involved in extra-oral infections. The objective of this study was to investigate the role of T. denticola cell length and motility during mouse peritoneal macrophages in vitro uptake. Macrophages, incubated under aerobic and anaerobic conditions, produced a similar amount of TNF-alpha when stimulated with Escherichia coli LPS. The uptake of FlgE- and CfpA-deficient mutants of T. denticola was significantly increased compared with the wild-type strain, due to cell size or lack of motility. Opsonization with specific antibodies considerably improved the treponemes' uptake. These results suggest that macrophages, in addition to other phagocytes, could play an important role in the control of T. denticola infection, and that the raising of specific antibodies could improve the efficacy of the immune response toward T. denticola, either at an oral site or during dissemination.

Nodal inflammatory pseudotumor caused by luetic infection

Inflammatory pseudotumor of lymph nodes (IPT-LN) represents an unusual cause of lymphadenitis of unknown etiology. Upon the observation of a case of IPT-LN associated with Treponema pallidum (Tp) infection, we analyzed a series of 9 IPT-LN and 9 extranodal IPT (spleen, 4 cases; lung, orbit, gut, skin, and liver) for the presence of Tp, using a polyclonal antibody anti-Tp. At the time of biopsy, none of the patients was suspected for luetic infection, nor specific serologic tests were available. IPT-LN areas extensively involved the nodal parenchyma in 4 cases, whereas they were focal in the remaining 5 cases. Capsular thickening and inflammation (6/9), venulitis (3/9), small granulomas (3/9), and follicular hyperplasia (7/9) were observed in the associated lymphoid parenchyma. Tp were detected in 4/9 cases of LN-IPT and in none of the extranodal IPT. Tp were extremely abundant within the IPT areas and in the perivascular tissues in the surrounding parenchyma, whereas they were scattered within the capsule. In Tp+ cases, marked follicular hyperplasia was the single distinctively associated feature. Double immunostains revealed that Tp were predominantly contained in the cytoplasm of CD11c+ CD163+ macrophages, some of which co-expressed HLA-DR. In addition, scattered S100+ interdigitating dendritic cells also showed intracytoplasmic Tp. This study shows that a significant number of IPT-LN is associated with Tp infection. A spirochetal etiology can be suspected in cases of IPT-LN, independently from the extension of the lesions, especially when pronounced follicular hyperplasia is found. Infection by Tp of macrophages and dendritic cells are in keeping with in vitro data and indicate that immune mediated mechanisms may be involved in the pathogenesis of the lesions.

Phagocytosis of Treponema pallidum and reactive oxygen species production by isolated rat Kupffer cells

The in vitro phagocytosis of viable Treponema pallidum subsp. pallidum by isolated rat Kupffer cells, studied by immunofluorescence staining of Kupffer cells-associated bacteria, showed that ingestion of live, unopsonized treponemes was slow: in fact, Kupffer cells started to be positive 1 h after infection, when only 4% of the cells presented small round fluorescent inclusion-like bodies. Thereafter, the number of positive cells progressively increased with time: 7%, 17%, 36%, and 69% of Kupffer cells were positive, respectively, 2, 4, 6 and 8 h after infection. Opsonization of T. pallidum with human immune serum did not substantially modify the percentage (8%) of Kupffer cells ingesting T. pallidum 1 h after infection, whereas opsonization significantly ( P<0.01) increased phagocytosis after 2, 4 and 6 h of incubation, when 44%, 58%, and 68% of Kupffer cells were positive, respectively. At 8 h after infection of Kupffer cells by opsonized T. pallidum, 75% of the cells were positive by immunofluorescence. Heat-inactivation of T. pallidum slightly enhanced phagocytosis. In contrast, opsonization of heat-inactivated spirochetes with specific antibodies significantly ( P<0.01) increased the phagocytosis of bacteria by Kupffer cells, beginning as early as 30 min after infection, when 65% of the cells were positive by immunofluorescence. The reactive oxygen species (ROS) production by Kupffer cells following incubation with spirochetes was also determined by chemiluminescence. Treponemes induced an oxidative burst in Kupffer cells in a dose-dependent manner and the generation of ROS was already detectable 20 min after the exposure of the Kupffer cells to treponemes and peaked at 35 min of incubation. Live, as well as live and opsonized, and heat-inactivated treponemes, induced an O(2)(-) production lower than that induced by heat-inactivated and opsonized spirochetes.

Dendritic cells phagocytose and are activated by Treponema pallidum

Cell-mediated immune processes play a prominent role in the clinical manifestations of syphilis, a sexually transmitted disease of humans caused by spirochetal bacterium Treponema pallidum. The immune cell type that initiates the early immune response to T. pallidum thus far has not been identified. However, dendritic cells (DCs) are the first immune-competent cells to encounter antigens within skin or mucous membranes, the principal sites of early syphilitic infection. In the present study, immature DC line XS52, derived from murine skin, was utilized to examine T. pallidum-DC interactions and subsequent DC activation (maturation). Electron microscopy revealed that T. pallidum was engulfed by DCs via both coiling and conventional phagocytosis and was delivered to membrane-bound vacuoles. The XS52 DC line expressed surface CD14 and mRNA for Toll-like receptors 2 and 4, molecules comprising important signaling components for immune cell activation by bacterial modulins. Both T. pallidum and a synthetic lipopeptide (corresponding to the 47-kDa major membrane lipoprotein) activated the XS52 DC line, as indicated by the secretion of interleukin-12 (IL-12), IL-1beta, tumor necrosis factor alpha, and IL-6 and elevated surface expression of CD54. The combined data support the contention that DCs stimulated by T. pallidum and/or its proinflammatory membrane lipoproteins are involved in driving the cellular immune processes that typify syphilis.

Activation of Human Monocytic Cells by Borrelia burgdorferi and Treponema pallidum Is Facilitated by CD14 and Correlates with Surface Exposure of Spirochetal Lipoproteins

Here we examined the involvement of CD14 in monocyte activation by motile Borrelia burgdorferi and Treponema pallidum. B. burgdorferi induced secretion of IL-8 by vitamin D3-matured THP-1 cells, which was inhibited by a CD14-specific mAb known to block cellular activation by LPS and the prototypic spirochetal lipoprotein, outer surface protein A. Enhanced responsiveness to B. burgdorferi also was observed when THP-1 cells were transfected with CD14. Because borreliae within the mammalian host and in vitro-cultivated organisms express different lipoproteins, experiments also were performed with “host-adapted” spirochetes grown within dialysis membrane chambers implanted into the peritoneal cavities of rabbits. Stimulation of THP-1 cells by host-adapted organisms was CD14 dependent and, interestingly, was actually greater than that observed with in vitro-cultivated organisms grown at either 34°C or following temperature shift from 23°C to 37°C. Consistent with previous findings that transfection of Chinese hamster ovary cells with CD14 confers responsiveness to LPS but not to outer surface protein A, B. burgdorferi failed to stimulate CD14-transfected Chinese hamster ovary cells. T. pallidum also activated THP-1 cells in a CD14-dependent manner, although its stimulatory capacity was markedly less than that of B. burgdorferi. Moreover, cell activation by motile T. pallidum was considerably less than that induced by treponemal sonicates. Taken together, these findings support the notion that lipoproteins are the principle component of intact spirochetes responsible for monocyte activation, and they indicate that surface exposure of lipoproteins is an important determinant of a spirochetal pathogen’s proinflammatory capacity.

Surface antigens of the syphilis spirochete and their potential as virulence determinants

A unique physical feature of Treponema pallidum, the venereally transmitted agent of human syphilis, is that its outer membrane contains 100-fold less membrane-spanning protein than the outer membranes of typical gram-negative bacteria, a property that has been related to the chronicity of syphilitic infection. These membrane-spanning T. pallidum rare outer membrane proteins, termed TROMPs, represent potential surface-exposed virulence determinants and targets of host immunity. Only recently has the outer membrane of T. pallidum been isolated and its constituent proteins identified. Five proteins of molecular mass 17-, 28-, 31-, 45-, and 65-kDa were outer membrane associated. The 17- and 45-kDa proteins, which are also present in greater amounts with the T. pallidum inner membrane protoplasmic cylinder complex, had been previously characterized lipoproteins and are, therefore, not membrane-spanning but rather membrane-anchored by their lipid moiety. In contrast, the 28-, 31-, and 65-kDa proteins are exclusively associated with the outer membrane. Both the purified native and an Escherichia coli recombinant outer membrane form of the 31-kDa protein, designated Tromp1, exhibit porin activity, thereby confirming the membrane-spanning outer membrane topology of Tromp1. The 28-kDa protein, designated Tromp2, has sequence characteristics in common with membrane-spanning outer membrane proteins and has also been recombinantly expressed in E. coli, where it targets exclusively to the E. coli outer membrane. The 65-kDa protein, designated Tromp3, is present in the least amount relative to Tromps1 and 2. Tromps 1, 2, and 3 were antigenic when tested with serum from infection and immune syphilitic rabbits and humans. These newly identified TROMPs provide a molecular foundation for the future study of syphilis pathogenesis and immunity.

Recombinant Treponema pallidum rare outer membrane protein 1 (Tromp1) expressed in Escherichia coli has porin activity and surface antigenic exposure

We recently reported the cloning and sequencing of the gene encoding a 31-kDa Treponema pallidum subsp. pallidum rare outer membrane porin protein, designated Tromp1 (D. R. Blanco, C. I. Champion, M. M. Exner, H. Erdjument-Bromage, R. E. W. Hancock, P. Tempst, J. N. Miller, and M. A. Lovett, J. Bacteriol. 177:3556-3562, 1995). Here, we report the stable expression of recombinant Tromp1 (rTromp1) in Escherichia coli. rTromp1 expressed without its signal peptide and containing a 22-residue N-terminal fusion resulted in high-level accumulation of a nonexported soluble protein that was purified to homogeneity by fast protein liquid chromatography (FPLC). Specific antiserum generated to the FPLC-purified rTromp1 fusion identified on immunoblots of T. pallidum the native 31-kDa Tromp1 protein and two higher-molecular-mass oligomeric forms of Tromp1 at 55 and 80 kDa. rTromp1 was also expressed with its native signal peptide by using an inducible T7 promoter. Under these conditions, rTromp1 fractionated predominantly with the E. coli soluble and outer membrane fractions, but not with the inner membrane fraction. rTromp1 isolated from the E. coli outer membrane and reconstituted into planar lipid bilayers showed porin activity based on average single-channel conductances of 0.4 and 0.8 nS in 1 M KCl. Whole-mount immunoelectron microscopy using infection-derived immune serum against T. pallidum indicated that rTromp1 was surface exposed when expressed in E. coli. These findings demonstrate that rTromp1 can be targeted to the E. coli outer membrane, where it has both porin activity and surface antigenic exposure.

Treponema pallidum and the quest for outer membrane proteins

Treponema pallidum, the syphilis spirochaete, has a remarkable ability to evade the humoral and cellular responses it elicits in infected hosts. Although formerly attributed to the presence of an outer coat comprised of serum proteins and/or mucopolysaccharides, current evidence indicates that the immuno-evasiveness of this bacterium is largely the result of its unusual molecular architecture. Based upon a combination of molecular, biochemical, and ultrastructural data, it is now believed that the T. pallidum outer membrane (OM) contains a paucity of poorly immunogenic transmembrane proteins ('rare outer membrane proteins') and that its highly immunogenic proteins are lipoproteins anchored predominantly to the periplasmic leaflet of the cytoplasmic membrane. The presence in the T. pallidum OM of a limited number of transmembrane proteins has profound implications for understanding syphilis pathogenesis as well as treponemal physiology. Two major strategies for molecular characterization of rare outer membrane proteins have evolved. The first involves the identification of candidate OM proteins as fusions with Escherichia coli alkaline phosphatase. The second involves the characterization of candidate OM proteins identified in outer membranes isolated from virulent T. pallidum. Criteria to define candidate OM proteins and for definitive identification of rare OM proteins are proposed as a guide for future studies.

Treponema pallidum rare outer membrane proteins: analysis of mobility by freeze-fracture electron microscopy

Freeze-fracture and deep-etch electron microscopy were used to investigate the molecular architecture of the Treponema pallidum outer membrane (OM). Freeze-fracture electron microscopy of treponemes freshly harvested from rabbit testes revealed that the intramembranous particles (IMPs) in both the concave and convex OM leaflets were distributed into alternating areas of relatively high and low particle density; in many OM fractures, IMPs formed rows that ran either parallel to or obliquely across the fracture faces. Statistical analysis (runs test) confirmed that the IMPs were nonrandomly distributed in both OM leaflets. Examination of deep-etched specimens revealed that the particles observed in freeze-fractured OMs also were surface exposed. Combined analysis of deep-etched and cross-fractured treponemes revealed that the OM particles were located in regions of the OM away from the endoflagella and closely apposed to the cytoplasmic membrane-peptidoglycan complex. When treponemes were incubated for extended periods with heat-inactivated immune rabbit syphilitic serum, no alteration in the distribution of OM IMPs was detected. In further experiments, approximately 1:1 mixtures of T. pallidum and Escherichia coli or separate suspensions of the nonpathogenic Treponema phagedenis biotype Reiter were fixed at 34 degrees C or after cooling to 0 degree C (to induce lateral phase separations that would aggregate IMPs). Only particles in the T. pallidum OM failed to aggregate in cells fixed at the lower temperature. The combined data suggest that the mobility of T. pallidum rare OM proteins is limited, perhaps as a result of interactions between their periplasmic domains and components of the peptidoglycan-cytoplasmic membrane complex.

Characterization of the serum requirement for macrophage-mediated killing of Treponema pallidum ssp. pallidum: relationship to the development of opsonizing antibodies

Serum pools were collected from rabbits bled at various times after intra-testicular infection with Treponema pallidum ssp. pallidum. These were tested for their ability to opsonize T. pallidum and promote killing of the organisms by macrophages. Compared to normal sera, significant opsonization was first seen on day 10 of infection as measured by both ingestion (P < 0.001) and macrophage-mediated killing (P = 0.006); significant levels of functional antibodies persisted through 300 days of infection. Although opsonic activity peaked early in infection, antibodies that promoted optimal macrophage-mediated killing developed much later, suggesting that these two functions may represent activities of antibodies with differing specificities or affinities. The initial development of antibodies that augment both phagocytosis and killing corresponds with the in vivo clearance of treponemes from the primary site of infection. These observations support the hypothesis that macrophages are the major effector mechanism for elimination of T. pallidum during early syphilis infection.

Opsonization of Treponema pallidum is mediated by immunoglobulin G antibodies induced only by pathogenic treponemes

Rabbit antisera to Leptospira interrogans, Borrelia hermsii, and Treponema phagedenis biotype Reiter, reactive to shared spirochetal antigens, failed to enhance phagocytosis of Treponema pallidum by macrophages, while immunoglobulin G to Treponema pallidum subsp. pertenue and Treponema paraluiscuniculi promoted phagocytosis. Opsonic antibodies are directed to pathogen-restricted, not shared spirochetal, antigens.

Antimicrobial activity of rabbit leukocyte defensins against Treponema pallidum subsp. pallidum

Defensins, which are peptides with broad antimicrobial activity, are major constituents of rabbit neutrophils and certain macrophages. We tested six rabbit defensins, NP-1, NP-2, NP-3a, NP-3b, NP-4, and NP-5, for activity against Treponema pallidum subsp. pallidum. Mixtures of T. pallidum and defensin in 10% normal rabbit serum (NRS) or heat-inactivated NRS (HI-NRS) were incubated anaerobically for various time periods ranging between 0 and 16 h and then examined by dark-field microscopy for treponemal motility or inoculated intradermally into rabbits to assess treponemal virulence. Immobilization of T. pallidum by NP-1 (400 micrograms/ml) occurred after 4 and 8 h of coincubation in mixtures containing NRS and HI-NRS, respectively. Similarly, neutralization of T. pallidum by NP-1 occurred more rapidly and was complete when incubations were performed in NRS as compared with that in HI-NRS. Endpoint titration confirmed the augmentation of NP-1 antitreponemal activity by heat-labile serum factors; NP-1 showed neutralizing activity at 4 micrograms/ml (about 1 microM) in NRS and at 40 micrograms/ml in HI-NRS. When NP-1 was tested in serum that was deficient in C6, the T. pallidum neutralizing activity of NP-1 was reduced to levels slightly greater than that observed in HI-NRS. NP-1 that had been reduced and alkylated was inactive against T. pallidum. When NP-2, NP-3a, NP-3b, NP-4, and NP-5 were tested at 400 micrograms/ml, all exerted potent treponemicidal activity, manifested by abrogation or delayed development of cutaneous lesions relative to that of controls. These data suggest that defensins may equip certain macrophages and neutrophils to participate in host defense against T. pallidum, that the direct activity of defensins against T. pallidum is enhanced by heat-labile serum factors (presumably complement), and that conformational factors influence the biological activity of the defensin molecule.

Factors in virulence expression and their role in periodontal disease pathogenesis

The classic progression of the development of periodontitis with its associated formation of an inflammatory lesion is characterized by a highly reproducible microbiological progression of a Gram-positive microbiota to a highly pathogenic Gram-negative one. While this Gram-negative microbiota is estimated to consist of at least 300 different microbial species, it appears to consist of a very limited number of microbial species that are involved in the destruction of periodontal diseases. Among these "putative periodontopathic species" are members of the genera Porphyromonas, Bacteroides, Fusobacterium, Wolinella, Actinobacillus, Capnocytophaga, and Eikenella. While members of the genera Actinomyces and Streptococcus may not be directly involved in the microbial progression, these species do appear to be essential to the construction of the network of microbial species that comprise both the subgingival plaque matrix. The temporal fluctuation (emergence/disappearance) of members of this microbiota from the developing lesion appears to depend upon the physical interaction of the periodontal pocket inhabitants, as well as the utilization of the metabolic end-products of the respective species intimately involved in the disease progression. A concerted action of the end-products of prokaryotic metabolism and the destruction of host tissues through the action of a large number of excreted proteolytic enzymes from several of these periodontopathogens contribute directly to the periodontal disease process.(ABSTRACT TRUNCATED AT 250 WORDS)