Relationship of cell surface morphology and composition of Streptococcus salivarius K+ to adherence and hydrophobicity

Oral streptococci: modulators of health and disease

Streptococci are primary colonizers of the oral cavity where they are ubiquitously present and an integral part of the commensal oral biofilm microflora. The role oral streptococci play in the interaction with the host is ambivalent. On the one hand, they function as gatekeepers of homeostasis and are a prerequisite for the maintenance of oral health - they shape the oral microbiota, modulate the immune system to enable bacterial survival, and antagonize pathogenic species. On the other hand, also recognized pathogens, such as oral Streptococcus mutans and Streptococcus sobrinus, which trigger the onset of dental caries belong to the genus Streptococcus. In the context of periodontitis, oral streptococci as excellent initial biofilm formers have an accessory function, enabling late biofilm colonizers to inhabit gingival pockets and cause disease. The pathogenic potential of oral streptococci fully unfolds when their dissemination into the bloodstream occurs; streptococcal infection can cause extra-oral diseases, such as infective endocarditis and hemorrhagic stroke. In this review, the taxonomic diversity of oral streptococci, their role and prevalence in the oral cavity and their contribution to oral health and disease will be discussed, focusing on the virulence factors these species employ for interactions at the host interface.

Identification of a functional capsule locus in Streptococcus mitis

The polysaccharide capsule of Streptococcus pneumoniae is a hallmark for virulence in humans. In its close relative Streptococcus mitis, a common human commensal, analysis of the sequenced genomes of six strains revealed the presence of a putative capsule locus in four of them. We constructed an isogenic S. mitis mutant from the type strain that lacked the 19 open reading frames in the capsule locus (Δcps mutant), using a deletion strategy similar to previous capsule functional studies in S. pneumoniae. Transmission electron microscopy and atomic force microscopy revealed a capsule-like structure in the S. mitis type strain that was absent or reduced in the Δcps mutant. Since S. mitis are predominant oral colonizers of tooth surfaces, we addressed the relevance of the capsule locus for the S. mitis overall surface properties, autoaggregation and biofilm formation. The capsule deletion resulted in a mutant with approximately two-fold increase in hydrophobicity. Binding to the Stains-all cationic dye was reduced by 40%, suggesting a reduction in the overall negative surface charge of the mutant. The mutant exhibited also increased autoaggregation in coaggregation buffer, and up to six-fold increase in biofilm levels. The results suggested that the capsule locus is associated with production of a capsule-like structure in S. mitis and indicated that the S. mitis capsule-like structure may confer surface attributes similar to those associated with the capsule in S. pneumoniae.

Streptococcus adherence and colonization

Streptococci readily colonize mucosal tissues in the nasopharynx; the respiratory, gastrointestinal, and genitourinary tracts; and the skin. Each ecological niche presents a series of challenges to successful colonization with which streptococci have to contend. Some species exist in equilibrium with their host, neither stimulating nor submitting to immune defenses mounted against them. Most are either opportunistic or true pathogens responsible for diseases such as pharyngitis, tooth decay, necrotizing fasciitis, infective endocarditis, and meningitis. Part of the success of streptococci as colonizers is attributable to the spectrum of proteins expressed on their surfaces. Adhesins enable interactions with salivary, serum, and extracellular matrix components; host cells; and other microbes. This is the essential first step to colonization, the development of complex communities, and possible invasion of host tissues. The majority of streptococcal adhesins are anchored to the cell wall via a C-terminal LPxTz motif. Other proteins may be surface anchored through N-terminal lipid modifications, while the mechanism of cell wall associations for others remains unclear. Collectively, these surface-bound proteins provide Streptococcus species with a "coat of many colors," enabling multiple intimate contacts and interplays between the bacterial cell and the host. In vitro and in vivo studies have demonstrated direct roles for many streptococcal adhesins as colonization or virulence factors, making them attractive targets for therapeutic and preventive strategies against streptococcal infections. There is, therefore, much focus on applying increasingly advanced molecular techniques to determine the precise structures and functions of these proteins, and their regulatory pathways, so that more targeted approaches can be developed.

Role of Hydrophobicity in Adhesion of the Dissimilatory Fe(III)-Reducing Bacterium Shewanella alga to Amorphous Fe(III) Oxide

The mechanisms by which the dissimilatory Fe(III)-reducing bacterium Shewanella alga adheres to amorphous Fe(III) oxide were examined through comparative analysis of S. alga BrY and an adhesion-deficient strain of this species, S. alga RAD20. Approximately 100% of S. alga BrY cells typically adhered to amorphous Fe(III) oxide, while less than 50% of S. alga RAD20 cells adhered. Bulk chemical analysis, isoelectric point analysis, and cell surface analysis by time-of-flight secondary-ion mass spectrometry and electron spectroscopy for chemical analysis demonstrated that the surfaces of S. alga BrY cells were predominantly protein but that the surfaces of S. alga RAD20 cells were predominantly exopolysaccharide. Physicochemical analyses and hydrophobic interaction assays demonstrated that S. alga BrY cells were more hydrophobic than S. alga RAD20 cells. This study represents the first quantitative analysis of the adhesion of a dissimilatory Fe(III)-reducing bacterium to amorphous Fe(III) oxide, and the results collectively suggest that hydrophobic interactions are a factor in controlling the adhesion of this bacterium to amorphous Fe(III) oxide. Despite having a reduced ability to adhere, S. alga RAD20 reduced Fe(III) oxide at a rate identical to that of S. alga BrY. This result contrasts with results of previous studies by demonstrating that irreversible cell adhesion is not requisite for microbial reduction of amorphous Fe(III) oxide. These results suggest that the interaction between dissimilatory Fe(III)-reducing bacteria and amorphous Fe(III) oxide is more complex than previously believed.

Interactions between Campylobacter jejuni and lipids

We previously showed that motility plays several key roles in Campylobacter jejuni pathogenesis, including increasing the efficiency of C. jejuni attachment to host epithelial cells. To further characterize C. jejuni attachment, we first examined the role of carbohydrates. Experiments with Chinese hamster ovary (CHO) cell mutants with defined defects in complex carbohydrate biosynthesis revealed that oligosaccharide sequences probably play a subordinate role in C. jejuni attachment to eukaryotic cells. Simple sugars such as mannose, fucose, glucose, N-acetylglucosamine, maltose, and galactose also did not significantly alter the binding of C. jejuni to CHO cells. Thin-layer chromatography overlay analysis with lipids extracted from CHO cells suggested that C. jejuni binds to lipids. Lipid binding was further investigated using a receptor-based enzyme-linked immunosorbent assay. Hydrophobic interactions were determined to play a minor role in binding, since tetramethylurea, a strong inhibitor of hydrophobic interactions, did not significantly decrease binding between C. jejuni and lipids. The interaction was dissected further by comparing the binding of C. jejuni to lipids and their derivatives. The results showed that binding was greatest to the entire lipid structure and decreased in affinity when portions of the lipid were removed. Thin-layer chromatography overlay analysis showed that lipids with unsaturated fatty acids were bound with the highest affinity. Our results suggest that C. jejuni may interact with lipids in host cell membranes. However, lipids only partially inhibited C. jejuni binding to CHO cells, suggesting that multiple interactions occur between the bacteria and host cells.

Inhibition of saliva-induced oral streptococcal aggregation by blood group glycoproteins

The inhibition of saliva-induced oral streptococcal aggregation with anti-sera (anti-A, anti-B, anti-AB and anti-B treated with galactose), normal human serum (NHS), blood group-specific lectins (UEA-I, HBA, GPA, BSI-B4, GS-I), non-specific blood group lectins (MPA, SBA) and carbohydrates (galactose, N-acetylgalactosamine, L-fucose) was studied. Streptococcal species and strains included S. mutans 318, S. mutans 10449, S. mutans NG-8, S. salivarius and S. cricetus HS-6. The saliva was obtained from three subjects with secretor status (2 blood group B persons, 1 blood group A person). The data obtained from experiments performed with S. mutans 10449 and S. mutans NG-8 suggest the involvement of the H-antigenic determinant in the aggregation mechanism of the first strain and of the group B determinant for the second strain. The aggregation of S. salivarius only by B saliva might be related to a galactose-specific lectin on this strain and to some properties of its cell surface (hydrophobicity and the fibrillar surface layer). S. cricetus HS-6 aggregation was inhibited in different degrees by all the inhibitors used. The results demonstrate that interactions between oral streptococci and salivary components depend on the strain and species and on the individual saliva samples.

Proteins found within porcine respiratory tract secretions bind lipopolysaccharides of Actinobacillus pleuropneumoniae

Affinity for porcine respiratory tract secretions was found in some isolates of Actinobacillus pleuropneumoniae and involved lipopolysaccharides (LPS) (M. Bélanger, S. Rioux, B. Foiry, and M. Jacques, FEMS Microbiol. Lett. 97:119-126, 1992). In the present study, the affinity for a crude preparation of porcine respiratory tract mucus of isolates of the Pasteurellaceae family, i.e., Actinobacillus, Haemophilus, and Pasteurella spp., and of some unrelated gram-negative bacteria was examined. Affinity for crude porcine respiratory tract mucus was not a property shared by all Pasteurellaceae isolates tested. Furthermore, affinity for the porcine crude mucus preparation was not unique to the Pasteurellaceae group and did not seem to be restricted to bacteria originating from pigs. Different surface properties of A. pleuropneumoniae isolates in relation to their adherence to crude mucus were examined. The capsular layer seemed to mask the adhesin and interfered with adherence to crude mucus. Two poorly capsulated isolates, which had a more hydrophobic surface and bound Congo red, were also heavily labeled by gold particles coated with polymyxin, which is known to interact with the lipid A-core region of LPS, and adhered strongly to respiratory tract secretions. Tetramethylurea, charged polymers, divalent cations, chelators, monosaccharides and amino sugars, or lectins were unable to inhibit adherence of A. pleuropneumoniae to the crude mucus preparation. To identify the receptor(s) recognized by the lipopolysaccharidic adhesin of A. pleuropneumoniae, affinity chromatography was used. Two bands, which were proteinaceous in nature, of 10 and 11 kDa were recovered. Our results suggest that two low-molecular-mass proteins present in porcine respiratory tract secretions bind A. pleuropneumoniae LPS.

Interaction of the salivary glycoprotein EP-GP with the bacterium Streptococcus salivarius HB

The interaction of the human salivary glycoprotein EP-GP with a number of oral bacterial species, following incubation with human whole saliva, has been investigated. EP-GP could be detected with a specific monoclonal antibody, by means of ELISA or by electrophoresis in combination with Western Transfer. The results indicated that EP-GP is bound only by Streptococcus salivarius, and not by the other tested strains of bacteria, Actinomyces viscosus, A. naeslundii, Actinobacillus actinomycetemcomitans, Bacteroides fragilis, S. gordonii, S. oralis, S. sanguis, S. mitis, S. mutans, S. sobrinus, S. rattus, S. constellatus, and S. anginosus. Binding of EP-GP to S. salivarius is mediated by a protein-protein interaction, which was found to be pH-dependent with a maximum binding between pH 5 and 6. For further characterization of the binding of EP-GP to S. salivarius, four mutants were tested, each of them lacking different cell wall antigens. EP-GP was bound to all mutants in amounts comparable with the wildtype, in spite of the different surface antigen compositions. We were able to identify a 27-kD EP-GP binding protein, by extraction of S. salivarius-cell wall antigens and electrophoretic techniques. In addition to EP-GP, S. salivarius also bound two other salivary proteins, namely, secretory IgA and low-molecular-weight mucin (MG-2).

Binding of nonmucoid Pseudomonas aeruginosa to normal human intestinal mucin and respiratory mucin from patients with cystic fibrosis

Lung infections due to Pseudomonas aeruginosa and Pseudomonas cepacia are common in patients with cystic fibrosis. Initial colonization is due to nonmucoid P. aeruginosa, while later mucoid variants emerge and are associated with chronic infection. P. cepacia colonization tends to be more prevalent in older patients. The present study was conducted to discover whether highly purified mucins (from cystic fibrosis sputum and control intestinal secretions) exhibited specific binding of nonmucoid P. aeruginosa. In vitro solid phase microtiter binding assays (with or without a blocking agent) as well as solution phase assays were conducted. Bacteria bound to both mucins via bacterial pili, but no differences in binding capacity were noted between the mucins. Unlike P. cepacia (described in the accompanying manuscript) there was also no preferential binding of P. aeruginosa to mucins versus bovine serum albumin, casein, gelatin, or a host of structurally unrelated proteins and glycoproteins. Carbohydrate hapten inhibition studies did not suggest the existence of specific mucin carbohydrate receptors for P. aeruginosa. In solid phase assays a low concentration (0.05 M) of tetramethylurea abolished P. aeruginosa bacterial binding to both mucins as well as to BSA, whereas in solution phase assays mucin binding to bacteria was not completely disrupted by tetramethylurea. Specific monoclonal antipilus antibodies did not inhibit binding to a greater extent than did Fab fragments of normal mouse IgG. Binding of strains PAO1 and PAK (and isolated PAK pili) to buccal epithelial cells was not influenced by the presence of mucin in binding assay mixtures. Our findings do not support the widely held notion that specific mucin receptors are responsible for the attachment of P. aeruginosa pili, nor do they support the idea that there is a competitive interference by mucins of bacterial binding to respiratory cells. In patients with cystic fibrosis, it would seem unlikely therefore that initial colonization of the lungs by P. aeruginosa is due to a 'selective tropism' of these bacteria for respiratory mucin.

The effects of three non-antibiotic, antimicrobial agents on the surface hydrophobicity of certain micro-organisms evaluated by different methods

The effects of three non-antibiotic, antimicrobial agents (taurolidine, chlorhexidine acetate and providone-iodine) on the surface hydrophobicity of the clinical strains Escherichia coli, Staphylococcus saprophyticus, Staphylococcus epidermidis and Candida albicans were examined. Three recognized techniques for hydrophobicity measurements, Bacterial Adherence to Hydrocarbons (BATH), the Salt Aggregation Test (SAT) and Hydrophobic Interaction Chromatography (HIC) were compared. At concentrations reported to interfere with microbial-epithelial cell adherence, all three agents altered the cell surface hydrophobicity. However, these effects failed to exhibit a uniform relationship. Generally, taurolidine and povidone-iodine treatments decreased the hydrophobicity of the strains examined whereas chlorhexidine acetate effects depended upon the micro-organism treated. Subsequently, the exact contribution of altered cell surface hydrophobicity to the reported microbial anti-adherence effects is unclear. Comparison of the three techniques revealed a better correlation between the results obtained with the BATH test and HIC than the results obtained with the BATH and SAT or SAT and HIC. However, these differences may be due to the inaccuracy associated with the visual assessment of results employed by the SAT.

In vitro surface properties of the newly recognized gastric pathogen Helicobacter pylori

There appears to be a particular association between Helicobacter pylori and the gastric antrum, but the mechanisms by which the organism adheres to and colonizes the gastric mucosa are unclear. Surface hydrophobicity and surface charge mediate the adherence of other bacterial pathogens to mucosal epithelial cell surfaces. Therefore, in this study we characterized both the surface hydrophobicity and the surface charge of 10 H. pylori strains grown in broth culture. Four complementary methods were used to determine hydrophobicity: hydrophobic interaction chromatography, the salt aggregation test, comparison of bacterial adherence to polystyrene with adherence to sulfonated polystyrene, and measurement of contact angle with droplets of water. Three of the methods (salt aggregation test, adherence to polystyrene, and contact angles) indicated that each of the 10 strains expressed a relatively hydrophilic cell surface. In contrast, hydrophobic interaction chromatography determinations with both phenyl- and octyl-Sepharose suggested that the H. pylori strains were relatively hydrophobic. However, tetramethyl urea (0.4 M) did not reduce the binding of H. pylori to phenyl-Sepharose columns. DEAE-cellulose ion-exchange chromatography showed that each of the 10 strains of H. pylori had a surface which, overall, was highly negatively charged. We conclude that H. pylori expresses an overall relatively hydrophilic and negatively charged surface in vitro.

Saliva-induced aggregation of oral streptococci and the influence of blood group reactive substances

Aggregation of strains of Streptococcus rattus, Strep. mutans and Strep. salivarius by saliva from individuals of blood groups A, B and O was investigated. Blood group A salivas had a significantly higher aggregation activity with Strep. rattus than blood group B salivas (P less than 0.05). However, Strep. mutans and Strep. salivarius were better aggregated by blood group B saliva and this was significant for Strep. mutans (P less than 0.05). For all three strains, the variance within blood group O was too large to give significant differences with either blood group A or B. The blood group A-specific carbohydrate, N-acetyl-D-galactosamine, inhibited aggregation of Strep. rattus, but not of the other strains. The blood group B-specific carbohydrate, D-galactose, inhibited aggregation of Strep. mutans but not of Strep. rattus or Strep. salivarius. L-Fucose, specific for blood group O failed to inhibit aggregation of any of the three strains. These findings suggest that blood group-specific substances may be involved in bacterial aggregation.

Bacterial cell surface hydrophobicity properties in the mediation of in vitro adhesion by the rabbit enteric pathogen Escherichia coli strain RDEC-1

The role of hydrophobicity in the attachment of enteropathogens to gastrointestinal mucosa is controversial. In vitro binding of Escherichia coli RDEC-1 to rabbit intestine is dependent on the expression of pili. We examined in vitro adherence of piliated RDEC-1 after altering either the hydrophobicity of the organisms, the hydrophobicity of the substrate for attachment, or the surface tension of the suspending liquid. Hydrophobicity of RDEC-1 was determined using four complementary methods. In each assay piliated RDEC-1 demonstrated relatively more hydrophobic properties compared with both organisms grown to suppress pilus expression and a mutant that cannot express mannose-resistant pili. When piliated RDEC-1 were pretreated with tetramethyl urea to disrupt hydrophobic bonds surface hydrophobicity decreased. Concurrently, bacterial adherence to rabbit ileal microvillus membranes, mucus and mucin was reduced. Binding of piliated organisms to hydrophobic surfaces was significantly higher compared to both nonpiliated bacteria and the adherence of piliated RDEC-1 to relatively hydrophilic surfaces. Addition of propanol reduced the surface tension of the suspending liquid, and decreased adhesion of piliated RDEC-1 to polystyrene by 80%. Conversely, adherence of piliated organisms to a hydrophilic surface increased 12-fold after lowering the surface tension of the suspending liquid. We conclude that hydrophobic properties have a role in mediating in vitro adherence of this E. coli enteric pathogen.

Aggregation of 27 oral bacteria by human whole saliva. Influence of culture medium, calcium, and bacterial cell concentration, and interference by autoaggregation

Twenty-seven oral strains of the genera Actinomyces (5), Bacteroides (3), and Streptococcus (19) were tested for aggregation by human whole saliva, as well as the effect of culture medium, Ca-ions, and bacteria concentration thereupon. Of the media tested, GF-broth gave rise to less interference by autoaggregation or higher aggregation titers than BHI and TSB, and was used throughout this study. In most cases, Ca-ions (1 mM) only enhanced the rate of induced aggregation, whereas raising the bacteria concentration increased the rate of both induced- and autoaggregation. The final titers, ranging from 1-64, were hardly affected by these parameters, except those of S. rattus HG 59 and S. mutans HG 199, which were respectively increased and decreased by Ca-ions. Saliva-induced aggregation was observed for 21 strains of A. viscosus, A. naeslundii, A. israelii, B. gingivalis, B. intermedius, S. cricetus, S. mutans, S. rattus, S. sanguis, and S. sobrinus, mostly within 15 min to 3 h. Seventeen of these strains also showed autoaggregation, usually well after the onset of induced aggregation. Any potential induced aggregation of B. gingivalis HG 91 was always masked by autoaggregation, as well as that of the S. mutans strains under a particular set of conditions. The aggregation rate and titer varied considerably in a mutually unrelated and strain-dependent way. These microtiterplate data were matched by the 5 spectrophotometric patterns observed for saliva-bacterial interaction, which moreover, gave the better differentiation between induced and autoaggregation. In conclusion, most strains tested can show rapid saliva-induced aggregation in a strain-dependent way, yet strongly affected by the experimental conditions and interference from autoaggregation.

Surface properties of Streptococcus salivarius HB and nonfibrillar mutants: measurement of zeta potential and elemental composition with X-ray photoelectron spectroscopy

To characterize the functional cell surface, the zeta potentials and elemental surface composition of Streptococcus salivarius HB and a range of mutants with known molecular surface structures were determined. Zeta potentials of fully hydrated cells were measured as a function of pH in dilute potassium phosphate solutions, yielding isoelectric points of the strains. Elemental composition (O, C, N, and P) of the outer 2 to 5 nm of the freeze-dried cell surfaces were measured by X-ray photoelectron spectroscopy. An increasing loss of proteinaceous fibrillar surface antigens of the mutants was found to be accompanied by a progressive decrease in the N/C ratio from 0.104 in the parent strain HB to 0.053 in mutant HBC12. Simultaneously, the value of the isoelectric point shifted from 3.0 to 1.3. In a previous study (A.H. Weerkamp, H.C. van der Mei, and J. W. Slot, Infect. Immun. 55:438-455, 1987) on the cell surfaces of the same strains, it was shown that removal of fibrils led to increased exposure of (lipo)teichoic acid at the surface, which explains the low isoelectric point caused by the low pKa of the phosphate groups.