Activities of bismuth thiols against staphylococci and staphylococcal biofilms

A sheep model for the study of biofilms in rhinosinusitis

BACKGROUND

Bacterial biofllms have been shown in chronic diseases such as cystic fibrosis, cholesteatoma, and otitis media with effusion. Recently, their detection on the mucosal tissue of sinusitis patients has implicated them in the pathogenesis of this condition. We present an animal model using sheep experimentally infected with Staphylococcus aureus to study the possible association between biofilm and sinusitis.

METHODS

Twenty-four sheep underwent bilateral endoscopic sinus surgery to identify their frontal ostia. The frontal sinuses were treated in one of the following ways according to preoperative randomization: (1) ostium left patent, (2) ostium left patent and bacteria instilled, (3) ostium occluded, or (4) ostium occluded and bacteria instilled. The frontal mucosa was harvested at day 7 and examined for biofilm presence using confocal scanning laser microscopy (CSLM) as well as scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

RESULTS

All three modalities showed different rates of biofilm detection. Three-dimensional structures that could be interpreted as biofilms were documented in 86% (n = 36) of the sinuses analyzed using SEM. These structures were seen in all four study groups. The detection rate using the other two modalities was much lower with CSLM, showing biofilms in 48% (n = 20) and TEM in only 29% (n = 12) of the sinuses analyzed. Unlike SEM, these two modalities only detected bacterial biofilms in sinuses randomized to bacterial instillation.

CONCLUSION

The demonstration of bacterial biofilms in this animal model of sinusitis further supports the hypotheses that biofilms may play a role in the pathogenesis of this condition. There is an obvious discrepancy in the sensitivity and specificity of biofilm detection using the three modalities mentioned. CSLM appears to be the most objective technique. The inherent flaws, sampling error, and subjectivity involved in SEM and TEM make these less reliable in documenting biofilm existence.

Antibiofilm activity of GlmU enzyme inhibitors against catheter-associated uropathogens

The colonization of uropathogenic bacteria on urinary catheters resulting in biofilm formation frequently leads to the infection of surrounding tissue and often requires removal of the catheter. Infections associated with biofilms are difficult to treat since they may be more than 1,000 times more resistant to antibiotics than their planktonic counterparts. We have developed an antibiofilm composition comprising an N-acetyl-D-glucosamine-1-phosphate acetyltransferase (GlmU) inhibitor and protamine sulfate, a cationic polypeptide. The antibiofilm activity of GlmU inhibitors, such as iodoacetamide (IDA), N-ethyl maleimide (NEM), and NEM analogs, including N-phenyl maleimide, N,N'-(1,2-phenylene)dimaleimide (oPDM), and N-(1-pyrenyl)maleimide (PyrM), was tested against that of catheter-associated uropathogens. Both IDA and NEM inhibited biofilm formation in Escherichia coli. All NEM analogs showed antibiofilm activity against clinical isolates of E. coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, Staphylococcus epidermidis, and Enterococcus faecalis. The combination of oPDM with protamine sulfate (PS) enhanced its antibiofilm activity and reduced its effective concentration to as low as 12.5 microM. In addition, we found that the in vitro inhibitory activity of oPDM-plus-PS-coated silicone catheters against P. aeruginosa and S. epidermidis colonization was superior to that of catheters coated with silver hydrogel. Confocal scanning laser microscopy further confirmed that the oPDM-plus-PS-coated silicone catheters were almost free from bacterial colonization. Thus, a broad-spectrum antibiofilm composition comprising a GlmU inhibitor and protamine sulfate shows promise for use in anti-infective coatings for medical devices, including urinary catheters.

Antibacterial properties of some cyclic organobismuth(III) compounds

Bismuth compounds are known for their low levels of toxicity in mammals, and various types of bismuth salts have been used to treat medical disorders. As part of our program to probe this aspect of bismuth chemistry, cyclic organobismuth compounds 1 to 8 bearing a nitrogen or sulfur atom as an additional ring member have been synthesized, and their antimicrobial activities against five standard strains of gram-negative and gram-positive bacteria were assessed. The eight-membered-ring compounds, compounds 1 to 3, exhibited MICs of less than 0.5 microg/ml against Staphylococcus aureus and were more active than the six-membered ones, compounds 5 to 8 (MICs, 4.0 to 16 microg/ml). The gram-positive bacteria (Staphylococcus aureus, Bacillus subtilis, and Enterococcus faecalis) were more susceptible to both types of ring compounds than the gram-negative ones (Escherichia coli and Pseudomonas aeruginosa). Treatment with polymyxin B nonapeptide increased the susceptibility of E. coli to cyclic organobismuth compounds, indicating the low permeability of the outer membrane of gram-negative bacteria to the compounds. Compound 1 also had activity against methicillin-resistant S. aureus, which had an MIC for 90% of the hospital stock strains of 1.25 microg/ml. The killing curves for S. aureus treated with compound 1 or 3 revealed a static effect at a low dose (2x the MIC). However, when S. aureus was treated with 10x the MIC of compound 1 or 3, there was an approximately 3-log reduction in the viable cell number after 48 h of treatment. Electron microscopic inspection demonstrated a considerable increase in the size of S. aureus and the proportion of cells undergoing cell division after treatment with compound 1 at 0.5x the MIC.

Inhibition of bacterial adherence on the surface of stents and bacterial growth in bile by bismuth dimercaprol

Bacterial infection and biofilm formation on the surface of biliary stents is believed to be one of the main factors in stent occlusion. This study explored the role of the new reagent, bismuth dimercaprol, in preventing bacterial adherence and bacterial biofilm formation on the surface of biliary stents. Sterile porcine bile preparations, infected separately with Escherichia coli, Klebsiella pneumoniae, Enterobacter, and Enterococcus, were used as the perfusion media in an in vitro perfusion system. The bacterial growth in the media and the bacterial adherence on the surface of stents were tested when different concentrations of bismuth dimercaprol were used in the perfusion media. BisBAL (5 microM) did not inhibit the growth of any of the tested bacterial species. It did, however, significantly decrease the amount of bacteria adhering to the surface of stents for all bacterial strains except Escherichia coli. Bismuth dimercaprol (20 microM) significantly inhibited the growth of Escherichia coli, Klebsiella pneumoniae, and Enterobacter and, thereby, significantly decreased the amount of these bacteria adhering to the surface of stents. The unique bactericidal and anitbiofilm activities of bismuth thiols might contribute to delaying the process of biliary stent occlusion if the effective concentrations of bismuth thiols could be delivered to the target sites. The feasibility of this application of bismuth thiols deserves further investigation.

Bismuth–dithiol inhibition of the Escherichia coli rho transcription termination factor

Bismuth-dithiol mixtures are proven antimicrobial agents with unknown mechanism(s) of action. We show that select bismuth-dithiol solutions inhibit the Escherichia coli rho transcription termination factor. Rho is an essential enzyme in most Gram-negative prokaryotes and without rho function the cells are not viable. Bismuth complexes with 2,3-dimercapto-1-propanol (BiBAL) (3:1 solutions) functioned as a noncompetitive inhibitor with respect to ATP in the rho poly(C)-dependent ATPase assay (I50=60 microM) and as a competitive inhibitor with respect to ribo(C)10 in the poly(dC)-ribo(C)10-dependent ATPase assay. The minimum inhibitory concentration (MIC) of bacterial growth for BiBAL (3:1) in the liquid culture assay using E. coli W3350 was 16 microM. Using the tnaA/lacZ fusion reporter assay we showed that sublethal amounts (3 microM) of BiBAL (3:1 solution) led to a small increase (37%) in in vivo beta-galactosidase activity in E. coli SVS1144, which corresponds to antitermination of the tna operon as a result of rho inhibition. We concluded that BiBAL was a potent in vitro rho inhibitor but its effect on in vivo rho processes was modest indicating that other mechanisms contributed to the antibacterial activity of BiBAL. Our study suggests that structural changes in the dithiol unit that provide greater bismuth binding may improve rho specificity, a macromolecular target not previously recognized for bismuth therapy.

BisEDT and RIP act in synergy to prevent graft infections by resistant staphylococci

Staphylococci are a major cause of infections associated with indwelling medical devices. Biofilm formation on these devices adds to the antibiotic resistance seen among clinical isolates. RNAIII-inhibiting peptide (RIP) is a heptapeptide that inhibits staphylococcal pathogenesis, including biofilm formation, by obstructing quorum sensing mechanisms. Bismuth ethanedithiol (BisEDT) also prevents biofilm formation at subinhibitory concentrations. RIP and BisEDT were combined to prevent infections in a rat graft model, using antibiotic sensitive and resistant strains of Staphylococcus aureus and Staphylococcus epidermidis. BisEDT, RIP, or rifampin, or their combinations reduced the graft associated bacterial load over seven days. BisEDT-RIP was the best combination, reducing bacterial load to undetectable levels. BisEDT-RIP may prove useful for coating medical devices to prevent staphylococcal infections.

The action of bismuth against Helicobacter pylori mimics but is not caused by intracellular iron deprivation

Helicobacter pylori is highly susceptible to bismuth, a heavy metal with antimicrobial activity linked to its effect on bacterial iron uptake. Three strains of H. pylori were analyzed for indicators of iron limitation following exposure to the MIC of colloidal bismuth subcitrate (MIC(CBS)). Similar morphologic and outer membrane changes were observed following growth in iron-limiting medium and at the MIC(CBS) that inhibited the growth of all three strains. These changes, which were also observed for iron-limited bacteria, were alleviated by the addition of iron to the cultures. H. pylori ATP levels, reduced in iron-limiting medium, were below the limits of detection in two of the three strains following exposure to bismuth. The addition of iron partially restored bacterial ATP levels in these two strains, although not to normal concentrations. In contrast, exposure of the same strains to the MIC(CBS) failed to deplete intracellular levels of iron, which were significantly reduced by culturing in iron-limiting medium. Thus, the antimicrobial effect of bismuth and of iron limitation on H. pylori may be similar. However, the respective mechanisms of intracellular action would appear to be mediated by different pathways within the cell.

Assessment of bismuth thiols and conventional disinfectants on drinking water biofilms

AIMS

Biofilms in water distribution systems represent a far more significant reservoir of micro-organisms than the water phase. Biofilms are (i) resistant to disinfectants, (ii) nuclei for microbial regrowth, (iii) a refuge for pathogens, (iv) accompanied by taste and odour problems, and (v) corrode surfaces. The effects of the current strategies for disinfection of drinking water systems in large buildings (chlorination, copper and silver ionization, and hyper-heating) were compared with a new generation of bismuth thiol (BT) biocides.

METHODS AND RESULTS

Multispecies biofilms were treated with 0.8 mg l(-1) of free chlorine, 400 and 40 microg l(-1) of copper and silver ions, respectively, at 55 and 70 degrees C, and bismuth-2,3-dimercaptopropanol (BisBAL). Furthermore, the effect of combined heat and BisBAL on planktonic cell viability was examined in monoculture using Escherichia coli suspensions. Inactivation rates for BisBAL were similar to copper-silver ions, where the effects were slower than for free chlorine or temperature. The BisBAL effect on E. coli monocultures was augmented greatly by increasing temperatures.

CONCLUSIONS

Like copper-silver ions, BTs show more persistent residual effects than chlorine and hyper-heating in water systems. BT efficiency increased with temperature. Like copper-silver ions, BT action is relatively slow.

SIGNIFICANCE AND IMPACT OF THE STUDY

BT presents a new approach to containing water biofilms. BT action is not as rapid, but is more thorough than chlorine, and less caustic. BTs may also be more efficacious in hot water systems. At sub-minimum inhibition concentration levels, BTs uniquely inhibit bacterial exopolysaccharide, thereby retarding biofilm formation. Thus, the combination of bactericidal and residual effects may prevent slime build-up in hot water systems.

Subinhibitory bismuth-thiols reduce virulence of Pseudomonas aeruginosa

Pseudomonas aeruginosa is a common pathogen in mechanically ventilated patients and produces a wide array of virulence factors. Bismuth-thiols (BTs) are active in vitro against all bacterial lung pathogens, including P. aeruginosa. The objective of these studies was to examine the biochemical and morphologic effects of sublethal BT concentrations on P. aeruginosa and to evaluate virulence in cell culture. Bismuth-dimercaprol, at a fraction of the minimal inhibitory concentration, reduced alginate expression by 67% in P. aeruginosa, whereas subinhibitory bismuth-ethanedithiol (BisEDT) reduced alginate by 92% in P. syringae. BisEDT effects on lipopolysaccharide content and type III secreted cytoxins were examined by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Subinhibitory BisEDT reduced cell-associated lipopolysaccharide, and inhibited processing of the secreted cytotoxic protein ExoU. BisEDT-induced outer membrane blebbing and aggregation of cytoplasmic material was noted in electron microscopy. Virulence of P. aeruginosa was assessed by adherence to epithelial cells and sensitivity to serum killing. BisEDT inhibited adherence of P. aeruginosa to 16HBE14o- cells by 28% and to a collagen matrix by 53%. BisEDT-treated bacteria were also 100-fold more sensitive to serum bactericidal activity. In summary, low BT concentrations affect P. aeruginosa in a variety of ways, the combination of which may help prevent or resolve respiratory tract infection.