The Natural Surfactant Glycerol Monolaurate Significantly Reduces Development of Staphylococcus aureus and Enterococcus faecalis Biofilms.

BACKGROUND: Bacterial biofilms are involved in a large proportion of clinical infections, including device-related infections. Unfortunately, biofilm-associated bacteria are typically less susceptible to antibiotics, and infected devices must often be removed. On the basis of a recent observation that lipid-rich biofilm matrix material is present in early biofilm formation and may protect a population of bacteria from interacting with ordinarily diffusible small molecules, we hypothesized that surfactants may be useful in preventing biofilm development. METHODS: Experimental Staphylococcus aureus or Enterococcus faecalis biofilms were cultivated on surgical suture suspended in a growth medium supplemented with the natural surfactant glycerol monolaurate (GML) or with a component molecule, lauric acid. After 16 h incubation, the numbers of viable biofilm-associated bacteria were measured by standard microbiologic techniques and biofilm biomass was measured using the colorimetric crystal violet assay. RESULTS: Both GML and lauric acid were effective in inhibiting biofilm development as measured by decreased numbers of viable biofilm-associated bacteria as well as decreased biofilm biomass. Compared with lauric acid on a molar basis, GML represented a more effective inhibitor of biofilms formed by either S. aureus or E. faecalis. CONCLUSIONS: Because the natural surfactant GML inhibited biofilm development, resulting data were consistent with the hypothesis that lipids may play an important role in biofilm growth, implying that interfering with lipid formation may help control development of clinically relevant biofilms.

Antibacterial synergy of glycerol monolaurate and aminoglycosides in Staphylococcus aureus biofilms.

Glycerol monolaurate (GML) is a natural surfactant with antimicrobial properties. At ∼0.3 mM, both GML and its component lauric acid were bactericidal for antibiotic-resistant Staphylococcus aureus biofilms. With the use of MICs of antibiotics obtained from planktonic cells, GML and lauric acid acted synergistically with gentamicin and streptomycin, but not ampicillin or vancomycin, to eliminate detectable viable biofilm bacteria. Images of GML-treated biofilms suggested that GML may facilitate antibiotic interaction with matrix-embedded bacteria.

Aminoglycoside inhibition of Staphylococcus aureus biofilm formation is nutrient dependent.

Biofilms represent microbial communities, encased in a self-produced matrix or extracellular polymeric substance. Microbial biofilms are likely responsible for a large proportion of clinically significant infections and the multicellular nature of biofilm existence has been repeatedly associated with antibiotic resistance. Classical in vitro antibiotic-susceptibility testing utilizes artificial growth media and planktonic microbes, but this method may not account for the variability inherent in environments subject to biofilm growth in vivo. Experiments were designed to test the hypothesis that nutrient concentration can modulate the antibiotic susceptibility of Staphylococcus aureus biofilms. Developing S. aureus biofilms initiated on surgical sutures, and in selected experiments planktonic cultures, were incubated for 16 h in 66 % tryptic soy broth, 0.2 % glucose (1× TSBg), supplemented with bactericidal concentrations of gentamicin, streptomycin, ampicillin or vancomycin. In parallel experiments, antibiotics were added to growth medium diluted one-third (1/3× TSBg) or concentrated threefold (3× TSBg). Following incubation, viable bacteria were enumerated from planktonic cultures or suture sonicates, and biofilm biomass was assayed using spectrophotometry. Interestingly, bactericidal concentrations of gentamicin (5 µg gentamicin ml(-1)) and streptomycin (32 µg streptomycin ml(-1)) inhibited biofilm formation in samples incubated in 1/3× or 1× TSBg, but not in samples incubated in 3× TSBg. The nutrient dependence of aminoglycoside susceptibility is not only associated with biofilm formation, as planktonic cultures incubated in 3× TSBg in the presence of gentamicin also showed antibiotic resistance. These findings appeared specific for aminoglycosides because biofilm formation was inhibited in all three growth media supplemented with bactericidal concentrations of the cell wall-active antibiotics, ampicillin and vancomycin. Additional experiments showed that the ability of 3× TSBg to overcome the antibacterial effects of gentamicin was associated with decreased uptake of gentamicin by S. aureus. Uptake is known to be decreased at low pH, and the kinetic change in pH of growth medium from biofilms incubated in 5 µg gentamicin ml(-1) in the presence of 3× TSBg was decreased when compared with pH determinations from biofilms formed in 1/3× or 1× TSBg. These studies underscore the importance of environmental factors, including nutrient concentration and pH, on the antibiotic susceptibility of S. aureus planktonic and biofilm bacteria.

Anoxia inhibits biofilm development and modulates antibiotic activity.

BACKGROUND: Many infections involve bacterial biofilms that are notoriously antibiotic resistant. Unfortunately, the mechanism for this resistance is unclear. We tested the effect of oxygen concentration on development of Staphylococcus aureus biofilms, and on the ability of gentamicin and vancomycin to inhibit biofilm development. MATERIALS AND METHODS: To mimic catheter-associated biofilms, silastic coupons were inoculated with 10(7)S aureus and incubated either aerobically (∼21% O2) or anaerobically (10% CO2, 5% H2, 85% N2) for 16 h at 37°C with varying concentrations of gentamicin and vancomycin. Viable colony-forming units were quantified from sonicated biofilms, and the crystal violet assay quantified biofilm biomass. Metabolomic profiles probed biochemical differences between aerobic and anaerobic biofilms. RESULTS: Control biofilms (no antibiotic) cultivated aerobically contained 8.1-8.6 log10S aureus. Anaerobiasis inhibited biofilm development, quantified by viable bacterial numbers and biomass (P < 0.05). Bactericidal concentrations of gentamicin inhibited biofilm development in normoxia but not anoxia, likely because bacterial uptake of gentamicin is oxygen dependent. The inhibitory effect of vancomycin was more uniform aerobically and anaerobically, although at high bactericidal concentrations, vancomycin effectiveness was decreased under anoxia. There were notable differences in the metabolomic profiles of biofilms cultivated under normoxia versus anoxia. CONCLUSIONS: Compared with aerobic incubation, anaerobiasis resulted in decreased biofilm development, and metabolomics is a promising tool to identify key compounds involved in biofilm formation. The effectiveness of a specific antibiotic depended on its mode of action, as well as on the oxygen concentration in the environment.

Ultrastructure of a novel bacterial form located in Staphylococcus aureus in vitro and in vivo catheter-associated biofilms.

Bacterial biofilms are ubiquitous in nature, industry, and medicine, and understanding their development and cellular structure is critical in controlling the unwanted consequences of biofilm growth. Here, we report the ultrastructure of a novel bacterial form observed by scanning electron microscopy in the luminal vegetations of catheters from patients with active Staphylococcus aureus bacteremia. This novel structure had the general appearance of a normal staphylococcal cell but up to 10 to 15 times as large. Transmission electron microscopy indicated that these structures appeared as sacs enclosing multiple normal-sized (~0.6 µm) staphylococcal forms. Using in vitro cultivated biofilms, cytochemical studies using fluorescent reagents revealed that these structures were rich in lipids and appeared within 15 min after S. aureus inoculation onto clinically relevant abiotic surfaces. Because they appeared early in biofilm development, these novel bacterial forms may represent an unappreciated mechanism for biofilm surface adherence, and their prominent lipid expression levels could explain the perplexing increased antimicrobial resistance of biofilm-associated bacteria.

Interplay of antibiotics and bacterial inoculum on suture-associated biofilms.

BACKGROUND: Biofilms are often antibiotic resistant, and it is unclear if prophylactic antibiotics can effectively prevent biofilm formation. Experiments were designed to test the ability of high (bactericidal) concentrations of ampicillin (AMP), vancomycin (VAN), and oxacillin (OXA) to prevent formation of suture-associated biofilms initiated with low (10(4)) and high (10(7)) numbers of Staphylococcus aureus. MATERIALS AND METHODS: S. aureus biofilms were cultivated overnight on silk suture incubated in biofilm growth medium supplemented with bactericidal concentrations of AMP, VAN, or OXA. Standard microbiological methods were used to quantify total numbers of viable suture-associated S. aureus. Crystal violet staining followed by spectroscopy was used to quantify biofilm biomass, which includes bacterial cells plus matrix components. To observe the effects of antibiotics on the microscopic appearance of biofilm formation, biofilms were cultivated on glass slides, then stained with fluorescent dyes, and observed by confocal microscopy. RESULTS: In the presence of a relatively low inoculum (10(4)) of S. aureus cells, bactericidal concentrations of AMP, VAN, or OXA were effective in preventing development of suture-associated biofilms. However, similar concentrations of these antibiotics were typically ineffective in preventing biofilm development on sutures inoculated with 10(7)S. aureus, a concentration relevant to contaminated skin. Confocal microscopy confirmed that bactericidal concentrations of AMP, VAN, or OXA inhibited, but did not prevent, development of S. aureus biofilms. CONCLUSION: Bactericidal concentrations of AMP, VAN, or OXA inhibited formation of suture-associated biofilms initiated with low numbers (10(4)), but not high numbers (10(7)), of S. aureus cells.

Relation between antibiotic susceptibility and ultrastructure of Staphylococcus aureus biofilms on surgical suture.

BACKGROUND: Infectious biofilms are recalcitrant to antimicrobial therapy, but the mechanism(s) responsible for the greater resistance are unclear. Experiments were designed to clarify the association between antibiotic resistance and biofilm ultrastructure. METHODS: Staphylococcus aureus was cultivated for 24 h on silk suture, where robust biofilms formed. Initial experiments compared the susceptibilities of planktonic (free-living) cells and mechanically dispersed biofilm cells to ampicillin, oxacillin, and vancomycin. Antibiotics in bactericidal concentrations were then incubated overnight with 24-h biofilms, and subsequent assays determined the viability of cells in mechanically dispersed biofilms, biofilm metabolic capacity and biomass, and biofilm ultrastructure (scanning electron microscopy). RESULTS: Planktonic and biofilm cells had similar intrinsic antibiotic susceptibility. Nonetheless, a stable population of bacteria remained viable after biofilms were incubated with inhibitory drug concentrations, although biofilm metabolic capacity often was not detected, and biomass generally was reduced. Electron microscopy revealed that control (no drug) biofilms consisted primarily of bacterial clusters amid fibrillar elements. Antibiotic-treated biofilms had some staphylococci with smooth cells walls similar to planktonic cells, but other cocci were encased in extracellular material. This material was more abundant in antibiotic-treated than in control biofilms. CONCLUSIONS: In the presence of high antibiotic concentrations, dense extracellular material may inhibit interaction of antibiotics with their bacterial targets.

Gentamicin promotes Staphylococcus aureus biofilms on silk suture.

BACKGROUND: Communities of bacteria, termed biofilms, develop on biotic and abiotic surfaces, including medical devices and surgical suture. Biofilm-associated bacteria are typically recalcitrant to antibiotic therapy, and the effects of antibiotics on microbial biofilms are not clearly understood. There is emerging evidence that under specific conditions, aminoglycosides may actually promote biofilm development. Experiments were designed to study the effects of gentamicin on suture-associated Staphylococcus aureus biofilms. MATERIALS AND METHODS: S. aureus biofilms were formed after 24 h incubation of bacteria with silk suture. Susceptibility of planktonic S. aureus (from broth culture) to gentamicin was compared with the susceptibility of cells from mechanically dispersed S. aureus biofilms. Subinhibitory and inhibitory concentrations of gentamicin were subsequently incubated with intact suture-associated biofilms. S. aureus viability and metabolic capacity were assessed, and biofilm biomass was quantified with crystal violet (binds negatively charged surface molecules) and with the nucleic acid stain Syto 9. Scanning electron microscopy was used to assess the effect of gentamicin on the ultrastructure of suture-associated S. aureus biofilms. RESULTS: Planktonic cells and S. aureus cells from mechanically dispersed biofilms had similar susceptibility to gentamicin. However, after incubation of high concentrations of gentamicin with intact biofilms, high numbers of S. aureus remained both viable and metabolically active; biofilm biomass was increased and biofilm ultrastructure showed staphylococcal cells within copious amounts of extracellular material. CONCLUSION: Gentamicin does not effectively kill S. aureus within intact suture-associated biofilms, and gentamicin also promotes the biomass of S. aureus biofilms.

Role of Staphylococcus aureus protein A in adherence to silastic catheters.

BACKGROUND: Communities of bacteria, termed biofilms, frequently develop on central venous catheters, and bacterial contamination of central venous catheters is the most common cause of nosocomial bloodstream infections. Little is known about the initial events in bacterial adherence to the catheter surface, and experiments were designed to clarify the role of staphylococcal protein A, serum, and immunoglobulin in adherence of Staphylococcus aureus to silastic catheters. We hypothesized that S. aureus protein A in the presence of serum and immunoglobulin would alter S. aureus adherence to silastic catheters. MATERIALS AND METHODS: Three strains of S. aureus with varying expression of staphylococcal protein A were incubated 15 min at room temperature with silastic catheters, and bacterial adherence to the catheter surface was quantified. In addition, the effects of serum, albumin, and purified IgG on bacterial adherence were assessed. RESULTS: Both serum and albumin had an inhibitory effect on S. aureus adherence to the catheter surface, and protein A expression did not appreciably modulate these effects. Purified serum IgG also inhibited S. aureus adherence, with IgG having a greater inhibitory effect on the adherence of an S. aureus strain deficient in protein A compared with an S. aureus strain expressing high levels of protein A. CONCLUSION: S. aureus adherence to silastic catheters was inhibited by whole serum, albumin, and purified IgG. Expression of staphylococcal protein A interfered with IgG mediated inhibition of bacterial binding to the catheter surface. Protein A altered S. aureus adherence to silastic catheters in the presence of immunoglobulin, but not in the presence of serum or albumin.

Bacterial contamination of surgical suture resembles a biofilm.

BACKGROUND: Although much attention is currently directed to studying microbial biofilms on a variety of surfaces, few studies are designed to study bacterial growth on surgical suture. The purpose of this study was to compare the kinetic development of Staphylococcus aureus and Enterococcus faecalis on five surgical suture materials and to clarify factors that might influence this growth. METHODS: Pure cultures of S. aureus and E. faecalis were incubated with five types of suture for four days using either tissue culture medium or a bacterial growth medium. Suture-associated bacteria were quantified daily. In selected experiments, the bacterial growth medium was supplemented with heparin, a substance known to promote S. aureus biofilm formation. The ultrastructure of S. aureus biofilm developing on braided suture was studied with scanning electron microscopy. RESULTS: Staphylococcus aureus and E. faecalis were recovered in greater numbers (typically p < 0.01) from braided than from monofilament suture, and the numbers of bacteria were greater (often p < 0.01) on sutures incubated in bacterial growth medium rather than tissue culture medium. Addition of heparin 1,000 U/mL to silk or braided polyglactin 910 suture incubated three days with S. aureus resulted in greater numbers of bacteria on day one but not on subsequent days. Scanning electron microscopy showed a maturing S. aureus biofilm that developed from small clusters of cells among amorphous material and fibrillar elements to larger clusters of cells that appeared covered by more consolidated extracellular material. CONCLUSIONS: Bacterial growth was favored on braided vs. monofilament suture, and heparin enhanced bacterial adherence after day one, but not at subsequent times. Staphylococcus aureus adhered to suture material and formed a structure consistent with a bacterial biofilm.

Selected factors affecting Staphylococcus aureus within silastic catheters.

BACKGROUND: Catheter-related infections are frequent complications in hospitalized patients, and Staphylococcus aureus is a frequent etiologic agent. Little is known about factors that contribute to the growth and viability of S. aureus within contaminated catheters. MATERIALS AND METHODS: In vitro experiments assessed the ability of S. aureus to adhere to silastic catheter tubing. The effects of heparin, serum, and calcium on initial bacterial adherence were also assessed. Additional experiments quantified the effect of ethanol locking on S. aureus viability within catheter-associated biofilms produced after 48 to 72 h incubation of S. aureus with catheters under conditions of nutrient flow. Scanning electron microscopy visualized the effect of ethanol locking on the morphology of bacterial vegetations adherent to the catheter wall. RESULTS: S. aureus readily adhered (in a dose dependent manner) to silastic catheters incubated with bacteria for 15 min, and adherence was not affected by calcium or heparin (even though heparin adhered to the silastic tubing and S. aureus is known to express heparin-binding proteins). S. aureus adherence was inhibited by serum but not albumin. Ethanol locking (5 min to 24 h) of catheters containing mature 48 to 72 h S. aureus biofilms resulted in no detectable bacterial viability, although scanning electron microscopy revealed similar bacterial vegetations adherent to control and ethanol-treated catheters. CONCLUSION: S. aureus adherence to silastic catheters was inhibited by serum, but the active inhibitory component was not albumin. Ethanol locking efficiently sterilized S. aureus contaminated catheters, although nonviable bacterial vegetations remained on the ethanol-treated catheters.

Acceleration of Enterococcus faecalis biofilm formation by aggregation substance expression in an ex vivo model of cardiac valve colonization.

Infectious endocarditis involves formation of a microbial biofilm in vivo. Enterococcus faecalis Aggregation Substance (Asc10) protein enhances the severity of experimental endocarditis, where it has been implicated in formation of large vegetations and in microbial persistence during infection. In the current study, we developed an ex vivo porcine heart valve adherence model to study the initial interactions between Asc10(+) and Asc10(-)E. faecalis and valve tissue, and to examine formation of E. faecalis biofilms on a relevant tissue surface. Scanning electron microscopy of the infected valve tissue provided evidence for biofilm formation, including growing masses of bacterial cells and the increasing presence of exopolymeric matrix over time; accumulation of adherent biofilm populations on the cardiac valve surfaces during the first 2-4 h of incubation was over 10-fold higher than was observed on abiotic membranes incubated in the same culture medium. Asc10 expression accelerated biofilm formation via aggregation between E. faecalis cells; the results also suggested that in vivo adherence to host tissue and biofilm development by E. faecalis can proceed by Asc10-dependent or Asc10-independent pathways. Mutations in either of two Asc10 subdomains previously implicated in endocarditis virulence reduced levels of adherent bacterial populations in the ex vivo system. Interference with the molecular interactions involved in adherence and initiation of biofilm development in vivo with specific inhibitory compounds could lead to more effective treatment of infectious endocarditis.

Escherichia coli and TNF-alpha modulate macrophage phagocytosis of Candida glabrata.

BACKGROUND: The incidence of systemic nonalbicans Candida (especially C. glabrata) infections is increasing dramatically in intensive care units, but relatively little is known about the pathogenesis or host defenses associated with these life threatening infections. MATERIALS AND METHODS: The course of systemic C. glabrata infection was assessed as the fungal burden in the kidneys and livers of mice sacrificed 1, 8, and 15 d after intravenous C. glabrata. Sixteen hours before each sacrifice, half of the mice were injected intraperitoneally with intact viable or nonviable E. coli cells, or with E. coli lipopolysaccharide (LPS), or with tumor necrosis factor (TNF)-alpha. To clarify the effect of LPS and TNF-alpha on phagocytosis, resident (unstimulated) mouse peritoneal macrophages were harvested, cultivated ex vivo, and some cultures were treated with LPS or TNF-alpha prior to 30 min incubation with C. glabrata. RESULTS: Compared with mice injected with vehicle, each agent (intact E. coli cells or E. coli LPS or TNF-alpha) was consistently associated with decreased numbers of tissue C. glabrata, and some of these decreases were significant (P < 0.05). Compared with untreated macrophages, phagocytosis of C. glabrata was increased with LPS-treated macrophages (P < 0.01), and phagocytosis was also increased in the presence of TNF-alpha (P < 0.01). CONCLUSION: E. coli LPS and TNF-alpha may participate in host defense against C. glabrata by a mechanism involving increased macrophage phagocytosis, suggesting that stimulation of inflammatory cytokines may facilitate host clearance of C. glabrata.

Polyethylene glycol influences microbial interactions with intestinal epithelium.

There is emerging evidence that polyethylene glycol (PEG), widely used as a bowel preparation before surgery, may protect the intestinal epithelium from microbial invasion. Experiments were designed to study the effects of both low-molecular-weight (LMW; 3.35 kd) and high-molecular-weight (HMW; 15-20 kd) PEG on interactions of Escherichia coli, Candida albicans, and Candida glabrata with intestinal epithelium (these three intestinal microbes are frequently involved in systemic infection in shock and trauma patients.) In vitro experiments studied the effects of PEG on mature Caco-2 enterocytes. Using the gentamicin protection assay, both HMW and LMW PEG inhibited E. coli internalization by Caco-2 enterocytes. Using an immunosorbent assay, both HMW and LMW PEG inhibited C. albicans and C. glabrata adherence to Caco-2 enterocytes. Scanning electron micrographs of Caco-2 cells incubated in HMW or LMW PEG showed globular material distributed randomly over the epithelial surface, and apical microvilli seemed distorted. As an in vivo correlate to these experiments, separate groups of antibiotic-treated mice were orally associated with either E. coli, C. albicans, or C. glabrata, and cohort groups were given drinking water containing 5% HMW or 5% LMW PEG. Cecal colonization of E. coli was decreased in mice given HMW but not LMW PEG. Cecal colonization with C. albicans or C. glabrata was decreased in mice given either HMW or LMW PEG. These data provide further evidence that PEG may decrease microbial colonization and microbial interactions with intestinal epithelium.

An internal polarity landmark is important for externally induced hyphal behaviors in Candida albicans.

Directional growth is a function of polarized cells such as neurites, pollen tubes, and fungal hyphae. Correct orientation of the extending cell tip depends on signaling pathways and effectors that mediate asymmetric responses to specific environmental cues. In the hyphal form of the eukaryotic fungal pathogen Candida albicans, these responses include thigmotropism and galvanotropism (hyphal turning in response to changes in substrate topography and imposed electrical fields, respectively) and penetration into semisolid substrates. During vegetative growth in C. albicans, as in the model yeast Saccharomyces cerevisiae, the Ras-like GTPase Rsr1 mediates internal cellular cues to position new buds in a prespecified pattern on the mother cell cortex. Here, we demonstrate that Rsr1 is also important for hyphal tip orientation in response to the external environmental cues that induce thigmotropic and galvanotropic growth. In addition, Rsr1 is involved in hyphal interactions with epithelial cells in vitro and its deletion diminishes the hyphal invasion of kidney tissue during systemic infection. Thus, Rsr1, an internal polarity landmark in yeast, is also involved in polarized growth responses to asymmetric environmental signals, a paradigm that is different from that described for the homologous protein in S. cerevisiae. Rsr1 may thereby contribute to the pathogenesis of C. albicans infections by influencing hyphal tip responses triggered by interaction with host tissues.

Candida glabrata colonizes but does not often disseminate from the mouse caecum.

Candida glabrata is the second or third most frequent cause of candidaemia. The gastrointestinal tract is considered to be a major portal of entry for systemic candidiasis, but relatively few studies have investigated the pathogenesis of C. glabrata. Experiments were designed to clarify the ability of C. glabrata to disseminate from the mouse intestinal tract. Following oral inoculation, C. glabrata readily colonized the caeca [approx. 10(7) cells (g caecum)(-1)] of antibiotic-treated mice, but extraintestinal dissemination was not detected. Superimposing several mouse models of trauma and/or immunosuppression known to induce dissemination of Candida albicans and other intestinal microbes did not cause C. glabrata to disseminate often, although one exception was mice given high doses of dexamethasone for 4 days. These data support the hypothesis that the antibiotic-treated mouse intestine may be an epidemiological reservoir for C. glabrata and that this yeast tends to disseminate under specific clinical conditions.

Synergistic effect of tumor necrosis factor-alpha and interferon-gamma on enterocyte shedding of syndecan-1 and associated decreases in internalization of Listeria monocytogenes and Staphylococcus aureus.

Syndecan-1 is a heparan sulfate proteoglycan expressed on epithelia, and its ectodomain can be shed into the extracellular milieu, affecting a variety of cellular functions. Using two bacteria known to react with heparan sulfate, Listeria monocytogenes and Staphylococcus aureus, experiments were designed to clarify the effect of syndecan-1 shedding on bacterial internalization by human HT-29 enterocytes. Mature enterocytes were incubated with tumor necrosis factor (TNF)-alpha and/or interferon (IFN)-gamma for 16h prior to addition of bacteria. These cytokines acted synergistically to decrease syndecan-1 expression, assessed by visual observations of syndecan-1 expression on enterocytes using immunohistochemistry and a monoclonal antibody to the syndecan-1 core protein, by quantifying this fluorescent intensity, and by quantifying the concentration of shed syndecan-1 using an enzyme-linked immunoabsorbent assay. Neither IFN-gamma nor TNF-alpha alone had a noticeable effect on L. monocytogenes internalization, but a mixture of both cytokines resulted in decreased (P<0.01) internalization. Enterocyte preincubation with TNF-alpha alone, and with both cytokines, was associated with decreased S. aureus internalization, at P<0.05 and P<0.01, respectively. Thus, TNF-alpha and IFN-gamma acted synergistically to shed syndecan-1 ectodomains from HT-29 enterocytes, and shedding was associated with decreased internalization of two pathogenic bacteria, suggesting that syndecan-1 shedding may modulate the pathogenesis of specific microbes.

Heparan sulfate proteoglycans mediate Staphylococcus aureus interactions with intestinal epithelium.

Staphylococcus aureus can be internalized by non-professional phagocytes, and may colonize the intestine in normal and antibiotic-treated individuals. Intestinal colonization may depend on the interactions of S. aureus with the intestinal epithelium. The best described mechanism of S. aureus binding to eukaryotic cells involves S. aureus fibronectin binding proteins (FnBPs), using fibronectin as a bridging molecule to beta1 integrins on the eukaryotic cell surface. Because S. aureus can be internalized by enterocytes, and because S. aureus is known to bind heparan sulfate (HS), we hypothesized that heparan sulfate proteoglycans (HSPGs) widely expressed on epithelia may mediate S. aureus interactions with intestinal epithelial cells. Internalization of S. aureus RN6390 by cultured intestinal epithelial cells was inhibited in a dose-dependent fashion by the HS mimic heparin, and by HS itself. Internalization of S. aureus DU5883, which lacks expression of staphylococcal FnBPs, was also inhibited by heparin. S. aureus adherence to ARH-77 cells, transfected to express the HSPG syndecan-1, was greatly increased when compared to adherence to plasmid control ARH-77 cells which have little detergent extractable HS. In addition, compared to wild-type HS-expressing Chinese hamster ovary (CHO) cells, internalization of S. aureus was decreased using mutant CHO cells with decreased HS expression. These findings are consistent with a model wherein S. aureus internalization by intestinal epithelial cells (and perhaps other epithelia) is mediated by S. aureus binding to the HS moiety of cell-surface HSPGs, and this interaction appears independent of fibronectin binding.

Ability of the heparan sulfate proteoglycan syndecan-1 to participate in bacterial translocation across the intestinal epithelial barrier.

Although hundreds of microbial species reside in the human intestinal tract, comparatively few (e.g., Escherichia coli and other enterobacteria, Enterococcus faecalis, etc.) are typically associated with systemic infection in postsurgical, shock, and trauma patients. Syndecan-1 is the predominant cell surface heparan sulfate proteoglycan expressed on epithelia, and there is substantial evidence that heparan sulfate participates in interactions of a variety of frankly pathogenic microbes with mammalian cells. To investigate the role of syndecan-1 in interactions of enteric flora with intestinal epithelium, bacteria that might use the enterocyte as a portal of entry for systemic infection (including E. faecalis, E. coli, and other enterobacteria, and several species of staphylococci and streptococci) were studied for their abilities to interact with syndecan-1. Streptococcus bovis, S. agalactiae, S. pyogenes, Staphylococcus aureus, and S. epidermidis showed increased adherence to ARH-77 cells transfected to express syndecan-1. Heparin, a heparan sulfate analog, inhibited internalization of S. bovis, S. agalactiae, S. pyogenes, and S. aureus by HT-29 enterocytes (prominent syndecan-1 expression), but not Caco-2 enterocytes (relatively low syndecan-1 expression). Data from experiments with Chinese hamster ovary cells with altered glycosaminoglycan expression indicated that heparan sulfate and chondroitin sulfate (glycosaminoglycans on the syndecan-1 ectodomain) participated in bacterial interactions with mammalian cells. Thus, although E. faecalis, E. coli, and other gram-negative enterobacteria did not appear to interact with syndecan-1, this heparan sulfate proteoglycan may mediate enterocyte interactions with some staphylococci and streptococci that are known to cause systemic infections in specific populations of high-risk, immunosuppressed, postsurgical, and trauma patients.