Increased production of aureolysin and staphopain A is a primary determinant of the reduced virulence of Staphylococcus aureus sarA mutants in osteomyelitis

We previously demonstrated that mutation of sarA in Staphylococcus aureus limits biofilm formation, cytotoxicity for osteoblasts and osteoclasts, and virulence in osteomyelitis, and that all of these phenotypes can be attributed to the increased production of extracellular proteases. Here we extend these studies to assess the individual importance of these proteases alone and in combination with each other using the methicillin-resistant USA300 strain LAC, the methicillin-susceptible USA200 strain UAMS-1, and isogenic sarA mutants that were also unable to produce aureolysin (Aur), staphopain A (ScpA), staphylococcal serine protease A (subsp.), staphopain B (SspB), and the staphylococcal serine protease-like proteins A-F (SplA-F). Biofilm formation was restored in LAC and UAMS-1 sarA mutants by subsequent mutation of aur and scpA, while mutation of aur had the greatest impact on cytotoxicity to mammalian cells, particularly with conditioned medium (CM) from the more cytotoxic strain LAC. However, SDS-PAGE and western blot analysis of CM confirmed that mutation of sspAB was also required to mimic the phenotype of sarA mutants unable to produce any extracellular proteases. Nevertheless, in a murine model of post-traumatic osteomyelitis, mutation of aur and scpA had the greatest impact on restoring the virulence of LAC and UAMS-1 sarA mutants, with concurrent mutation of sspAB and the spl operon having relatively little effect. These results demonstrate that the increased production of Aur and ScpA in combination with each other is a primary determinant of the reduced virulence of S. aureus sarA mutants in diverse clinical isolates including both methicillin-resistant and methicillin-susceptible strains.IMPORTANCEPrevious work established that SarA plays a primary role in limiting the production of extracellular proteases to prevent them from limiting the abundance of S. aureus virulence factors. Eliminating the production of all 10 extracellular proteases in the methicillin-resistant strain LAC has also been shown to enhance virulence in a murine sepsis model, and this has been attributed to the specific proteases Aur and ScpA. The importance of this work lies in our demonstration that the increased production of these same proteases largely accounts for the decreased virulence of sarA mutants in a murine model of post-traumatic osteomyelitis not only in LAC but also in the methicillin-susceptible human osteomyelitis isolate UAMS-1. This confirms that sarA-mediated repression of Aur and ScpA production plays a critical role in the posttranslational regulation of S. aureus virulence factors in diverse clinical isolates and diverse forms of S. aureus infection.

Comparative evaluation of small molecules reported to be inhibitors of Staphylococcus aureus biofilm formation

IMPORTANCE

Because biofilm formation is such a problematic feature of Staphylococcus aureus infections, much effort has been put into identifying biofilm inhibitors. However, the results observed with these compounds are often reported in isolation, and the methods used to assess biofilm formation vary between labs, making it impossible to assess relative efficacy and prioritize among these putative inhibitors for further study. The studies we report address this issue by directly comparing putative biofilm inhibitors using a consistent in vitro assay. This assay was previously shown to maximize biofilm formation, and the results observed with this assay have been proven to be relevant in vivo. Of the 19 compounds compared using this method, many had no impact on biofilm formation under these conditions. Indeed, only one proved effective at limiting biofilm formation without also inhibiting growth.

The major role of sarA in limiting Staphylococcus aureus extracellular protease production in vitro is correlated with decreased virulence in diverse clinical isolates in osteomyelitis

We previously demonstrated that MgrA, SarA, SarR, SarS, SarZ, and Rot bind at least three of the four promoters associated with genes encoding primary extracellular proteases in Staphylococcus aureus (Aur, ScpA, SspA/SspB, SplA-F). We also showed that mutation of sarA results in a greater increase in protease production, and decrease in biofilm formation, than mutation of the loci encoding any of these other proteins. However, these conclusions were based on in vitro studies. Thus, the goal of the experiments reported here was to determine the relative impact of the regulatory loci encoding these proteins in vivo. To this end, we compared the virulence of mgrA, sarA, sarR, sarS, sarZ, and rot mutants in a murine osteomyelitis model. Mutants were generated in the methicillin-resistant USA300 strain LAC and the methicillin-sensitive USA200 strain UAMS-1, which was isolated directly from the bone of an osteomyelitis patient during surgical debridement. Mutation of mgrA and rot limited virulence to a statistically significant extent in UAMS-1, but not in LAC, while the sarA mutant exhibited reduced virulence in both strains. The reduced virulence of the sarA mutant was correlated with reduced cytotoxicity for osteoblasts and osteoclasts, reduced biofilm formation, and reduced sensitivity to the antimicrobial peptide indolicidin, all of which were directly attributable to increased protease production in both LAC and UAMS-1. These results illustrate the importance of considering diverse clinical isolates when evaluating the impact of regulatory mutations on virulence and demonstrate the significance of SarA in limiting protease production in vivo in S. aureus.

Limiting protease production plays a key role in the pathogenesis of the divergent clinical isolates of Staphylococcus aureus LAC and UAMS-1

Using the USA300, methicillin-resistant Staphylococcus aureus strain LAC, we previously examined the impact of regulatory mutations implicated in biofilm formation on protease production and virulence in a murine sepsis model. Here we examined the impact of these mutations in the USA200, methicillin-sensitive strain UAMS-1. Mutation of agr, mgrA, rot, sarA and sigB attenuated the virulence of UAMS-1. A common characteristic of codY, rot, sigB, and sarA mutants was increased protease production, with mutation of rot having the least impact followed by mutation of codY, sigB and sarA, respectively. Protein A was undetectable in conditioned medium from all four mutants, while extracellular nuclease was only present in the proteolytically cleaved NucA form. The abundance of high molecular weight proteins was reduced in all four mutants. Biofilm formation was reduced in codY, sarA and sigB mutants, but not in the rot mutant. Eliminating protease production partially reversed these phenotypes and enhanced biofilm formation. This was also true in LAC codY, rot, sarA and sigB mutants. Eliminating protease production enhanced the virulence of LAC and UAMS-1 sarA, sigB and rot mutants in a murine sepsis model but did not significantly impact the virulence of the codY mutant in either strain. Nevertheless, these results demonstrate that repressing protease production plays an important role in defining critical phenotypes in diverse clinical isolates of S. aureus and that Rot, SigB and SarA play critical roles in this regard.

Staphylococcal infection prevention using antibiotic-loaded mannitol-chitosan paste in a rabbit model of implant-associated osteomyelitis

Antibiotic-loaded chitosan pastes have shown advantages in the treatment and coverage of complex musculoskeletal defects. We added mannitol, previously shown to increase antibiotic susceptibility of biofilm, to an injectable chitosan/polyethylene glycol paste for delivery of antibiotics. Ground sponges (0.85% acetic acid solution, 1% chitosan, 0% or 2% mannitol, 1% polyethylene glycol) were hydrated using phosphate-buffered saline with 10 mg/ml amikacin and 10 mg/ml vancomycin added to form pastes. We inoculated rabbit radial defects with 10⁵ colony-forming units of Staphylococcus aureus (UAMS-1) and inserted titanium pins into the cortical bone. Groups compared included mannitol blend pastes, non-mannitol blends, antibiotic-loaded bone cement, vancomycin powder, and no treatment controls. We harvested tissue samples and retrieved the pins retrieved at 3 weeks. All antibiotic-loaded groups lowered bacterial growth and colony-forming unit counts in soft and bone tissue and on titanium pins in in vivo studies. The results indicate this biomaterial is capable of eluting active antibiotics at concentrations that reduce bacterial growth on biomaterials and tissue, which, in turn, may prevent biofilm formation. Blends of chitosan and mannitol may be useful in prevention and treatment of osteomyelitis and implant-associated infections.

SarA plays a predominant role in controlling the production of extracellular proteases in the diverse clinical isolates of Staphylococcus aureus LAC and UAMS-1

Using DNA affinity chromatography we demonstrate that the S. aureus regulatory proteins MgrA, Rot, SarA, and SarS bind DNA baits derived from the promoter regions associated with the genes encoding aureolysin, ScpAB, SspABC, and SplA-F. Three of four baits also bound SarR and SarZ, the exception in both cases being the ScpAB-associated bait. Using the USA300, methicillin-resistant strain LAC and the USA200, methicillin-sensitive strain UAMS-1, we generated mutations in the genes encoding each of these proteins alone and in combination with sarA and examined the impact on protease production, the accumulation of high molecular weight proteins, and biofilm formation. These studies confirmed that multiple regulatory loci are involved in limiting protease production to a degree that impacts all of these phenotypes, but also demonstrate that sarA plays a predominant role in this regard. Using sarA mutants unable to produce individual proteases alone and in combination with each other, we also demonstrate that the increased production of aureolysin and ScpA is particularly important in defining the biofilm-deficient phenotype of LAC and UAMS-1 sarA mutants, while aureolysin alone plays a key role in defining the reduced accumulation of alpha toxin and overall cytotoxicity as assessed using both osteoblasts and osteoclasts.

Exploiting Correlations between Protein Abundance and the Functional Status of saeRS and sarA To Identify Virulence Factors of Potential Importance in the Pathogenesis of Staphylococcus aureus Osteomyelitis

We used a murine model of postsurgical osteomyelitis (OM) to evaluate the relative virulence of the Staphylococcus aureus strain LAC and five isogenic variants that differ in the functional status of saeRS and sarA relative to each other. LAC and a variant in which saeRS activity is increased (sae^C) were comparably virulent to each other, while ΔsaeRS, ΔsarA, ΔsaeRS/ΔsarA, and sae^C/ΔsarA mutants were all attenuated to a comparable degree. Phenotypic comparisons including a mass-based proteomics approach that allowed us to assess the number and abundance of full-length proteins suggested that mutation of saeRS attenuates virulence in our OM model owing primarily to the decreased production of S. aureus virulence factors, while mutation of sarA does so owing to protease-mediated degradation of these same virulence factors. This was confirmed by demonstrating that eliminating protease production restored virulence to a greater extent in a LAC sarA mutant than in the isogenic saeRS mutant. Irrespective of the mechanism involved, mutation of saeRS or sarA was shown to result in reduced accumulation of virulence factors of potential importance. Thus, using our proteomics approach we correlated the abundance of specific proteins with virulence in these six strains and identified 14 proteins that were present in a significantly increased amount (log₂ ≥ 5.0) in both virulent strains by comparison to all four attenuated strains. We examined biofilm formation and virulence in our OM model using a LAC mutant unable to produce one of these 14 proteins, specifically staphylocoagulase. The results confirmed that mutation of coa limits biofilm formation and, to a lesser extent, virulence in our OM model, although in both cases the limitation was reduced by comparison to the isogenic sarA mutant.

Ciprofloxacin and Rifampin Dual Antibiotic-Loaded Biopolymer Chitosan Sponge for Bacterial Inhibition

Complex extremity wounds in Wounded Warriors can become contaminated with microbes, which may cause clinical outcomes resulting in amputation, morbidity, or even fatality. Local delivery of multiple or broad-spectrum antibiotics allows practicing clinicians treatment solutions that may inhibit biofilm formation. Propagation of vancomycin-resistant Staphylococcus aureus is also a growing concern. The development of vancomycin-resistant S. aureus has become a critical challenge in nosocomial infection prevention in the USA, but to date has seen little occurrence in osteomyelitis. As an alternative, locally delivered ciprofloxacin and rifampin were investigated in a preclinical model for the prevention of biofilm in complex extremity wounds with implanted fixation device. In vitro assays demonstrated ciprofloxacin and rifampin possess an additive effect against Gram-negative Pseudomonas aeruginosa and were actively eluted from a chitosan sponge based local delivery system. In an in vivo orthopedic hardware-associated polymicrobial model (S. aureus and Escherichia coli) the combination was able to achieve complete clearance of both bacterial strains. E. coli was detected in bone of untreated animals, but did not form biofilm on wires. Results reveal the clinical potential of antibiotic-loaded chitosan sponges to inhibit infection through tailored antibiotic selection at desired concentrations with efficacy towards biofilm inhibition.

Local Delivery of Amikacin and Vancomycin from Chitosan Sponges Prevent Polymicrobial Implant-Associated Biofilm

Military personnel have high risk for infection, particularly those with combat-related extremity trauma. Administration of multiple or broad-spectrum antibiotics provides clinicians with a strategy for preventing biofilm-based medical device infections. Selection of effective antibiotic combinations based on common pathogens may be used to improve chitosan wound dressing sponge-based local antibiotic delivery systems. In vitro assays in this study demonstrate that vancomycin and amikacin have a synergistic relationship against a strain of osteomyelitis-producing Gram-positive Staphylococcus aureus, although an indifferent relationship was observed against Gram-negative Pseudomonas aeruginosa. In an in vivo model of orthopedic hardware-associated polymicrobial (S. aureus and Escherichia coli) biofilm, chitosan sponges loaded with a combination of vancomycin and amikacin at 5 mg/mL each showed a greater percentage of complete clearance, 50%, than either antibiotic alone, 8.33%. Doubling the loading concentration of the combination achieved a complete clearance rate of 100%, a four log-fold reduction of S. aureus on the wire and a six log-fold reduction in bone. E. coli was detected in bone of untreated animals but did not form biofilm on wires. Results demonstrate the clinical potential of chitosan sponges to prevent infection and illustrates antibiotic selection and loading concentrations necessary for effective biofilm prevention.

Label-Free Proteomic Approach to Characterize Protease-Dependent and -Independent Effects of sarA Inactivation on the Staphylococcus aureus Exoproteome

The staphylococcal accessory regulator A ( sarA) impacts the extracellular accumulation of Staphylococcus aureus virulence factors at the level of intracellular production and extracellular protease-mediated degradation. We previously used a proteomics approach that measures protein abundance of all proteoforms to demonstrate that mutation of sarA results in increased levels of extracellular proteases and assesses the impact of this on the accumulation of S. aureus exoproteins. Our previous approach was limited as it did not take into account that large, stable proteolytic products from a given protein could result in false negatives when quantified by total proteoforms. Here, our goal was to use an expanded proteomics approach utilizing a dual quantitative method for measuring abundance at both the total proteoform and full-length exoprotein levels to alleviate these false negatives and thereby provide for characterization of protease-dependent and -independent effects of sarA mutation on the S. aureus exoproteome. Proteins present in conditioned medium from overnight, stationary phase cultures of the USA300 strain LAC, an isogenic sarA mutant, and a sarA mutant unable to produce any of the known extracellular proteases ( sarA/protease) were resolved using one-dimensional gel electrophoresis. Quantitative proteomic comparisons of sarA versus sarA/protease mutants identified proteins that were cleaved in a protease-dependent manner owing to mutation of sarA, and comparisons of sarA/protease mutant versus the LAC parent strain identified proteins in which abundance was altered in a sarA mutant in a protease-independent manner. Furthermore, the proteins uniquely identified by the full-length data analysis approach eliminated false negatives observed in the total proteoform analysis. This expanded approach provided for a more comprehensive analysis of the impact of mutating sarA on the S. aureus exoproteome.

Versatility of targeted antibiotic-loaded gold nanoconstructs for the treatment of biofilm-associated bacterial infections

BACKGROUND

We previously demonstrated that a photoactivatable therapeutic approach employing antibiotic-loaded, antibody-conjugated, polydopamine (PDA)-coated gold nanocages (AuNCs) could be used for the synergistic killing of bacterial cells within a biofilm. The approach was validated with a focus on Staphylococcus aureus using an antibody specific for staphylococcal protein A (Spa) and an antibiotic (daptomycin) active against Gram-positive cocci including methicillin-resistant S. aureus (MRSA). However, an important aspect of this approach is its potential therapeutic versatility.

METHODS

In this report, we evaluated this versatility by examining the efficacy of AuNC formulations generated with alternative antibodies and antibiotics targeting S. aureus and alternative combinations targeting the Gram-negative pathogen Pseudomonas aeruginosa.

RESULTS

The results confirmed that daptomycin-loaded AuNCs conjugated to antibodies targeting two different S. aureus lipoproteins (SACOL0486 and SACOL0688) also effectively kill MRSA in the context of a biofilm. However, our results also demonstrate that antibiotic choice is critical. Specifically, ceftaroline and vancomycin-loaded AuNCs conjugated to anti-Spa antibodies were found to exhibit reduced efficacy relative to daptomycin-loaded AuNCs conjugated to the same antibody. In contrast, gentamicin-loaded AuNCs conjugated to an antibody targeting a conserved outer membrane protein were highly effective against P. aeruginosa biofilms.

CONCLUSIONS

These results confirm the therapeutic versatility of our approach. However, to the extent that its synergistic efficacy is dependent on the ability to achieve both a lethal photothermal effect and the thermally controlled release of a sufficient amount of antibiotic, they also demonstrate the importance of carefully designing appropriate antibody and antibiotic combinations to achieve the desired therapeutic synergy.

Impact of Staphylococcus aureus regulatory mutations that modulate biofilm formation in the USA300 strain LAC on virulence in a murine bacteremia model

Staphylococcus aureus causes acute and chronic forms of infection, the latter often associated with formation of a biofilm. It has previously been demonstrated that mutation of atl, codY, rot, sarA, and sigB limits biofilm formation in the USA300 strain LAC while mutation of agr, fur, and mgrA has the opposite effect. Here we used a murine sepsis model to assess the impact of these same loci in acute infection. Mutation of agr, atl, and fur had no impact on virulence, while mutation of mgrA and rot increased virulence. In contrast, mutation of codY, sarA, and sigB significantly attenuated virulence. Mutation of sigB resulted in reduced accumulation of AgrA and SarA, while mutation of sarA resulted in reduced accumulation of AgrA, but this cannot account for the reduced virulence of sarA or sigB mutants because the isogenic agr mutant was not attenuated. Indeed, as assessed by accumulation of alpha toxin and protein A, all of the mutants we examined exhibited unique phenotypes by comparison to an agr mutant and to each other. Attenuation of the sarA, sigB and codY mutants was correlated with increased production of extracellular proteases and global changes in extracellular protein profiles. These results suggest that the inability to repress the production of extracellular proteases plays a key role in attenuating the virulence of S. aureus in acute as well as chronic, biofilm-associated infections, thus opening up the possibility that strategies aimed at the de-repression of protease production could be used to broad therapeutic advantage. They also suggest that the impact of codY, sarA, and sigB on protease production occurs via an agr-independent mechanism.

Evaluation of a chitosan-polyethylene glycol paste as a local antibiotic delivery device

AIM

To investigate the efficacy of a chitosan/polyethylene glycol blended paste as a local antibiotic delivery device, particularly in musculoskeletal wounds.

METHODS

Acidic (A) chitosan sponges and neutralized (N) chitosan/polyethylene glycol (PEG) blended sponges were combined in ratios of 3A:2N, 1A:1N, and 2A:3N; then hydrated with phosphate buffered saline to form a chitosan/PEG paste (CPP). Both in vitro and in vivo studies were conducted to determine the potential CPP has as a local antibiotic delivery device. In vitro biocompatibility was assessed by the cytotoxic response of fibroblast cells exposed to the experimental groups. Degradation rate was measured as the change in dry mass due to lysozyme based degradation over a 10-d period. The antibiotic elution profiles and eluate activity of CPP were evaluated over a 72-h period. To assess the in vivo antimicrobial efficacy of the CPP, antibiotic-loaded paste samples were exposed to subcutaneously implanted murine catheters inoculated with Staphylococcus aureus. Material properties of the experimental paste groups were evaluated by testing the ejection force from a syringe, as well as the adhesion to representative musculoskeletal tissue samples.

RESULTS

The highly acidic CPP group, 3A:2N, displayed significantly lower cell viability than the control sponge group. The equally distributed group, 1A:1N, and the highly neutral group, 2A:3N, displayed similar cell viability to the control sponge group and are deemed biocompatible. The degradation studies revealed CPP is more readily degradable than the chitosan sponge control group. The antibiotic activity studies indicated the CPP groups released antibiotics at a constant rate and remained above the minimum inhibitory concentrations of the respective test bacteria for a longer time period than the control chitosan sponges, as well as displaying a minimized burst release. The in vivo functional model resulted in complete bacterial infection prevention in all catheters treated with the antibiotic loaded CPP samples. All experimental paste groups exhibited injectability and adhesive qualities that could be advantageous material properties for drug delivery to musculoskeletal injuries.

CONCLUSION

CPP is an injectable, bioadhesive, biodegradable, and biocompatible material with potential to allow variable antibiotic loading and active, local antibiotic release to prevent bacterial contamination.

Antibiotic-loaded phosphatidylcholine inhibits staphylococcal bone infection

AIM: To test antibiotic-loaded coating for efficacy in reducing bacterial biofilm and development of osteomyelitis in an orthopaedic model of implant infection.

METHODS: Phosphatidylcholine coatings loaded with 25% vancomycin were applied to washed and sterilized titanium wires 20 mm in length. A 10 mm segment was removed from rabbit radius (total = 9; 5 coated, 4 uncoated), and the segment was injected with 1 × 10⁶ colony forming units (CFUs) of Staphylococcus aureus (UAMS-1 strain). Titanium wires were inserted through the intramedullary canal of the removed segment and into the proximal radial segment and the segment was placed back into the defect. After 7 d, limbs were removed, X-rayed, swabbed for tissue contamination. Wires were removed and processed to determine attached CFUs. Tissue was swabbed and streaked on agar plates to determine bacteriological score.

RESULTS: Antibiotic-loaded coatings resulted in significantly reduced biofilm formation (4.7 fold reduction in CFUs; P < 0.001) on titanium wires and reduced bacteriological score in surrounding tissue (4.0 ± 0 for uncoated, 1.25 ± 0.5 for coated; P = 0.01). Swelling and pus formation was evident in uncoated controls at the 7 d time point both visually and radiographically, but not in antibiotic-loaded coatings.

CONCLUSION: Active antibiotic was released from coated implants and significantly reduced signs of osteomyelitic symptoms. Implant coatings were well tolerated in bone. Further studies with additional control groups and longer time periods are warranted. Antibiotic-loaded phosphatidylcholine coatings applied at the point of care could prevent implant-associated infection in orthopaedic defects.

Synergistic Photothermal and Antibiotic Killing of Biofilm-Associated Staphylococcus aureus Using Targeted Antibiotic-Loaded Gold Nanoconstructs

Resistance to conventional antibiotics is a growing public health concern that is quickly outpacing the development of new antibiotics. This has led the Infectious Diseases Society of America (IDSA) to designate Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species as "ESKAPE pathogens" on the basis of the rapidly decreasing availability of useful antibiotics. This emphasizes the urgent need for alternative therapeutic strategies to combat infections caused by these and other bacterial pathogens. In this study, we used Staphylococcus aureus (S. aureus) as a proof-of-principle ESKAPE pathogen to demonstrate that an appropriate antibiotic (daptomycin) can be incorporated into polydopamine-coated gold nanocages (AuNC@PDA) and that daptomycin-loaded AuNC@PDA can be conjugated to antibodies targeting a species-specific surface protein (staphylococcal protein A; Spa) as a means of achieving selective delivery of the nanoconstructs directly to the bacterial cell surface. Targeting specificity was confirmed by demonstrating a lack of binding to mammalian cells, reduced photothermal and antibiotic killing of the Spa-negative species Staphylococcus epidermidis, and reduced killing of S. aureus in the presence of unconjugated anti-Spa antibodies. We demonstrate that laser irradiation at levels within the current safety standard for use in humans can be used to achieve both a lethal photothermal effect and controlled release of the antibiotic, thus resulting in a degree of therapeutic synergy capable of eradicating viable S. aureus cells. The system was validated using planktonic bacterial cultures of both methicillin-sensitive and methicillin-resistant S. aureus strains and subsequently shown to be effective in the context of an established biofilm, thus indicating that this approach could be used to facilitate the effective treatment of intrinsically resistant biofilm infections.

Regulatory Mutations Impacting Antibiotic Susceptibility in an Established Staphylococcus aureus Biofilm

We previously determined the extent to which mutations of different Staphylococcus aureus regulatory loci impact biofilm formation as assessed under in vitro conditions. Here we extend these studies to determine the extent to which those regulatory loci that had the greatest effect on biofilm formation also impact antibiotic susceptibility. The experiments were done under in vitro and in vivo conditions using two clinical isolates of S. aureus (LAC and UAMS-1) and two functionally diverse antibiotics (daptomycin and ceftaroline). Mutation of the staphylococcal accessory regulator (sarA) or sigB was found to significantly increase susceptibilities to both antibiotics and in both strains in a manner that could not be explained by changes in the MICs. The impact of a mutation in sarA was comparable to that of a mutation in sigB and greater than the impact observed with any other mutant. These results suggest that therapeutic strategies targeting sarA and/or sigB have the greatest potential to facilitate the ability to overcome the intrinsic antibiotic resistance that defines S. aureus biofilm-associated infections.

Comparative impact of diverse regulatory loci on Staphylococcus aureus biofilm formation

The relative impact of 23 mutations on biofilm formation was evaluated in the USA300, methicillin-resistant strain LAC. Mutation of sarA, atl, codY, rsbU, and sigB limited biofilm formation in comparison to the parent strain, but the limitation imposed by mutation of sarA was greater than that imposed by mutation of any of these other genes. The reduced biofilm formation of all mutants other than the atl mutant was correlated with increased levels of extracellular proteases. Mutation of fur- and mgrA-enhanced biofilm formation but in LAC had no impact on protease activity, nuclease activity, or accumulation of the polysaccharide intercellular adhesin (PIA). The increased capacity of these mutants to form a biofilm was reversed by mutation of sarA, and this was correlated with increased protease production. Mutation of sarA, mgrA, and sigB had the same phenotypic effect in the methicillin-sensitive strain UAMS-1, but mutation of codY increased rather than decreased biofilm formation. As with the UAMS-1 mgrA mutant, this was correlated with increased production of PIA. Examination of four additional clinical isolates suggests that the differential impact of codY on biofilm formation may be a conserved characteristic of methicillin-resistant versus methicillin-sensitive strains.

Characterization of local delivery with amphotericin B and vancomycin from modified chitosan sponges and functional biofilm prevention evaluation

Polymicrobial musculoskeletal wound infections are troublesome complications and can be difficult to treat when caused by invasive fungi or bacteria. However, few local antifungal delivery systems have been studied. Chitosan and polyethylene glycol (PEG) sponge local antifungal delivery systems have been developed for adjunctive therapy to reduce musculoskeletal wound contamination. This study evaluated the effects of blending PEG, at 6,000 or 8,000 g/mol, with chitosan in sponge form on in vitro amphotericin B and vancomycin elution, eluate activity, cytocompatibility, and in vivo prevention of a bacterial biofilm. Blended chitosan sponges released both amphotericin B and vancomycin in vitro. All tested amphotericin B eluates remained active against Candida albicans, and vancomycin eluates from blended sponges maintained activity against Staphylococcus aureus. Amphotericin B eluates obtained after 1 h from blended sponges elicited 62-95% losses in fibroblast viability, but 3 h eluates only caused 22-60% decreases in viability. In a Staphylococcus aureus infected mouse catheter biofilm prevention model, vancomycin loaded chitosan/PEG 6000 sponge cleared bacteria from 100% of the catheters, with reduced clearance rate observed in other sponges. These results indicate that the chitosan/PEG blended sponges have potential for local antifungal and/or antibiotic combination delivery as an adjunctive therapy to prevent wound infections.

Impact of individual extracellular proteases on Staphylococcus aureus biofilm formation in diverse clinical isolates and their isogenic sarA mutants

We demonstrate that the purified Staphylococcus aureus extracellular proteases aureolysin, ScpA, SspA, and SspB limit biofilm formation, with aureolysin having the greatest impact. Using protease-deficient derivatives of LAC, we confirmed that this is due to the individual proteases themselves. Purified aureolysin, and to a lesser extent ScpA and SspB, also promoted dispersal of an established biofilm. Mutation of the genes encoding these proteases also only partially restored biofilm formation in an FPR3757 sarA mutant and had little impact on restoring virulence in a murine bacteremia model. In contrast, eliminating the production of all of these proteases fully restored both biofilm formation and virulence in a sarA mutant generated in the closely related USA300 strain LAC. These results confirm an important role for multiple extracellular proteases in S. aureus pathogenesis and the importance of sarA in repressing their production. Moreover, purified aureolysin limited biofilm formation in 14 of 15 methicillin-resistant isolates and 11 of 15 methicillin-susceptible isolates, while dispersin B had little impact in UAMS-1, LAC, or 29 of 30 contemporary isolates of S. aureus. This suggests that the role of sarA and its impact on protease production is important in diverse strains of S. aureus irrespective of their methicillin resistance status.

Chitosan coating to enhance the therapeutic efficacy of calcium sulfate-based antibiotic therapy in the treatment of chronic osteomyelitis

We demonstrate that coating calcium sulfate with deacetylated chitosan enhances the elution profile of daptomycin by prolonging the period during which high concentrations of antibiotic are released. Coatings reduced initial bolus release of daptomycin by a factor of 10 to approximately 1000 µg/ml, and levels remained above 100 µg/ml for up to 10 days. Chitosan-coated and uncoated calcium sulfate implants with and without 15% daptomycin were evaluated in an experimental model of staphylococcal osteomyelitis through bacteriology scores, radiology, histopathology, and Gram staining. Significant reduction in bacteriology scores was observed for implants containing daptomycin and coated with chitosan compared with all the other groups. We confirm that the use of chitosan-coated calcium sulfate beads for local antibiotic delivery can be correlated with an improved therapeutic outcome following surgical debridement in the treatment of chronic osteomyelitis.

Impact of the functional status of saeRS on in vivo phenotypes of Staphylococcus aureus sarA mutants

We investigated the in vivo relevance of the impact of sarA and saeRS on protease production using derivatives of the USA300 strain LAC. The results confirmed that mutation of saeRS or sarA reduces virulence in a bacteremia model to a comparable degree. However, while eliminating protease production restored virulence in the sarA mutant, it had little impact in the saeRS mutant. Additionally, constitutive activation of saeRS (saeRS(C)) enhanced the virulence of LAC and largely restored virulence in the isogenic sarA mutant. Based on these results, together with our analysis of the representative virulence factors alpha toxin, protein A (Spa), and extracellular nucleases, we propose a model in which the attenuation of saeRS mutants is defined primarily by decreased production of such factors, while constitutive activation of saeRS increases virulence, and reverses the attenuation of sarA mutants, because it results in both increased production and decreased protease-mediated degradation of these same factors. This regulatory balance was also apparent in a murine model of catheter-associated infection, with the results suggesting that the impact of saeRS on nuclease production plays an important role during the early stages of these infections that is partially offset by increased protease production in sarA mutants.

Modeling the Tripartite Drug Efflux Pump Archetype: Structural and Functional Studies of the Macromolecular Constituents Reveal More Than Their Names Imply

It is a remarkable age in molecular biology when one can argue that our current understanding of a process is influenced as much by structural studies as it is by genetic and physiological manipulations. This statement is particularly poignant with membrane proteins for which structural knowledge has been long impeded by the inability to easily obtain crystal structures in a lipid matrix. Thus, several high-resolution structures of the components comprising tripartite multidrug efflux pumps from Escherichia coli and Pseudomonas aeruginosa are now available and were received with much acclaim over ever-evolving crystal structures of soluble, aqueous proteins. These structures, in conjunction with functional mutagenesis studies, have provided insight into substrate capture and binding domains and redefined the potential interactions between individual pump constituents. However, correct assembly of the components is still a matter of debate as is the functional contribution of each to the translocation of drug substrates over long distances spanning the Gram-negative cell envelope.

Quercus cerris extracts limit Staphylococcus aureus biofilm formation

ETHNOPHARMACOLOGICAL RELEVANCE

Quercus cerris L., Fagaceae has been used in traditional Mediterranean medicine for numerous purposes, including anti-infective therapies for diarrhea and wound care.

AIM OF THE STUDY

To evaluate the anti-staphylococcal activity of fractions of ethanolic extracts of Quercus cerris leaf and stem/fruit samples in models for biofilm and growth inhibition.

MATERIALS AND METHODS

Ethanolic extracts of Quercus cerris leaves and stems/fruits were prepared, resuspended in water and fractioned by successively partitioning with hexane, ethyl acetate and butanol. The ability of the fractions to inhibit Staphylococcus aureus biofilm formation was tested using static crystal violet staining methods and confocal laser scanning microscopy. Growth studies were conducted to determine if the diminished capacity to form a biofilm was related to growth inhibition.

RESULTS

The butanol extracts of both the leaf and stem/fruit samples were the most active, and at a dose of 200 μg/ml, the capacity to form a biofilm was limited to a level equivalent to that of the sarA mutant controls. Further examination of the impact of these fractions on Staphylococcus aureus growth revealed that biofilm inhibition by the leaf butanol fraction was due to its bacteriostatic activity. The stem/fruit butanol fraction, however, showed a limited impact on growth, thus demonstrating that biofilm inhibition in this case is not related to the bacteriostatic activity of the extract.

CONCLUSION

Our evaluation of a medicinal plant used in Mediterranean ethnotherapies for infectious disease has demonstrated significant activity in the inhibition of staphylococcal biofilm formation with a mechanism unrelated to staphylococcal growth inhibition. These results contribute towards validation of this botanical remedy and form the groundwork for future studies in the search for novel biofilm inhibiting drugs.

sarA-mediated repression of protease production plays a key role in the pathogenesis of Staphylococcus aureus USA300 isolates

Mutation of staphylococcal accessory regulator (sarA) results in increased production of extracellular proteases in Staphylococcus aureus, which has been correlated with decreased biofilm formation and decreased accumulation of extracellular toxins. We used murine models of implant-associated biofilm infection and S. aureus bacteraemia (SAB) to compare virulence of USA300 strain LAC, its isogenic sarA mutant, and derivatives of each of these strains with mutations in all 10 of the genes encoding recognized extracellular proteases. The sarA mutant was attenuated in both models, and this was reversed by eliminating production of extracellular proteases. To examine the mechanistic basis, we identified proteins impacted by sarA in a protease-dependent manner. We identified 253 proteins where accumulation was reduced in the sarA mutant compared with the parent strain, and was restored in the sarA/protease mutant. Additionally, in SAB, the LAC protease mutant exhibited a hypervirulent phenotype by comparison with the isogenic parent strain, demonstrating that sarA also positively regulates production of virulence factors, some of which are subject to protease-mediated degradation. We propose a model in which attenuation of sarA mutants is defined by their inability to produce critical factors and simultaneously repress production of extracellular proteases that would otherwise limit accumulation of virulence factors.

In vivo magnetic enrichment, photoacoustic diagnosis, and photothermal purging of infected blood using multifunctional gold and magnetic nanoparticles

Bacterial infections are a primary cause of morbidity and mortality worldwide. Bacteremia is a particular concern owing to the possibility of septic shock and the development of metastatic infections. Treatment of bacteremia is increasingly compromised by the emergence of antibiotic resistant strains, creating an urgent need for alternative therapy. Here, we introduce a method for in vivo photoacoustic (PA) detection and photothermal (PT) eradication of Staphylococcus aureus in tissue and blood. We show that this method could be applicable for label-free diagnosis and treatment of in the bloodstream using intrinsic near-infrared absorption of endogenous carotenoids with nonlinear PA and PT contrast enhancement. To improve sensitivity and specificity for detection of circulating bacteria cells (CBCs), two-color gold and multilayer magnetic nanoparticles with giant amplifications of PA and PT contrasts were functionalized with an antibody cocktail for molecular targeting of S. aureus surface-associated markers such as protein A and lipoprotein. With a murine model, the utility of this approach was demonstrated for ultrasensitive detection of CBCs with threshold sensitivity as low as 0.5 CBCs/mL, in vivo magnetic enrichment of CBCs, PT eradication of CBCs, and real-time monitoring of therapeutic efficacy by CBC counting. Our PA-PT nano-theranostic platform, which integrates in vivo multiplex targeting, magnetic enrichment, signal amplification, multicolor recognition, and feedback control, could be used as a biological tool to gain insights on dissemination pathways of CBCs, infection progression by bacteria re-seeding, and sepsis development and treatment, and could potentially be feasible in humans, especially using bypass schematic.

saeRS and sarA act synergistically to repress protease production and promote biofilm formation in Staphylococcus aureus

Mutation of the staphylococcal accessory regulator (sarA) limits biofilm formation in diverse strains of Staphylococcus aureus, but there are exceptions. One of these is the commonly studied strain Newman. This strain has two defects of potential relevance, the first being mutations that preclude anchoring of the fibronectin-binding proteins FnbA and FnbB to the cell wall, and the second being a point mutation in saeS that results in constitutive activation of the saePQRS regulatory system. We repaired these defects to determine whether either plays a role in biofilm formation and, if so, whether this could account for the reduced impact of sarA in Newman. Restoration of surface-anchored FnbA enhanced biofilm formation, but mutation of sarA in this fnbA-positive strain increased rather than decreased biofilm formation. Mutation of sarA in an saeS-repaired derivative of Newman (P18L) or a Newman saeRS mutant (ΔsaeRS) resulted in a biofilm-deficient phenotype like that observed in clinical isolates, even in the absence of surface-anchored FnbA. These phenotypes were correlated with increased production of extracellular proteases and decreased accumulation of FnbA and/or Spa in the P18L and ΔsaeRS sarA mutants by comparison to the Newman sarA mutant. The reduced accumulation of Spa was reversed by mutation of the gene encoding aureolysin, while the reduced accumulation of FnbA was reversed by mutation of the sspABC operon. These results demonstrate that saeRS and sarA act synergistically to repress the production of extracellular proteases that would otherwise limit accumulation of critical proteins that contribute to biofilm formation, with constitutive activation of saeRS limiting protease production, even in a sarA mutant, to a degree that can be correlated with increased enhanced capacity to form a biofilm. Although it remains unclear whether these effects are mediated directly or indirectly, studies done with an sspA::lux reporter suggest they are mediated at a transcriptional level.

The staphylococcal accessory regulator, SarA, is an RNA-binding protein that modulates the mRNA turnover properties of late-exponential and stationary phase Staphylococcus aureus cells

The modulation of mRNA turnover is gaining recognition as a mechanism by which Staphylococcus aureus regulates gene expression, but the factors that orchestrate alterations in transcript degradation are poorly understood. In that regard, we previously found that 138 mRNA species, including transcripts coding for the virulence factors protein A (spa) and collagen-binding protein (cna), are stabilized in a sarA-dependent manner during exponential phase growth, suggesting that SarA directly or indirectly affects the RNA turnover properties of these transcripts. Herein, we expanded our characterization of the effects of sarA on mRNA turnover during late-exponential and stationary phases of growth. Results revealed that the locus affects the RNA degradation properties of cells during both growth phases. Further, using gel mobility shift assays and RIP-Chip, it was found that SarA protein is capable of binding mRNA species that it stabilizes both in vitro and within bacterial cells. Taken together, these results suggest that SarA post-transcriptionally regulates S. aureus gene expression in a manner that involves binding to and consequently altering the mRNA turnover properties of target transcripts.

Ellagic acid derivatives from Rubus ulmifolius inhibit Staphylococcus aureus biofilm formation and improve response to antibiotics

BACKGROUND

Biofilms contribute to the pathogenesis of many forms of Staphylococcus aureus infection. Treatment of these infections is complicated by intrinsic resistance to conventional antibiotics, thus creating an urgent need for strategies that can be used for the prevention and treatment of biofilm-associated infections.

METHODOLOGY/PRINCIPAL FINDINGS

This study demonstrates that a botanical natural product composition (220D-F2) rich in ellagic acid and its derivatives can limit S. aureus biofilm formation to a degree that can be correlated with increased antibiotic susceptibility. The source of this composition is Rubus ulmifolius Schott. (Rosaceae), a plant used in complementary and alternative medicine in southern Italy for the treatment of skin and soft tissue infections. All S. aureus clonal lineages tested exhibited a reduced capacity to form a biofilm at 220D-F2 concentrations ranging from 50-200 µg/mL, which were well below the concentrations required to limit bacterial growth (530-1040 µg/mL). This limitation was therapeutically relevant in that inclusion of 220D-F2 resulted in enhanced susceptibility to the functionally-distinct antibiotics daptomycin, clindamycin and oxacillin. Testing with kidney and liver cell lines also demonstrated a lack of host cell cytotoxicity at concentrations of 220D-F2 required to achieve these effects.

CONCLUSIONS/SIGNIFICANCE

These results demonstrate that extract 220D-F2 from the root of Rubus ulmifolius can be used to inhibit S. aureus biofilm formation to a degree that can be correlated with increased antibiotic susceptibility without toxic effects on normal mammalian cells. Hence, 220D-F2 is a strong candidate for development as a botanical drug for use in the prevention and treatment of S. aureus biofilm-associated infections.

Small molecule inhibitors of Staphylococcus aureus RnpA alter cellular mRNA turnover, exhibit antimicrobial activity, and attenuate pathogenesis

Methicillin-resistant Staphylococcus aureus is estimated to cause more U.S. deaths annually than HIV/AIDS. The emergence of hypervirulent and multidrug-resistant strains has further amplified public health concern and accentuated the need for new classes of antibiotics. RNA degradation is a required cellular process that could be exploited for novel antimicrobial drug development. However, such discovery efforts have been hindered because components of the Gram-positive RNA turnover machinery are incompletely defined. In the current study we found that the essential S. aureus protein, RnpA, catalyzes rRNA and mRNA digestion in vitro. Exploiting this activity, high through-put and secondary screening assays identified a small molecule inhibitor of RnpA-mediated in vitro RNA degradation. This agent was shown to limit cellular mRNA degradation and exhibited antimicrobial activity against predominant methicillin-resistant S. aureus (MRSA) lineages circulating throughout the U.S., vancomycin intermediate susceptible S. aureus (VISA), vancomycin resistant S. aureus (VRSA) and other Gram-positive bacterial pathogens with high RnpA amino acid conservation. We also found that this RnpA-inhibitor ameliorates disease in a systemic mouse infection model and has antimicrobial activity against biofilm-associated S. aureus. Taken together, these findings indicate that RnpA, either alone, as a component of the RNase P holoenzyme, and/or as a member of a more elaborate complex, may play a role in S. aureus RNA degradation and provide proof of principle for RNA catabolism-based antimicrobial therapy.

The last decade has witnessed a mass downsizing in pharmaceutical antibiotic drug discovery initiatives. This has posed a major healthcare issue that will likely worsen with time; antibiotic resistant bacteria continue to emerge while advances in new therapeutic options languish. In the current body of work, we show that agents that limit bacterial RNA turnover have potential as a new class of antibiotics. More specifically, our findings indicate the essential bacterial protein, RnpA, exhibits in vitro ribonuclease activity and either alone and/or as a member of the RNase P holoenzyme, may contribute to the RNA degradation properties of Staphylococcus aureus, a predominant cause of hospital and community bacterial infections. Accordingly, using high throughput screening we identified small molecule inhibitors of RnpA's in vitro RNA degradation activity. One of these agents, RNPA1000, was shown to limit S. aureus mRNA turnover and growth. RNPA1000 also limited growth of other important Gram-positive bacterial pathogens, exhibited antimicrobial efficacy against biofilm associated S. aureus and protected against the S. aureus pathogenesis in an animal model of infection. When taken together, our results illustrate that components of the bacterial RNA degradation machinery have utility as antibiotic drug-discovery targets and that RNPA1000 may represent a progenitor of this new class of antibiotics.

Epistatic relationships between sarA and agr in Staphylococcus aureus biofilm formation

BACKGROUND

The accessory gene regulator (agr) and staphylococcal accessory regulator (sarA) play opposing roles in Staphylococcus aureus biofilm formation. There is mounting evidence to suggest that these opposing roles are therapeutically relevant in that mutation of agr results in increased biofilm formation and decreased antibiotic susceptibility while mutation of sarA has the opposite effect. To the extent that induction of agr or inhibition of sarA could potentially be used to limit biofilm formation, this makes it important to understand the epistatic relationships between these two loci.

METHODOLOGY/PRINCIPAL FINDINGS

We generated isogenic sarA and agr mutants in clinical isolates of S. aureus and assessed the relative impact on biofilm formation. Mutation of agr resulted in an increased capacity to form a biofilm in the 8325-4 laboratory strain RN6390 but had little impact in clinical isolates S. aureus. In contrast, mutation of sarA resulted in a reduced capacity to form a biofilm in all clinical isolates irrespective of the functional status of agr. This suggests that the regulatory role of sarA in biofilm formation is independent of the interaction between sarA and agr and that sarA is epistatic to agr in this context. This was confirmed by demonstrating that restoration of sarA function restored the ability to form a biofilm even in the corresponding agr mutants. Mutation of sarA in clinical isolates also resulted in increased production of extracellular proteases and extracellular nucleases, both of which contributed to the biofilm-deficient phenotype of sarA mutants. However, studies comparing different strains with and without proteases inhibitors and/or mutation of the nuclease genes demonstrated that the agr-independent, sarA-mediated repression of extracellular proteases plays a primary role in this regard.

CONCLUSIONS AND SIGNIFICANCE

The results we report suggest that inhibitors of sarA-mediated regulation could be used to limit biofilm formation in S. aureus and that the efficacy of such inhibitors would not be limited by spontaneous mutation of agr in the human host.

Factors contributing to the biofilm-deficient phenotype of Staphylococcus aureus sarA mutants

Mutation of sarA in Staphylococcus aureus results in a reduced capacity to form a biofilm, but the mechanistic basis for this remains unknown. Previous transcriptional profiling experiments identified a number of genes that are differentially expressed both in a biofilm and in a sarA mutant. This included genes involved in acid tolerance and the production of nucleolytic and proteolytic exoenzymes. Based on this we generated mutations in alsSD, nuc and sspA in the S. aureus clinical isolate UAMS-1 and its isogenic sarA mutant and assessed the impact on biofilm formation. Because expression of alsSD was increased in a biofilm but decreased in a sarA mutant, we also generated a plasmid construct that allowed expression of alsSD in a sarA mutant. Mutation of alsSD limited biofilm formation, but not to the degree observed with the corresponding sarA mutant, and restoration of alsSD expression did not restore the ability to form a biofilm. In contrast, concomitant mutation of sarA and nuc significantly enhanced biofilm formation by comparison to the sarA mutant. Although mutation of sspA had no significant impact on the ability of a sarA mutant to form a biofilm, a combination of protease inhibitors (E-64, 1-10-phenanthroline, and dichloroisocoumarin) that was shown to inhibit the production of multiple extracellular proteases without inhibiting growth was also shown to enhance the ability of a sarA mutant to form a biofilm. This effect was evident only when all three inhibitors were used concurrently. This suggests that the reduced capacity of a sarA mutant to form a biofilm involves extracellular proteases of all three classes (serine, cysteine and metalloproteases). Inclusion of protease inhibitors also enhanced biofilm formation in a sarA/nuc mutant, with the combined effect of mutating nuc and adding protease inhibitors resulting in a level of biofilm formation with the sarA mutant that approached that of the UAMS-1 parent strain. These results demonstrate that the inability of a sarA mutant to repress production of extracellular nuclease and multiple proteases have independent but cumulative effects that make a significant contribution to the biofilm-deficient phenotype of an S. aureus sarA mutant.

Transcriptional profiling of a Staphylococcus aureus clinical isolate and its isogenic agr and sarA mutants reveals global differences in comparison to the laboratory strain RN6390

The production of Staphylococcus aureus virulence factors is under the control of complex regulatory circuits. Most studies aimed at defining these regulatory networks have focused on derivatives of the strain NCTC 8325, most notably RN6390. However, all NCTC 8325 derivatives, including RN6390, possess an 11 bp deletion in rsbU. This deletion renders NCTC 8325 derivatives naturally sigma-factor-B deficient. Recent studies have shown that RN6390 is also deficient, in comparison to clinical isolates, with respect to biofilm formation, a process which is important for both pathogenesis and antimicrobial resistance. Based on these considerations, the authors carried out genome-scale transcriptional profiling, comparing RN6390 with the virulent rsbU-positive clinical isolate UAMS-1. The results revealed significant genome-wide differences in expression patterns between RN6390 and UAMS-1, and suggested that the overall transcriptional profile of UAMS-1 is geared toward expression of factors that promote colonization and biofilm formation. In contrast, the transcriptional profile of RN6390 was heavily influenced by RNAIII expression, resulting in a phenotype characterized by increased production of exoproteins, and decreased capacity to form a biofilm. The greater influence of agr in RN6390 relative to UAMS-1 was also evident when the transcriptional profile of UAMS-1 was compared with that of its isogenic sarA and agr mutants. Specifically, the results indicate that, in contrast to NCTC 8325 derivatives, agr plays a limited role in overall regulation of gene expression in UAMS-1, when compared with sarA. Furthermore, by defining the sarA regulon in a biofilm-positive clinical isolate, and comparing the results with transcriptional profiling experiments defining biofilm-associated gene expression patterns in the same strain, the authors identified a sarA-regulated operon (alsSD) that is also induced in biofilms, and demonstrated that mutation of alsSD results in reduced capacity to form a biofilm.

Modeling the Tripartite Drug Efflux Pump Archetype: Structural and Functional Studies of the Macromolecular Constituents Reveal More Than Their Names Imply

It is a remarkable age in molecular biology when one can argue that our current understanding of a process is influenced as much by structural studies as it is by genetic and physiological manipulations. This statement is particularly poignant with membrane proteins for which structural knowledge has been long impeded by the inability to easily obtain crystal structures in a lipid matrix. Thus, several highresolution structures of the components comprising tripartite multidrug efflux pumps from Escherichia coli and Pseudomonas aeruginosa are now available and were received with much acclaim over ever-evolving crystal structures of soluble, aqueous proteins. These structures, in conjunction with functional mutagenesis studies, have provided insight into substrate capture and binding domains and redefined the potential interactions between individual pump constituents. However, correct assembly of the components is still a matter of debate as is the functional contribution of each to the translocation of drug substrates over long distances spanning the Gram-negative cell envelope.

Global gene expression in Staphylococcus aureus biofilms

We previously demonstrated that mutation of the staphylococcal accessory regulator (sarA) in a clinical isolate of Staphylococcus aureus (UAMS-1) results in an impaired capacity to form a biofilm in vitro (K. E. Beenken, J. S. Blevins, and M. S. Smeltzer, Infect. Immun. 71:4206-4211, 2003). In this report, we used a murine model of catheter-based biofilm formation to demonstrate that a UAMS-1 sarA mutant also has a reduced capacity to form a biofilm in vivo. Surprisingly, mutation of the UAMS-1 ica locus had little impact on biofilm formation in vitro or in vivo. In an effort to identify additional loci that might be relevant to biofilm formation and/or the adaptive response required for persistence of S. aureus within a biofilm, we isolated total cellular RNA from UAMS-1 harvested from a biofilm grown in a flow cell and compared the transcriptional profile of this RNA to RNA isolated from both exponential- and stationary-phase planktonic cultures. Comparisons were done using a custom-made Affymetrix GeneChip representing the genomic complement of six strains of S. aureus (COL, N315, Mu50, NCTC 8325, EMRSA-16 [strain 252], and MSSA-476). The results confirm that the sessile lifestyle associated with persistence within a biofilm is distinct by comparison to the lifestyles of both the exponential and postexponential phases of planktonic culture. Indeed, we identified 48 genes in which expression was induced at least twofold in biofilms over expression under both planktonic conditions. Similarly, we identified 84 genes in which expression was repressed by a factor of at least 2 compared to expression under both planktonic conditions. A primary theme that emerged from the analysis of these genes is that persistence within a biofilm requires an adaptive response that limits the deleterious effects of the reduced pH associated with anaerobic growth conditions.

Mutation of sarA in Staphylococcus aureus limits biofilm formation

Mutation of sarA resulted in a reduced capacity to form a biofilm in six of the eight Staphylococcus aureus strains we tested (UAMS-1, UAMS-601, SA113, SC-01, S6C, and DB). The exceptions were Newman, which formed a poor biofilm under all conditions, and RN6390, which consistently formed a biofilm only after mutation of agr. Mutation of agr in other strains had little impact on biofilm formation. In every strain other than Newman, including RN6390, simultaneous mutation of sarA and agr resulted in a phenotype like that observed with the sarA mutants. Complementation studies using a sarA clone confirmed that the defect in biofilm formation was due to the sarA mutation.

Role of sarA in the pathogenesis of Staphylococcus aureus musculoskeletal infection

We recently demonstrated that mutation of sarA in clinical isolates of Staphylococcus aureus results in a phenotype that is distinct by comparison to sarA mutants generated in the laboratory strain RN6390 (J. S. Blevins, K. E. Beenken, M. O. Elasri, B. K. Hurlburt, and M. S. Smeltzer, Infect. Immun. 70:470-480, 2002). This raises the possibility that studies demonstrating that RN6390 sarA mutants are attenuated do not accurately reflect the role of sarA in the pathogenesis of staphylococcal disease. To test this hypothesis, we used a murine model of musculoskeletal infection to assess the virulence of sarA and agr mutants generated in a clinical isolate of S. aureus (UAMS-1). By using this model, we confirmed that mutation of sarA and/or agr results in a reduced capacity to cause both septic arthritis and osteomyelitis.

Strain-dependent differences in the regulatory roles of sarA and agr in Staphylococcus aureus

The accessory gene regulator (agr) and the staphylococcal accessory regulator (sar) are central regulatory elements that control the production of Staphylococcus aureus virulence factors. To date, the functions of these loci have been defined almost exclusively using RN6390, which is representative of the laboratory strain 8325-4. However, RN6390 was recently shown to have a mutation in rsbU that results in a phenotype resembling that of a sigB mutant (I. Kullik et al., J. Bacteriol. 180:4814-4820, 1998). For that reason, it remains unclear whether the regulatory events defined in RN6390 are representative of the events that take place in clinical isolates of S. aureus. To address this issue, we generated mutations in the sarA and agr loci of three laboratory strains (RN6390, Newman, and S6C) and four clinical isolates (UAMS-1, UAMS-601, DB, and SC-1). Mutation of sarA in the cna-positive strains UAMS-1 and UAMS-601 resulted in an increased capacity to bind collagen, while mutation of agr had little impact. Northern blot analysis confirmed that the increase in collagen binding was due to increased cna transcription. Without exception, mutation of sarA resulted in increased production of proteases and a decreased capacity to bind fibronectin. Mutation of agr had the opposite effect. Although mutation of sarA resulted in a slight reduction in fnbA transcription, changes in the ability to bind fibronectin appeared to be more directly correlated with changes in protease activity. Lipase production was reduced in both sarA and agr mutants. While mutation of sarA in RN6390 resulted in reduced hemolytic activity, it had the opposite effect in all other strains. There appeared to be reduced levels of the sarC transcript in RN6390, but there was no difference in the overall pattern of sar transcription or the production of SarA. Although mutation of sarA resulted in decreased RNAIII transcription, this effect was not evident under all growth conditions. Taken together, these results suggest that studies defining the regulatory roles of sarA and agr by using RN6390 are not always representative of the events that occur in clinical isolates of S. aureus.

Staphylococcus aureus collagen adhesin contributes to the pathogenesis of osteomyelitis

To evaluate the role of the Staphylococcus aureus collagen-binding adhesin (Cna) in bone and joint infection, we generated a cna mutant in S. aureus UAMS-1, a strain that was originally isolated from the bone of a patient suffering from osteomyelitis. The mutant (UAMS-237) was unable to bind collagen but bound fibronectin at levels comparable to UAMS-1. The relative virulence of UAMS-1 and UAMS-237 was assessed using a murine model of acute hematogenous osteomyelitis. Specifically, 10(8) colony-forming units (cfu) were introduced into the bloodstream of NIH-Swiss mice via tail-vein injection. After 2 weeks, the left hind limb was harvested and examined histologically for evidence of osteomyelitis and septic arthritis. Osteomyelitis developed in 14 of 20 mice (70%) infected with UAMS-1, but only 1 of 20 (5%) infected with UAMS-237 (p < 0.001). In contrast, septic arthritis was observed in 12 of 20 mice (60%) infected with UAMS-1 and 14 of 20 (70%) infected with UAMS-237 (p < 0.75). These results indicate that Cna is not required to establish joint infection, but does make an important contribution to the ability of S. aureus to establish infection in bone through hematogenous spread.