An appraisal of the virulence factors associated with streptococcal endocarditis

Streptococcus gordonii FSS2 Challisin affects fibrin clot formation by digestion of the αC region and cleavage of the N-terminal region of the Bβ chains of fibrinogen

Bacteria within endocarditis vegetations are encased in fibrin matrix that is resistant to resolution. We have previously shown that FSS2 Challisin, a serine protease from Streptococcus gordonii, is able to hydrolyse the Aα and Bβ chains of fibrinogen and has potent angiotensin converting enzyme (ACE) activity. The alteration in the structure of fibrin formed from FSS2 Challisin-degraded fibrinogen may therefore contribute to the resistant fibrin matrix. To this end, we have investigated the specific interactions of FSS2 Challisin with fibrinogen. FSS2 Challisin extensively degrades the αC region of fibrinogen Aα chains, hydrolysing both the αC-domain and αC-connnector. Additionally, the N-terminal region of the Bβ chains is cleaved twice, at Leu19 and Ser28, removing the B fibrinopeptides and ‘B’ knobs. Substrate analysis indicates FSS2 Challisin has specific requirement for proline two residues before the cleavage point and a neutral or basic un-branched amino acid preceding the cleavage point. Fibrin formation by thrombin was modified and the initiation of fibrinolysis extended, in FSS2 Challisin-treated plasma clots. Digestion of fibrinogen by FSS2 Challisin prior to thrombin action increased fiber density and fiber branch point density. The velocity of fibrinolysis was significantly slower for fibrin formed from FSS2 Challisin-treated fibrinogen but was faster when data was normalised for the increased fibrin density. Thromboelastography of whole blood treated with FSS2 Challisin indicated reduced clot coagulation time and increased shear resistance. Combined ACE and fibrinogenase activities of FSS2 Challisin suggest a pro-coagulant effect of this virulence factor which is conserved in the viridans streptococci.

Streptococcus tigurinus is highly virulent in a rat model of experimental endocarditis

Streptococcus tigurinus is responsible for systemic infections in humans including infective endocarditis. We investigated whether the invasive trait of S. tigurinus in humans correlated with an increased ability to induce IE in rats. Rats with catheter-induced aortic vegetations were inoculated with 10⁴ CFU/ml of either of four S. tigurinus strains AZ_3a(T), AZ_4a, AZ_8 and AZ_14, isolated from patients with infective endocarditis or with the well known IE pathogen Streptococcus gordonii (Challis). Aortic infection was assessed after 24 h. S. tigurinus AZ_3a(T), AZ_4a and AZ_14 produced endocarditis in ≥80% of rats whereas S. gordonii produced endocarditis in only 33% of animals (P<0.05). S. tigurinus AZ_8 caused vegetation infection in 56% of the animals. The capacity of S. tigurinus to induce aortic infection was not related to their ability to bind extracellular matrix proteins (fibrinogen, fibronectin or collagen) or to trigger platelet aggregation. However, all S. tigurinus isolates showed an enhanced resistance to phagocytosis by macrophages and two of them had an increased ability to enter endothelial cells, key attributes of invasive streptococcal species.

Hsa, an adhesin of Streptococcus gordonii DL1, binds to alpha2-3-linked sialic acid on glycophorin A of the erythrocyte membrane

Bacterial recognition of host sialic acid-containing receptors plays an important role in microbial colonization of the human oral cavity. The aggregation of human platelets by Streptococcus gordonii DL1 is implicated in the pathogenesis of infective endocarditis. In addition, we consider that hemagglutination of this organism may act as an additive factor to increase the severity of this disease. We previously reported that this interaction requires the bacterial expression of a 203-kDa protein (Hsa), which has sialic acid-binding activity. In the present study, we confirmed that erythrocyte surface sialoglycoproteins are the receptors for Hsa. We examined the effects of proteinase K, chymotrypsin, phospholipase C, and alpha(2-3) or alpha(2-3, 6, 8) neuraminidase on hemagglutination activity and found that the interaction occurs between Hsa and alpha2-3-linked sialic acid-containing proteins of erythrocytes. We expressed recombinant NR2, which is the putative binding domain of Hsa, fused with GST in Escherichia coli BL21. Dot-blot analysis demonstrated that GST-HsaNR2 binds both glycophorin A (GPA) and band 3. Moreover, GPA and a small amount of band 3 were detected by GST pull-down assays. These findings indicate that S. gordonii Hsa specifically binds to GPA and band 3, alpha2-3-linked sialic acid membrane glycoproteins.

New concepts in the pathophysiology of infective endocarditis

Endocarditis pathogens colonize valves with pre-existing sterile vegetations or valves with minimal endothelial lesions. Inflamed endothelia produce cytokines, integrins, and tissue factor, which in turn attract fibronectin, monocytes, and platelets. Bacteria attaching to such structures further activate the cascade, becoming embedded and protected from host defenses. Staphylococcus aureus also actively invade the endothelium, causing apoptosis and endothelial damage. Knowledge of this interplay identifies host factors as potential therapeutic targets. Blocking infection by modulating host factors might be opportune because host factors are conserved. In contrast, interfering with bacterial virulence factors might be more complicated because they vary among different bacteria.

Characterization of an extracellular dipeptidase from Streptococcus gordonii FSS2

PepV, a dipeptidase found in culture fluids of Streptococcus gordonii FSS2, was purified and characterized, and its gene was cloned. PepV is a monomeric metalloenzyme of approximately 55 kDa that preferentially degrades hydrophobic dipeptides. The gene encodes a polypeptide of 467 amino acids, with a theoretical molecular mass of 51,114 Da and a calculated pI of 4.8. The S. gordonii PepV gene is homologous to the PepV gene family from Lactobacillus and Lactococcus spp.

Invasion and killing of human endothelial cells by viridans group streptococci

Colonization of the cardiovascular endothelium by viridans group streptococci can result in infective endocarditis and possibly atherosclerosis; however, the mechanisms of pathogenesis are poorly understood. We investigated the ability of selected oral streptococci to infect monolayers of human umbilical vein endothelial cells (HUVEC) in 50% human plasma and to produce cytotoxicity. Planktonic Streptococcus gordonii CH1 killed HUVEC over a 5-h period by peroxidogenesis (alpha-hemolysin) and by acidogenesis but not by production of protein exotoxins. HUVEC were protected fully by addition of supplemental buffers and bovine liver catalase to the culture medium. Streptococci were also found to invade HUVEC by an endocytic mechanism that was dependent on polymerization of actin microfilaments and on a functional cytoskeleton, as indicated by inhibition with cytochalasin D and nocodazole. Electron microscopy revealed streptococci attached to HUVEC surfaces via numerous fibrillar structures and bacteria in membrane-encased cytoplasmic vacuoles. Following invasion by S. gordonii CH1, HUVEC monolayers showed 63% cell lysis over 4 h, releasing 64% of the total intracellular bacteria into the culture medium; however, the bacteria did not multiply during this time. The ability to invade HUVEC was exhibited by selected strains of S. gordonii, S. sanguis, S. mutans, S. mitis, and S. oralis but only weakly by S. salivarius. Comparison of isogenic pairs of S. gordonii revealed a requirement for several surface proteins for maximum host cell invasion: glucosyltransferase, the sialic acid-binding protein Hsa, and the hydrophobicity/coaggregation proteins CshA and CshB. Deletion of genes for the antigen I/II adhesins, SspA and SspB, did not affect invasion. We hypothesize that peroxidogenesis and invasion of the cardiovascular endothelium by viridans group streptococci are integral events in the pathogenesis of infective endocarditis and atherosclerosis.

Pathogenesis of streptococcal and staphylococcal endocarditis

Although streptococcal and S. aureus IE share the same primary site of infection, their pathogenesis and clinical evolution present several major differences. Streptococci adhere to cardiac valves with pre-existing endothelial lesions. In contrast, S. aureus can colonize either damaged endothelium or invade physically intact endothelial cells. These interactions are mediated by multiple surface adhesins, some of which have been only partially characterized. Streptococci produce surface glucans (gtf and ftf), ECM adhesins (e.g., fibronectin-binding proteins, FimA), and platelet aggregating factors (phase I and phase II antigens, pblA, pblB, and pblT), all of which have been.

Extracellular arginine aminopeptidase from Streptococcus gordonii FSS2

Streptococcus gordonii is a primary etiological agent in the development of subacute bacterial endocarditis (SBE), producing thrombus formation and tissue damage on the surfaces of heart valves. This is ironic, considering its normal role as a benign inhabitant of the oral microflora. However, strain FSS2 of S. gordonii has been found to produce several extracellular aminopeptidase- and fibrinogen-degrading activities during growth in a pH-controlled batch culture. In this report, we describe the purification, characterization, and partial cloning of a predicted serine class arginine aminopeptidase (RAP) with some cysteine class characteristics. Isolation of this enzyme by anion-exchange, gel filtration, and isoelectric focusing chromatography yielded a protein monomer of approximately 70 kDa, as shown by matrix-assisted laser desorption ionization, gel filtration, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis under denaturing conditions. Nested-PCR cloning enabled the isolation of a 324-bp-long DNA fragment encoding the 108-amino-acid N terminus of RAP. Culture activity profiles and N-terminal sequence analysis indicated the export of this protein from the cell surface. Homology was found with a putative dipeptidase from Streptococcus pyogenes and nonspecific dipeptidases from Lactobacillus helveticus and Lactococcus lactis. We believe that RAP may serve as a critical factor for arginine acquisition during nutrient stress in vivo and also in the proteolysis of host proteins and peptides during SBE pathology.

Novel extracellular x-prolyl dipeptidyl-peptidase (DPP) from Streptococcus gordonii FSS2: an emerging subfamily of viridans Streptococcal x-prolyl DPPs

Streptococcus gordonii is generally considered a benign inhabitant of the oral microflora, and yet it is a primary etiological agent in the development of subacute bacterial endocarditis (SBE), an inflammatory state that propagates thrombus formation and tissue damage on the surface of heart valves. Strain FSS2 produced several extracellular aminopeptidase and fibrinogen-degrading activities during growth in culture. In this report we describe the purification, characterization, and cloning of a serine class dipeptidyl-aminopeptidase, an x-prolyl dipeptidyl-peptidase (Sg-xPDPP, for S. gordonii x-prolyl dipeptidyl-peptidase), produced in a pH-controlled batch culture. Purification of this enzyme by anion exchange, gel filtration, and hydrophobic interaction chromatography yielded a protein monomer of approximately 85 kDa, as shown by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (PAGE) under denaturing conditions. However, under native conditions, the protein appeared to be a homodimer on the basis of gel filtration and PAGE. Kinetic studies indicated that purified enzyme had a unique and stringent x-prolyl specificity that is comparable to both the dipeptidyl-peptidase IV/CD26 and lactococcal x-prolyl dipeptidyl-peptidase families. Nested PCR cloning from an S. gordonii library enabled the isolation and sequence analysis of the full-length gene. A 759-amino-acid polypeptide with a theoretical molecular mass of 87,115 Da and a calculated pI of 5.6 was encoded by this open reading frame. Significant homology was found with the PepX gene family from Lactobacillus and Lactococcus spp. and putative x-prolyl dipeptidyl-peptidases from other streptococcal species. Sg-xPDPP may serve as a critical factor for the sustained bacterial growth in vivo and furthermore may aid in the proteolysis of host tissue that is commonly observed during SBE pathology.

Concepts for the prophylaxis of infective endocarditis in dentistry

Oral commensal microorganisms are commonly associated with the pathogenesis of infective endocarditis. Despite modern antimicrobial and surgical treatment, infective endocarditis continues to cause substantial morbidity and mortality. Although dentistry is no longer considered a major risk factor for infective endocarditis, it is current standard for practice that dental procedures likely to produce significant bacteraemia in patients who are susceptible to this disease be prophylactically covered with an antimicrobial agent. The concepts of antimicrobial prophylaxis prior to invasive dental procedures are outlined in this review, with particular reference to the latest recommendations of the Australian Dental Association.

Fibronectin attachment protein homologue mediates fibronectin binding by Mycobacterium avium subsp. paratuberculosis

Attachment of Mycobacterium avium subsp. paratuberculosis to host tissue and penetration of mucosal surfaces are pivotal events in the pathogenesis of Johne's disease. Fibronectin (FN) binding is required for attachment and internalization of several mycobacteria by epithelial cells in vitro. The objective of this study was to further characterize the FN binding activity of M. avium subsp. paratuberculosis. Although the bacteria bound FN poorly at pH above 7, brief acid pretreatment greatly enhanced FN binding within the pH range (3 to 10) studied. A 4.6-kbp fragment from an M. avium subsp. paratuberculosis genomic library was found to contain a 1,107-bp open reading frame that shows very high nucleotide sequence identity with that of the FN attachment protein (FAP) gene of M. avium subsp. avium. Pretreatment of FN with an FN-binding peptide from M. avium subsp. avium FAP abolished FN binding, indicating that M. avium subsp. paratuberculosis binds FN in a FAP-dependent manner. Pretreatment of M. avium subsp. paratuberculosis with anti-FAP immunoglobulin G did not abrogate FN binding; blocking occurred only when anti-FAP was added together with FN. FAP was detected by immunofluorescence only in lipid-extracted M. avium subsp. paratuberculosis. Western blotting and immunoelectron microscopy revealed that FAP is located near the interior of the cell envelope of M. avium subsp. paratuberculosis. The results indicate that a FAP homologue mediates the attachment of FN to M. avium subsp. paratuberculosis. Further, given the subcellular location of FAP, it is considered that this protein operates at the terminus of a coordinated FN binding system in the cell envelope of M. avium subsp. paratuberculosis.

pH-regulated secretion of a glyceraldehyde-3-phosphate dehydrogenase from Streptococcus gordonii FSS2: purification, characterization, and cloning of the gene encoding this enzyme

Streptococcus gordonii and other viridans streptococci (VS) are primary etiologic agents of infective endocarditis, despite being part of the normal oral microflora. Recently, a surface-bound glyceraldehyde-3-phosphate dehydrogenase (GAPDH) has been found on the cells of all tested streptococcal species, where it has been implicated as a virulence factor. In contrast, we observed that a soluble extracellular GAPDH was the major secreted protein from S. gordonii FSS2, an endocarditis strain. The biochemical properties and gene sequence of S. gordonii GAPDH are almost identical to those of other streptococcal GAPDHs. Growth at defined pHs showed that secretion of GAPDH is regulated by environmental pH. GAPDH was primarily surface-associated at growth pH 6.5 and shifted to > 90% secreted at growth pH 7.5. Others have identified S. gordonii promoters that are up-regulated by a pH shift similar to that experienced by organisms entering the blood stream (neutral) from the oral cavity (slightly acid). Analysis of our results suggests that secretion of GAPDH may be a similar adaptation by S. gordonii.

Environmental regulation of glycosidase and peptidase production by Streptococcus gordonii FSS2

The synthesis of cell-associated and secreted proteins by Streptococcus gordonii FSS2, an infective endocarditis (IE) isolate, was influenced by both environmental pH and carbon source. Controlling the pH at 7.5 in stirred batch cultures showed that cell-associated and secreted protein concentrations were increased during late exponential and stationary phase by 68% and 125%, respectively, compared with similar cultures without pH control. The expression of five glycosidase and eight peptidase activities were examined using fluorogen-labelled synthetic substrates. Enzyme activities were significantly down-regulated during exponential growth, increasing during stationary phase (P<0.01) whether the culture pH was controlled at pH 7.5 or allowed to fall naturally to pH 4.4. Culture-supernatant activities were significantly increased (P<0.05) when the pH was maintained at 6.0 or 7.5, indicating modulation of enzyme activity by pH. Growth under nitrogen-limitation/glucose-excess conditions resulted in a significant repression of cell-associated glycosidase activities (P<0.01), whilst in the supernatant, activities were generally reduced. The expression of peptidase activities in the culture supernatant did not significantly change. The results suggest a possible role for catabolite repression by glucose in regulating enzyme expression. When S. gordonii FSS2 was cultured with 50% (v/v) added heat-inactivated foetal bovine serum, several cell-associated enzyme activities increased initially but were then reduced as the culture time was extended to 116 h. Culture-supernatant enzyme activities (N-acetyl-beta-D-glucosaminidase, N-acetyl-beta-D-galactosaminidase, thrombin, Hageman factor, collagenase and chymotrypsin), however, were significantly increased (P<0.01) over the same time period. The findings indicated that most of the important glycosidases synthesized by S. gordonii FSS2 were down-regulated by acid growth conditions and may also be subject to catabolite repression by glucose but conversely may be up-regulated by growth in serum. These results may have implications for streptococcal growth in an IE vegetation and in the mouth between meals or during sleep.

Neil Goldsworthy and his legacy: Sydney's Institute of Dental Research

Dr. Neil Goldsworthy had a profound effect on the development of dental research in the biological sciences in Australia, including measures for the prevention of dental caries. He played a major role in the establishment of the Institute of Dental Research in Sydney in 1946 and was Director until his sudden death in 1960. Reviewing the Institute's activities provides the opportunity for his achievements to receive due recognition and to show how they influenced its subsequent development. As one who has been associated with the Institute for most of the last 50 years, it is an honor for me to undertake this task.

Experimental endocarditis induction and platelet aggregation by Streptococcus anginosus, Streptococcus constellatus and Streptococcus intermedius

A total of 18 'Streptococcus milleri' strains including the ATCC type strains of Streptococcus anginosus, Streptococcus constellatus and Streptococcus intermedius were compared with Streptococcus oralis ATCC10557 for their ability to induce infective endocarditis in catheterized rats. Three days after intravenous injection of 10(8) colony-forming units all 8 S. anginosus strains tested produced infective vegetations and bacteremia in almost all rats whereas 5 S. constellatus strains and the S. oralis strain produced infective vegetations and bacteremia less frequently and 5 S. intermedius strains only occasionally. The vegetations infected with the S. anginosus strains harbored microbial cells in significantly higher numbers that those with the other strains. No strong correlation was found between the endocardial infectivity and platelet-aggregating capacity of these strains. The platelet-aggregating strains were members of the Lancefield groups F and G or ungroupable but not of group A or C.

Role of phagocytosis in activation of the coagulation system in Streptococcus sanguis endocarditis

The formation of vegetations consisting of fibrin, cellular elements, humoral factors, and bacteria is the central event in the pathogenesis of bacterial endocarditis. Fibrin formation occurs on the vegetation, the coagulation system being activated locally via the expression of tissue factor (TF) on fibrin-adherent monocytes. This study was performed to assess the importance of phagocytosis of fibrin-adherent Streptococcus sanguis in the stimulation of TF expression on fibrin-adherent monocytes, as well as a role for "frustrated" phagocytosis. With the latter process, these cells are unable to remove bacteria from the fibrin surface but nonetheless might be activated to generate TF. We found that serum was not required for the stimulation of TF expression by fibrin-adherent monocytes in the presence of S. sanguis in an in vitro model for bacterial endocarditis. The bacterial adhesin dextran did not influence the TF activity (TFA) of fibrin-adherent monocytes: TFA was the same after stimulation with a dextran-positive streptococcus as with its dextran-negative mutant. Furthermore, dextran did not influence the TFA of endocardial vegetations, which was the same for vegetations isolated from rabbits infected either with dextran-positive S. sanguis or its dextran-negative mutant. These results do not support the hypothesis that in bacterial endocarditis (frustrated) phagocytosis significantly contributes to TF expression on vegetation-adherent monocytes. Fibronectin, however, although not influencing the fibrin binding of the streptococci, did enhance the TFA of monocytes in a concentration-dependent manner. We conclude that although streptococci do enhance expression of TFA on monocytes, phagocytosis and bacterial adhesins do not play a major role in this process. Stimulation of monocyte TFA may be more dependent on interactions between monocytes and the vegetational surface via fibronectin receptors, such as VLA 4 and VLA 5 (very late antigens 4 and 5).

Platelet-streptococcal interactions in endocarditis

Infective endocarditis is characterized by the formation of septic masses of platelets on the surfaces of heart valves and is most commonly caused by viridans streptococci. Streptococcal virulence in endocarditis involves factors that promote infectivity and pathogenicity. Adhesins and exopolysaccharide (glycocalyx) contribute to infectivity. Although many factors may contribute to pathogenicity, the platelet aggregation-associated protein (PAAP) of Streptococcus sanguis contributes directly to the development of experimental endocarditis. PAAP is synthesized as a rhamnose-rich glycoprotein of 115 kDa and contains a collagen-like platelet-interactive domain, pro-gly-glu-gln-gly-pro-lys. Expressed on the cell wall of platelet aggregation-inducing strains (Agg+) of S. sanguis, PAAP apparently interacts with a signal-transducing receptor complex on platelets, which includes a novel 175-kDa alpha 2-integrin-associated protein and a 65-kDa collagen-binding component. From available data, the role of PAAP in the pathogenesis of experimental endocarditis may be explained by a proposed mechanistic model. On injured heart valves, PAAP first enhances platelet accumulation into a fibrin-enmeshed thrombus (vegetation), within which S. sanguis colonizes. Colonizing bacteria must resist platelet microbicidal protein (PMPR). The aggregation of platelets on the heart valve may be potentiated by an ectoATPase expressed on the surface of the S. sanguis and platelet alpha-adrenoreceptors that respond to endogenous catecholamines. The expression of PAAP may be modified during infection. Collagen is exposed on damaged heart valves; fever (heat shock) occurs during endocarditis. In response to heat shock or collagen in vitro, PAAP expression is altered. After colonization, streptococcal exotoxin(s) may cause fever. Proteases and other enzymes from streptococci and host sources may directly destroy the heart valves. When PAAP is unexpressed or neutralized with specific antibodies, experimental endocarditis runs a milder course and vegetations are smaller. The data suggest strongly, therefore, that the role of PAAP may overlap the colonization function of putative adhesins such as FimA or SsaB. Finally, PAAP also contributes to the development of the characteristic septic mural thrombus (vegetation) of infective endocarditis and the signs of valvular pathology.

Modulation of glycosidase and protease activities by chemostat growth conditions in an endocarditis strain of Streptococcus sanguis

The effects of growth conditions on the properties of the endocarditis-producing oral bacterium Streptococcus sanguis FSS2 were studied. This strain produces a variety of proteases and glycosidases, including a thrombin-like activity that is a potential virulence factor for endocarditis. Cultures were grown with limiting glucose or galactose in chemostats over a range of dilution rates and pH levels, and the following activities were measured at pH 7.5: thrombin-like, Hageman factor-like, N-acetyl-beta-D-galactosaminidase, beta-D-glucosidase, and beta-D-galactosidase. At growth pH 6.5, specific activities generally decreased as the dilution rate increased from 0.05 to 0.40 h(-1). At a dilution rate of 0.1 h(-1), specific activities generally were highest at growth pH 6.5 and lower and approximately equal at growth pH 5.5 and 7.5. The major exception was the thrombin-like activity, for which the specific activity at growth pH 7.5 was approximately 5-fold higher than at growth pH 5.5. Hageman factor-like activity was apparently glucose catabolite repressible, as its activity was 3-fold higher in galactose cultures. The measured activities changed as functions of growth conditions and thus were modulated by environment. Environmental regulation of thrombin-like activity by pH is consistent with an activity that is less important on tooth surfaces than in tissues.

Pathogenic potential of lactobacilli

Lactobacilli are often considered to be commensal or beneficial participants in human microbial ecology and considerable research is being carried out into the effects of the use of lactobacilli as additives in both human and animal diets. However, lactobacilli also cause some human diseases (e.g. dental caries, rheumatic vascular disease, septicaemia and infective endocarditis (IE)), and have recently been identified as potential emerging pathogens in elderly and immunocompromised patients, particularly those receiving broad spectrum antibiotic therapy. The identification of potential pathogenic traits amongst lactobacilli will therefore facilitate the use of the organisms for probiotic purposes. The ability to aggregate human platelets is considered to be a possible pathogenic trait in the progression of IE. A comparison of bacterial cell surface properties amongst L. rhamnosus strains showed that platelets were aggregated by 5/5 IE strains and 8/16 laboratory strains. For the L. paracasei subsp. paracasei strains the respective numbers were 2/5 and 2/9. However two strains, morphological mutants of a non-aggregating strain, which had been re-isolated after passaging through rats were found to aggregate platelets. No loss of aggregating function occurred on extensive subculturing of IE strains. Aggregation also occurred with 11/14 strains for five other species, namely, Lactobacillus acidophilus, Lactobacillus fermentum, Lactobacillus oris, Lactobacillus plantarum and Lactobacillus salvivarius, with each species being represented indicating that the property is not uncommon in the genus. A comparison of IE and oral isolates of L. rhamnosus and L. paracasei subsp. paracasei and seven other Lactobacillus species, has shown that the binding of both fibronectin and fibrinogen by lactobacilli is greatly increased, up to 50 fold, when the pH is reduced from 7.0 to 5.0. Re-exposing the lactobacilli to a neutral pH environment releases most of the bound proteins, but the amount still remaining bound to the cell is several times more than is bound at neutral pH. Lactobacilli will also bind to the proteins that make up the extracellular matrix of endothelial cells. Lactobacilli bound significantly better to collagen types I and V than to types III and IV (p < 0.01). Further, strains isolated from IE cases, particularly L. rhamnosus strains, bound significantly better to types I and V than did 'normal' strains (p < 0.02). Type V collagen has been demonstrated at the sites of endothelial damage. Thus the binding of lactobacilli, particularly L. rhamnosus to these collagen types may be of importance in the early stages of colonization of the damaged heart valve.(ABSTRACT TRUNCATED AT 400 WORDS)