Role of sarA in the pathogenesis of Staphylococcus aureus musculoskeletal infection

1H, 13C, and 15N resonance assignments of SarA monomer from Staphylococcus aureus in complex with DNA

SarA is a global transcription regulator in S. aureus which regulates the expression of over 120 genes related to quorum sensing, biofilm synthesis, drug resistance and many other important physiological processes during host infection. SarA can bind to the promoter region of agr and other target genes to activate or repress the transcription. The crystal structure of SarA uncovered a MarR protein-like conformation with two symmetrical winged helix domains, while its DNA binding mechanism is still unknown. We have constructed a monomeric DNA binding domain of SarA (SarAΔN19) for the study of the interaction between SarA and DNA with NMR spectroscopy. Here, we report the 1H, 13C and 15N NMR assignment of SarAΔN19/DNA complex which is the first step towards further structure and function analysis.

Choosing the right animal model for osteomyelitis research: Considerations and challenges

Osteomyelitis is a debilitating bone disorder characterized by an inflammatory process involving the bone marrow, bone cortex, periosteum, and surrounding soft tissue, which can ultimately result in bone destruction. The etiology of osteomyelitis can be infectious, caused by various microorganisms, or noninfectious, such as chronic nonbacterial osteomyelitis (CNO) and chronic recurrent multifocal osteomyelitis (CRMO). Researchers have turned to animal models to study the pathophysiology of osteomyelitis. However, selecting an appropriate animal model that accurately recapitulates the human pathology of osteomyelitis while controlling for multiple variables that influence different clinical presentations remains a significant challenge. In this review, we present an overview of various animal models used in osteomyelitis research, including rodent, rabbit, avian/chicken, porcine, minipig, canine, sheep, and goat models. We discuss the characteristics of each animal model and the corresponding clinical scenarios that can provide a basic rationale for experimental selection. This review highlights the importance of selecting an appropriate animal model for osteomyelitis research to improve the accuracy of the results and facilitate the development of novel treatment and management strategies.

Staphylococcus aureus From an Acute Fracture-related Infection Displays Important Bacteriological and Histopathologic Differences From a Chronic Equivalent in a Murine Bone Infection Model

BACKGROUND

Staphylococcus aureus is the leading pathogen in fracture-related infection. Previous in vitro experiments, in vivo testing in wax moth larvae, and genomic analysis of clinical S. aureu s isolates from fracture-related infection identified low-virulence (Lo-SA5464) and high-virulence (Hi-SA5458) strains. These findings correlated with acute fracture-related infection induced by Hi-SA5458, whereas Lo-SA5464 caused a chronic fracture-related infection in its human host. However, it remains unclear whether and to what extent the causative pathogen is attributable to these disparities in fracture-related infections.

QUESTION/PURPOSE

Are there differences in the course of infection when comparing these two different clinical isolates in a murine fracture-related infection model, as measured by (1) clinical observations of weight loss, (2) quantitative bacteriology, (3) immune response, and (4) radiographic and histopathologic morphology?

METHODS

Twenty-five (including one replacement animal) female (no sex-specific influences expected), skeletally mature C57Bl/6N inbred mice between 20 and 28 weeks old underwent femoral osteotomy stabilized by titanium locking plates. Fracture-related infection was established by inoculation of high-virulence S. aureus EDCC 5458 (Hi-SA5458) or low-virulence S. aureus EDCC 5464 (Lo-SA5464) in the fracture gap. Each of these groups consisted of 12 randomly assigned animals. Mice were euthanized 4 and 14 days postsurgery, resulting in six animals per group and timepoint. The severity and progression of infection were assessed in terms of clinical observation of weight loss, quantitative bacteriology, quantitative serum cytokine levels, qualitative analysis of postmortem radiographs, and semiquantitative histopathologic evaluation.

RESULTS

For clinical observations of weight change, no differences were seen at Day 4 between Hi-SA5458- and Lo-SA5464-infected animals (mean -0.6 ± 0.1 grams versus -0.8 ± 0.2 grams, mean difference -0.2 grams [95% CI -0.8 to 0.5 grams]; p =0.43), while at 14 days, the Hi-SA5458 group lost more weight than the Lo-SA5464 group (mean -1.55 ± 0.2 grams versus -0.8 ± 0.3 grams; mean difference 0.7 grams [95% CI 0.2 to 1.3 grams]; p = 0.02). Quantitative bacteriological results 4 days postoperatively revealed a higher bacterial load in soft tissue samples in Hi-SA5458-infected animals than in the Lo-SA5464-infected cohort (median 6.8 x 10 7 colony-forming units [CFU]/g, range 2.2 x 10 7 to 2.1 x 10 9 CFU/g versus median 6.0 x 10 6 CFU/g, range 1.8 x 10 5 to 1.3 x 10 8 CFU/g; difference of medians 6.2 x 10 7 CFU/g; p = 0.03). At both timepoints, mice infected with the Hi-SA5458 strain also displayed higher proportions of bacterial dissemination into organs than Lo-SA5464-infected animals (67% [24 of 36 organs] versus 14% [five of 36 organs]; OR 12.0 [95% CI 3.7 to 36]; p < 0.001). This was accompanied by a pronounced proinflammatory response on Day 14, indicated by increased serum cytokine levels of interleukin-1β (mean 9.0 ± 2.2 pg/mL versus 5.3 ± 1.5 pg/mL; mean difference 3.6 pg/mL [95% CI 2.0 to 5.2 pg/mL]; p < 0.001), IL-6 (mean 458.6 ± 370.7 pg/mL versus 201.0 ±89.6 pg/mL; mean difference 257.6 pg/mL [95% CI 68.7 to 446.5 pg/mL]; p = 0.006), IL-10 (mean 15.9 ± 3.5 pg/mL versus 9.9 ± 1.0 pg/mL; mean difference 6.0 pg/mL [95% CI 3.2 to 8.7 pg/mL]; p < 0.001), and interferon-γ (mean 2.7 ± 1.9 pg/mL versus 0.8 ± 0.3 pg/mL; mean difference 1.8 pg/mL [95% CI 0.5 to 3.1 pg/mL]; p = 0.002) in Hi-SA5458-infected compared with Lo-SA5464-infected animals. The semiquantitative histopathologic assessment on Day 4 revealed higher grades of granulocyte infiltration in Hi-SA5458-infected animals (mean grade 2.5 ± 1.0) than in Lo-SA5464-infected animals (mean grade 1.8 ± 1.4; mean difference 0.7 [95% CI 0.001 to 1.4]; p = 0.0498). On Day 14, bone healing at the fracture site was present to a higher extent in Lo-SA5464-infected animals than in Hi-SA5458-infected animals (mean grade 0.2 ± 0.4 versus 1.8 ± 1.2; mean difference -1.6 [95% CI -2.8 to -0.5]; p = 0.008).

CONCLUSION

Similar to septic infection in a human host, infection with Hi-SA5458 in this murine model was characterized by a higher bacterial load, more-pronounced systemic dissemination, and stronger systemic and local inflammation. Thus, there is strong support for the idea that pathogenic virulence plays a crucial role in fracture-related infections. To confirm our observations, future studies should focus on characterizing S. aureus virulence at the genomic and transcriptomic levels in more clinical isolates and patients. Comparing knockout and wildtype strains in vitro and in vivo, including the S. aureus strains studied, could confirm our findings and identify the genomic features responsible for S. aureus virulence in fracture-related infections.

CLINICAL RELEVANCE

For translational use, virulence profiles of S. aureus may be useful in guiding treatment decisions in the future. Once specific virulence targets are identified, one approach to fracture-related infections with high-virulence strains might be the development of antivirulence agents, particularly to treat or prevent septic dissemination. For fracture-related infections with low virulence, prolonged antimicrobial therapy or exchange of an indwelling implant might be beneficial owing to slower growth and persistence capacity.

A Journey into Animal Models of Human Osteomyelitis: A Review

Osteomyelitis is an infection of the bone characterized by progressive inflammatory destruction and apposition of new bone that can spread via the hematogenous route (hematogenous osteomyelitis (HO)), contiguous spread (contiguous osteomyelitis (CO)), and direct inoculation (osteomyelitis associated with peripheral vascular insufficiency (PVI)). Given the significant financial burden posed by osteomyelitis patient management, the development of new preventive and treatment methods is warranted. To achieve this objective, implementing animal models (AMs) of infection such as rats, mice, rabbits, avians, dogs, sheep, goats, and pigs might be of the essence. This review provides a literature analysis of the AMs developed and used to study osteomyelitis. Historical relevance and clinical applicability were taken into account to choose the best AMs, and some study methods are briefly described. Furthermore, the most significant strengths and limitations of each species as AM are discussed, as no single model incorporates all features of osteomyelitis. HO's clinical manifestation results in extreme variability between patients due to multiple variables (e.g., age, sex, route of infection, anatomical location, and concomitant diseases) that could alter clinical studies. However, these variables can be controlled and tested through different animal models.

Antibiofilm and staphyloxanthin inhibitory potential of terbinafine against Staphylococcus aureus: in vitro and in vivo studies

Background

Antimicrobial resistance is growing substantially, which necessitates the search for novel therapeutic options. Terbinafine, an allylamine antifungal agent that exhibits a broad spectrum of activity and is used in the treatment of dermatophytosis, could be a possible option to disarm S. aureus virulence.

Methods

Terbinafine inhibitory effect on staphyloxanthin was characterized by quantitative measurement of staphyloxanthin intermediates and molecular docking. The effect of terbinafine on S. aureus stress survival was characterized by viable counting. The anti-biofilm activity of terbinafine on S. aureus was assessed by the crystal violet assay and microscopy. Changes in S. aureus membrane following treatment with terbinafine were determined using Fourier transform infrared (FTIR) analysis. The synergistic action of terbinafine in combination with conventional antibiotics was characterized using the checkerboard assay. qRT-PCR was used to evaluate the impact of terbinafine on S. aureus gene expression. The influence of terbinafine on S. aureus pathogenesis was investigated in mice infection model.

Results

Terbinafine inhibits staphyloxanthin biosynthesis through targeting dehydrosqualene desaturase (CrtN). Docking analysis of terbinafine against the predicted active site of CrtN reveals a binding energy of − 9.579 kcal/mol exemplified by the formation of H-bonds, H-arene bonds, and hydrophobic/hydrophilic interactions with the conserved amino acids of the receptor pocket. Terbinafine treated S. aureus was more susceptible to both oxidative and acid stress as well as human blood killing as compared to untreated cells. Targeting staphyloxanthin by terbinafine rendered S. aureus more sensitive to membrane acting antibiotics. Terbinafine interfered with S. aureus biofilm formation through targeting cell autoaggregation, hydrophobicity, and exopolysaccharide production. Moreover, terbinafine demonstrated a synergistic interaction against S. aureus when combined with conventional antibiotics. Importantly, terbinafine attenuated S. aureus pathogenesis using mice infection model. qRT-PCR revealed that terbinafine repressed expression of the transcriptional regulators sigB, sarA, and msaB, as well as icaA in S. aureus.

Conclusions

Present findings strongly suggest that terbinafine could be used safely and efficiently as an anti-virulent agent to combat S. aureus infections.

Celastrol mitigates staphyloxanthin biosynthesis and biofilm formation in Staphylococcus aureus via targeting key regulators of virulence; in vitro and in vivo approach

Background

Staphylococcus aureus is a leading cause of human infections. The spread of antibiotic-resistant staphylococci has driven the search for novel strategies to supersede antibiotics use. Thus, targeting bacterial virulence rather than viability could be a possible alternative.

Results

The influence of celastrol on staphyloxanthin (STX) biosynthesis, biofilm formation, antibiotic susceptibility and host pathogenesis in S. aureus has been investigated. Celastrol efficiently reduced STX biosynthesis in S. aureus. Liquid chromatography-mass spectrometry (LC–MS) and molecular docking revealed that celastrol inhibits STX biosynthesis through its effect on CrtM. Quantitative measurement of STX intermediates showed a significant pigment inhibition via interference of celastrol with CrtM and accumulation of its substrate, farnesyl diphosphate. Importantly, celastrol-treated S. aureus was more sensitive to environmental stresses and human blood killing than untreated bacteria. Similarly, inhibition of STX upon celastrol treatment rendered S. aureus more susceptible to membrane targeting antibiotics. In addition to its anti-pigment capability, celastrol exhibits significant anti-biofilm activity against S. aureus as indicated by crystal violet assay and microscopy. Celastrol-treated cells showed deficient exopolysaccharide production and cell hydrophobicity. Moreover, celastrol markedly synergized the action of conventional antibiotics against S. aureus and reduced bacterial pathogenesis in vivo using mice infection model. These findings were further validated using qRT-PCR, demonstrating that celastrol could alter the expression of STX biosynthesis genes as well as biofilm formation related genes and bacterial virulence.

Conclusions

Celastrol is a novel anti-virulent agent against S. aureus suggesting, a prospective therapeutic role for celastrol as a multi-targeted anti-pathogenic agent.

A naturally occurring point mutation in the hyaluronidase gene (hysA1) of Staphylococcus aureus UAMS-1 results in reduced enzymatic activity

Hyaluronic acid is a high-molecular-weight polysaccharide that is widely distributed in animal tissues. Bacterial hyaluronidases degrade hyaluronic acid as secreted enzymes and have been shown to contribute to infection. Staphylococcus aureus UAMS-1 is a clinical isolate that codes for two hyaluronidases (hysA1 and hysA2). Previous research has shown the presence of a full-length HysA1 protein from the S. aureus UAMS-1 strain with no evidence of enzymatic activity. In this study, the coding and upstream promoter regions of hysA1 from the S. aureus UAMS-1 strain were cloned, sequenced, and compared to the hysA1 gene from the S. aureus Sanger 252 strain. A single base change resulting in an E480G amino acid change was identified in the hysA1 gene from the S. aureus UAMS-1 strain when compared to the hysA1 gene from S. aureus Sanger 252. A plasmid copy of hysA1 from S. aureus Sanger 252 transduced into an S. aureus UAMS-1 hysA2 deletion mutant strain restored near wild-type levels of enzymatic activity. Homology modeling of the HysA1 hyaluronidase was performed with SWISS-MODEL using hyaluronidase from Streptococcus pneumoniae as the template, followed by a series of structural analyses using PyMOL, PLIP, PDBsum, and HOPE servers. This glutamic acid is highly conserved among hyaluronidases from Staphylococcus and other gram-positive bacteria. A series of structural analyses suggested that Glu-480 in HysA1 is critically responsible for maintaining the structural and functional ensemble of the catalytic and tunnel-forming residues, which are essential for enzyme activity. The missense mutation of Glu-480 to Gly introduces a loss of side chain hydrogen bond interactions with key residues Arg-360 and Arg-364, which are responsible for the tunnel topology, resulting in displacement of the substrate from an ideal position for catalysis through a localized conformational change of the active site. There is a high degree of relatedness among several gram-positive bacterial hyaluronidases; the loss of enzymatic activity of HysA1 in the S. aureus UAMS-1 strain is most likely caused by the mutation identified in our study. The role of hyaluronidase in staphylococcal infection and the redundancy of this gene are yet to be determined.

Assessment of eugenol inhibitory effect on biofilm formation and biofilm gene expression in methicillin resistant Staphylococcus aureus clinical isolates in Egypt

Methicillin-resistant Staphylococcus aureus (MRSA) biofilm infection is a major threat in Healthcare facilities. The search for biofilm inhibitors is essential to overcome the antibiotic resistance. Eugenol is a phyto-compound that possesses many biological properties. In this study, the aim was to estimate the effect of eugenol on biofilms of MRSA through quantifying the level of gene expression of three genes (IcaA, IcaD and SarA) involved in biofilm development.. Fifty MRSA biofilm producers collected from the microbiology lab at Theodor Bilharz Research Institute were incubated with different concentrations of eugenol for 24 h. The minimum inhibitory concentration of eugenol (MIC) that eradicates the biofilms growth was detected. mRNA was extracted from all isolates before and after the application of eugenol at 0.5 x MIC, and then subjected to quantitative real-time PCR (qPCR). Results showed that fourteen isolates out of 50 (28%) exhibited intermediate biofilm formation ability, and 36 out of 50 (72%) were strong biofilm producers. The MIC values of eugenol for MRSA ranged from 3.125% to 0.01%. The mean values of MIC in both strong and intermediate biofilm forming MRSA isolates were statistically comparable (p = 0.202). qPCR results revealed that the levels of expression of the studied genes IcaA, IcaD, and SarA were decreased after eugenol treatment when compared with their corresponding values before treatment (p = 0.001). Eugenol inhibited the formation of biofilm of MRSA isolates, indicating it could be used to control infections associated with MRSA biofilms.

Quorum Sensing and Toxin Production in Staphylococcus aureus Osteomyelitis: Pathogenesis and Paradox

Staphylococcus aureus is a Gram-positive pathogen capable of infecting nearly every vertebrate organ. Among these tissues, invasive infection of bone (osteomyelitis) is particularly common and induces high morbidity. Treatment of osteomyelitis is notoriously difficult and often requires debridement of diseased bone in conjunction with prolonged antibiotic treatment to resolve infection. During osteomyelitis, S. aureus forms characteristic multicellular microcolonies in distinct niches within bone. Virulence and metabolic responses within these multicellular microcolonies are coordinated, in part, by quorum sensing via the accessory gene regulator (agr) locus, which allows staphylococcal populations to produce toxins and adapt in response to bacterial density. During osteomyelitis, the Agr system significantly contributes to dysregulation of skeletal homeostasis and disease severity but may also paradoxically inhibit persistence in the host. Moreover, the Agr system is subject to complex crosstalk with other S. aureus regulatory systems, including SaeRS and SrrAB, which can significantly impact the progression of osteomyelitis. The objective of this review is to highlight Agr regulation, its implications on toxin production, factors that affect Agr activation, and the potential paradoxical influences of Agr regulation on disease progression during osteomyelitis.

Staphylococcus aureus Osteomyelitis: Bone, Bugs, and Surgery

Osteomyelitis, or inflammation of bone, is most commonly caused by invasion of bacterial pathogens into the skeleton. Bacterial osteomyelitis is notoriously difficult to treat, in part because of the widespread antimicrobial resistance in the preeminent etiologic agent, the Gram-positive bacterium Staphylococcus aureus Bacterial osteomyelitis triggers pathological bone remodeling, which in turn leads to sequestration of infectious foci from innate immune effectors and systemically delivered antimicrobials. Treatment of osteomyelitis therefore typically consists of long courses of antibiotics in conjunction with surgical debridement of necrotic infected tissues. Even with these extreme measures, many patients go on to develop chronic infection or sustain disease comorbidities. A better mechanistic understanding of how bacteria invade, survive within, and trigger pathological remodeling of bone could therefore lead to new therapies aimed at prevention or treatment of osteomyelitis as well as amelioration of disease morbidity. In this minireview, we highlight recent developments in our understanding of how pathogens invade and survive within bone, how bacterial infection or resulting innate immune responses trigger changes in bone remodeling, and how model systems can be leveraged to identify new therapeutic targets. We review the current state of osteomyelitis epidemiology, diagnostics, and therapeutic guidelines to help direct future research in bacterial pathogenesis.

Structure-Activity Relationship Studies of Small Molecule Modulators of the Staphylococcal Accessory Gene Regulator

The accessory gene regulator (agr) quorum-sensing system is arguably the most important regulator of Staphylococcus virulence. The agr-system serves a crucial role in pathogenesis by triggering substantive gene expression alterations to up-regulate the production of a wide variety of virulence determinants such as exoenzymes (proteases, lipases, nucleases) and downregulate the expression of surface binding proteins. Accordingly, the agr-system represents a compelling target for the development of antivirulence therapeutics as potential adjuncts, or alternatives, to conventional bactericidal and bacteriostatic antibiotics. Despite this potential, to date, no agr-system inhibitors have progressed to the clinic; however, several promising lead compounds have been identified through screens of synthetic and natural product libraries. On the basis of the molecular components within the agr-system, the current contingent of regulating compounds can be clustered into three broad groups, AgrA-P3 activation inhibitors, AgrB-AgrD processing inhibitors, and AgrC-AIP interaction inhibitors. This review aims to provide an overview of the development, structure-activity-relationships, and limitations of compounds within each of these groups in addition to the current opportunities for developing next-generation anologs.

Hypericin enhances β-lactam antibiotics activity by inhibiting sarA expression in methicillin-resistant Staphylococcus aureus

Bacteremia is a life-threating syndrome often caused by methicillin-resistant Staphylococcus aureus (MRSA). Thus, there is an urgent need to develop novel approaches to successfully treat this infection. Staphylococcal accessory regulator A (SarA), a global virulence regulator, plays a critical role in pathogenesis and β-lactam antibiotic resistance in Staphylococcus aureus. Hypericin is believed to act as an antibiotic, antidepressant, antiviral and non-specific kinase inhibitor. In the current study, we investigated the impact of hypericin on β-lactam antibiotics susceptibility and mechanism(s) of its activity. We demonstrated that hypericin significantly decreased the minimum inhibitory concentrations of β-lactam antibiotics (e.g., oxacillin, cefazolin and nafcillin), biofilm formation and fibronectin binding in MRSA strain JE2. In addition, hypericin significantly reduced sarA expression, and subsequently decreased mecA, and virulence-related regulators (e.g., agr RNAⅢ) and genes (e.g., fnbA and hla) expression in the studied MRSA strain. Importantly, the in vitro synergistic effect of hypericin with β-lactam antibiotic (e.g., oxacillin) translated into in vivo therapeutic outcome in a murine MRSA bacteremia model. These findings suggest that hypericin plays an important role in abrogation of β-lactam resistance against MRSA through sarA inhibition, and may allow us to repurpose the use of β-lactam antibiotics, which are normally ineffective in the treatment of MRSA infections (e.g., oxacillin).

Screening of genes encoding adhesion factors and biofilm production in methicillin resistant strains of Staphylococcus aureus isolated from Palestinian patients

BACKGROUND

Intercellular adhesion and biofilm production by Staphylococcus aureus makes these bacteria resistant to antimicrobial therapy. Here, Methicillin-resistant Staphylococcus aureus (MRSA) strains were characterized and the prevalence of genes encoding adhesion factors and biofilm formation was determined.

RESULTS

All 248 MRSA isolates identified by cefoxitin disc diffusion were positive for the mecA gene. SCCmec-IV was the most frequently detected genotype (92.7%) and SCCmec-IVa was also very prevalent (84.3%). The quantitative microtiter plate assay showed that all the isolates were able to produce biofilm with levels ranging from high (21%) to moderate (46.4%) to low (32.7%). All the strains possessed the icaD/icaA genes and produced biofilm (P < 0.05). None of the isolates possessed the bap gene. Furthermore, 94.8% of the isolates were positive for eno, 80.2% for clfA and for clfB, 78.2% for fnbA, 76.2% for ebps, 62.2% for fib, 39.9% for cna and 29.0% for fnbB. Also, nearly 69.8% of the isolates were positive for the gene sarA. All four agr groups were present: agr group 1 was predominant with 39.5%; agr group 3. agr group 2 and 3 strains carried more toxin-producing genes, and frequently produced more toxin. Sixty-six (26.6%) of the strains were multidrug resistant. All were vancomycin sensitive. Agr group I is more resistant to ciprofloxacin and gentamicin while agr group III is more resistant to erythromycin. Maximum sensitivity was to gentamicin and SXT, and they could be considered drugs of choice for controlling MRSA mediated infections in this region.

CONCLUSIONS

Biofilm development in MRSA might be an ica dependent and one needs to investigate the involvement of other global regulators, agr and sarA, and their contribution to the biofilm phenotype, as the high rate of biofilm production among the studied strains of S. aureus.

Staphylococcus aureus Quorum Regulator SarA Targeted Compound, 2-[(Methylamino)methyl]phenol Inhibits Biofilm and Down-Regulates Virulence Genes

Staphylococcus aureus is a widely acknowledged Gram-positive pathogen for forming biofilm and virulence gene expressions by quorum sensing (QS), a cell to cell communication process. The quorum regulator SarA of S. aureus up-regulates the expression of many virulence factors including biofilm formation to mediate pathogenesis and evasion of the host immune system in the late phases of growth. Thus, inhibiting the production or blocking SarA protein might influence the down-regulation of biofilm and virulence factors. In this context, here we have synthesized 2-[(Methylamino)methyl]phenol, which was specifically targeted toward the quorum regulator SarA through in silico approach in our previous study. The molecule has been evaluated in vitro to validate its antibiofilm activity against clinical S. aureus strains. In addition, antivirulence properties of the inhibitor were confirmed with the observation of a significant reduction in the expression of representative virulence genes like fnbA, hla and hld that are governed under S. aureus QS. Interestingly, the SarA targeted inhibitor showed negligible antimicrobial activity and markedly reduced the minimum inhibitory concentration of conventional antibiotics when used in combination making it a more attractive lead for further clinical tests.

Insights into the regulation of small RNA expression: SarA represses the expression of two sRNAs in Staphylococcus aureus

The opportunistic pathogen Staphylococcus aureus expresses transcription factors (TFs) and regulatory small RNAs (sRNAs) which are essential for bacterial adaptation and infectivity. Until recently, the study of S. aureus sRNA gene expression regulation was under investigated, but it is now an expanding field. Here we address the regulation of Srn_3610_SprC sRNA, an attenuator of S. aureus virulence. We demonstrate that SarA TF represses srn_3610_sprC transcription. DNase I footprinting and deletion analyses show that the SarA binding site on srn_3610_sprC belongs to an essential 22 bp DNA region. Comparative analysis also revealed another possible site, this time in the srn_9340 promoter. SarA specifically binds these two sRNA promoters with high affinity in vitro and also represses their transcription in vivo. Chromatin immunoprecipitation (ChIP) assays confirmed SarA attachment to both promoters. ChIP and electrophoretic mobility shift assays targeting σ^A RNA polymerase subunit or using bacterial RNA polymerase holoenzyme suggested that SarA and the σ^A bind srn_3610_sprC and srn_9340 promoters in a mutually exclusive way. Beyond the mechanistic study of SarA repression of these two sRNAs, this work also suggests that some S. aureus sRNAs belong to the same regulon and act jointly in responding to environmental changes.

RpiRc Is a Pleiotropic Effector of Virulence Determinant Synthesis and Attenuates Pathogenicity in Staphylococcus aureus

In Staphylococcus aureus, metabolism is intimately linked with virulence determinant biosynthesis, and several metabolite-responsive regulators have been reported to mediate this linkage. S. aureus possesses at least three members of the RpiR family of transcriptional regulators. Of the three RpiR homologs, RpiRc is a potential regulator of the pentose phosphate pathway, which also regulates RNAIII levels. RNAIII is the regulatory RNA of the agr quorum-sensing system that controls virulence determinant synthesis. The effect of RpiRc on RNAIII likely involves other regulators, as the regulators that bind the RNAIII promoter have been intensely studied. To determine which regulators might bridge the gap between RpiRc and RNAIII, sarA, sigB, mgrA, and acnA mutations were introduced into an rpiRc mutant background, and the effects on RNAIII were determined. Additionally, phenotypic and genotypic differences were examined in the single and double mutant strains, and the virulence of select strains was examined using two different murine infection models. The data suggest that RpiRc affects RNAIII transcription and the synthesis of virulence determinants in concert with σ(B), SarA, and the bacterial metabolic status to negatively affect virulence.

Targeting Staphylococcus aureus Toxins: A Potential form of Anti-Virulence Therapy

Staphylococcus aureus is an opportunistic pathogen and the leading cause of a wide range of severe clinical infections. The range of diseases reflects the diversity of virulence factors produced by this pathogen. To establish an infection in the host, S. aureus expresses an inclusive set of virulence factors such as toxins, enzymes, adhesins, and other surface proteins that allow the pathogen to survive under extreme conditions and are essential for the bacteria’s ability to spread through tissues. Expression and secretion of this array of toxins and enzymes are tightly controlled by a number of regulatory systems. S. aureus is also notorious for its ability to resist the arsenal of currently available antibiotics and dissemination of various multidrug-resistant S. aureus clones limits therapeutic options for a S. aureus infection. Recently, the development of anti-virulence therapeutics that neutralize S. aureus toxins or block the pathways that regulate toxin production has shown potential in thwarting the bacteria’s acquisition of antibiotic resistance. In this review, we provide insights into the regulation of S. aureus toxin production and potential anti-virulence strategies that target S. aureus toxins.

Sigma Factor SigB Is Crucial to Mediate Staphylococcus aureus Adaptation during Chronic Infections

Staphylococcus aureus is a major human pathogen that causes a range of infections from acute invasive to chronic and difficult-to-treat. Infection strategies associated with persisting S. aureus infections are bacterial host cell invasion and the bacterial ability to dynamically change phenotypes from the aggressive wild-type to small colony variants (SCVs), which are adapted for intracellular long-term persistence. The underlying mechanisms of the bacterial switching and adaptation mechanisms appear to be very dynamic, but are largely unknown. Here, we analyzed the role and the crosstalk of the global S. aureus regulators agr, sarA and SigB by generating single, double and triple mutants, and testing them with proteome analysis and in different in vitro and in vivo infection models. We were able to demonstrate that SigB is the crucial factor for adaptation in chronic infections. During acute infection, the bacteria require the simultaneous action of the agr and sarA loci to defend against invading immune cells by causing inflammation and cytotoxicity and to escape from phagosomes in their host cells that enable them to settle an infection at high bacterial density. To persist intracellularly the bacteria subsequently need to silence agr and sarA. Indeed agr and sarA deletion mutants expressed a much lower number of virulence factors and could persist at high numbers intracellularly. SigB plays a crucial function to promote bacterial intracellular persistence. In fact, ΔsigB-mutants did not generate SCVs and were completely cleared by the host cells within a few days. In this study we identified SigB as an essential factor that enables the bacteria to switch from the highly aggressive phenotype that settles an acute infection to a silent SCV-phenotype that allows for long-term intracellular persistence. Consequently, the SigB-operon represents a possible target to develop preventive and therapeutic strategies against chronic and therapy-refractory infections.

5,5'-Methylenedisalicylic Acid (MDSA) Modulates SarA/MgrA Phosphorylation by Targeting Ser/Thr Phosphatase Stp1

SarA (staphylococcal accessory protein A), MgrA (MarR family of global transcriptional regulator A), and SarZ (a paralogue of SarA) play critical roles in modulating the virulence, drug resistance and autolysis of Staphylococcus aureus. Recently, eukaryotic-like Ser/Thr kinase/phosphatases (Stk1/Stp1) were found to modulate phosphorylation of these transcriptional regulators as well as staphylococcal virulence. Importantly, an stp1-deficient strain showed significant virulence reduction in mice, indicative of Stp1 as a potential drug target. Here, we report that MDSA, an inhibitor of MgrA, enhances phosphorylation of SarA/MgrA by inhibiting Stp1 in S. aureus. MDSA is a more-potent inhibitor (IC50 =9.68 ± 0.52 μM) of Stp1 than commonly used phosphatase inhibitors. We anticipate that MDSA could be a lead compound to develop new approaches for reducing staph virulence by targeting Stp1.

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.

Evolution of resistance to a last-resort antibiotic in Staphylococcus aureus via bacterial competition

Antibiotic resistance is a key medical concern, with antibiotic use likely being an important cause. However, here we describe an alternative route to clinically relevant antibiotic resistance that occurs solely due to competitive interactions among bacterial cells. We consistently observe that isolates of Methicillin-resistant Staphylococcus aureus diversify spontaneously into two distinct, sequentially arising strains. The first evolved strain outgrows the parent strain via secretion of surfactants and a toxic bacteriocin. The second is resistant to the bacteriocin. Importantly, this second strain is also resistant to intermediate levels of vancomycin. This so-called VISA (vancomycin-intermediate S. aureus) phenotype is seen in many hard-to-treat clinical isolates. This strain diversification also occurs during in vivo infection in a mouse model, which is consistent with the fact that both coevolved phenotypes resemble strains commonly found in clinic. Our study shows how competition between coevolving bacterial strains can generate antibiotic resistance and recapitulate key clinical phenotypes.

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.

Reconstruction of mreB expression in Staphylococcus aureus via a collection of new integrative plasmids

Protein localization has been traditionally explored in unicellular organisms, whose ease of genetic manipulation facilitates molecular characterization. The two rod-shaped bacterial models Escherichia coli and Bacillus subtilis have been prominently used for this purpose and have displaced other bacteria whose challenges for genetic manipulation have complicated any study of cell biology. Among these bacteria is the spherical pathogenic bacterium Staphylococcus aureus. In this report, we present a new molecular toolbox that facilitates gene deletion in staphylococci in a 1-step recombination process and additional vectors that facilitate the insertion of diverse reporter fusions into newly identified neutral loci of the S. aureus chromosome. Insertion of the reporters does not add any antibiotic resistance genes to the chromosomes of the resultant strains, thereby making them amenable for further genetic manipulations. We used this toolbox to reconstitute the expression of mreB in S. aureus, a gene that encodes an actin-like cytoskeletal protein which is absent in coccal cells and is presumably lost during the course of speciation. We observed that in S. aureus, MreB is organized in discrete structures in association with the membrane, leading to an unusual redistribution of the cell wall material. The production of MreB also caused cell enlargement, but it did not revert staphylococcal shape. We present interactions of MreB with key staphylococcal cell wall-related proteins. This work facilitates the use S. aureus as a model system in exploring diverse aspects of cellular microbiology.

Impact of vancomycin on sarA-mediated biofilm formation: role in persistent endovascular infections due to methicillin-resistant Staphylococcus aureus

BACKGROUND

Staphylococcus aureus is the most common cause of endovascular infections. The staphylococcal accessory regulator A locus (sarA) is a major virulence determinant that may potentially impact methicillin-resistant S. aureus (MRSA) persistence in such infections via its influence on biofilm formation.

METHODS

Two healthcare-associated MRSA isolates from patients with persistent bacteremia and 2 prototypical community-acquired MRSA strains, as well as their respective isogenic sarA mutants, were studied for in vitro biofilm formation, fibronectin-binding capacity, autolysis, and protease and nuclease activities. These assays were done in the presence or absence of sub-minimum inhibitory concentrations (MICs) of vancomycin. In addition, these strain pairs were compared for intrinsic virulence and responses to vancomycin therapy in experimental infective endocarditis, a prototypical biofilm model.

RESULTS

All sarA mutants displayed significantly reduced biofilm formation and binding to fibronectin but increased protease production in vitro, compared with their respective parental strains. Interestingly, exposure to sub-MICs of vancomycin significantly promoted biofilm formation and fibronectin-binding in parental strains but not in sarA mutants. In addition, all sarA mutants became exquisitely susceptible to vancomycin therapy, compared with their respective parental strains, in the infective endocarditis model.

CONCLUSIONS

These observations suggest that sarA activation is important in persistent MRSA endovascular infection, potentially in the setting of biofilm formation.

The Staphylococcus aureus SrrAB two-component system promotes resistance to nitrosative stress and hypoxia

Staphylococcus aureus is both a commensal and a pathogen of the human host. Survival in the host environment requires resistance to host-derived nitric oxide (NO·). However, S. aureus lacks the NO·-sensing transcriptional regulator NsrR that is used by many bacteria to sense and respond to NO·. In this study, we show that S. aureus is able to sense and respond to both NO· and hypoxia by means of the SrrAB two-component system (TCS). Analysis of the S. aureus transcriptome during nitrosative stress demonstrates the expression of SrrAB-dependent genes required for cytochrome biosynthesis and assembly (qoxABCD, cydAB, hemABCX), anaerobic metabolism (pflAB, adhE, nrdDG), iron-sulfur cluster repair (scdA), and NO· detoxification (hmp). Targeted mutations in SrrAB-regulated loci show that hmp and qoxABCD are required for NO· resistance, whereas nrdDG is specifically required for anaerobic growth. We also show that SrrAB is required for survival in static biofilms, most likely due to oxygen limitation. Activation by hypoxia, NO·, or a qoxABCD quinol oxidase mutation suggests that the SrrAB TCS senses impaired electron flow in the electron transport chain rather than directly interacting with NO· in the manner of NsrR. Nevertheless, like NsrR, SrrAB achieves the physiological goals of selectively expressing hmp in the presence of NO· and minimizing the potential for Fenton chemistry. Activation of the SrrAB regulon allows S. aureus to maintain energy production and essential biosynthetic processes, repair damage, and detoxify NO· in diverse host environments.

The Hmp flavohemoglobin is required for nitric oxide resistance and is widely distributed in bacteria. Hmp expression must be tightly regulated, because expression under aerobic conditions in the absence of nitric oxide can exacerbate oxidative stress. In most organisms, hmp expression is controlled by the Fe-S cluster-containing repressor NsrR, but this transcriptional regulator is absent in the human pathogen Staphylococcus aureus. We show here that S. aureus achieves hmp regulation in response to nitric oxide and oxygen limitation by placing it under the control of the SrrAB two-component system, which senses reduced electron flow through the respiratory chain. This provides a striking example of convergent evolution, in which the common physiological goals of responding to nitrosative stress while minimizing Fenton chemistry are achieved by distinct regulatory mechanisms.

Population genetics and the evolution of virulence in Staphylococcus aureus

Staphylococcus aureus is one of the most important human pathogens, causing life-threatening infection in the community and hospital setting. The population genetics of S. aureus and the evolution of virulence is the focus of this review. We describe the various techniques in determining S. aureus population structure and discuss the insights gained from whole genome sequencing of various S. aureus strains. The emergence of community-acquired, methicillin-resistant S. aureus provides a framework for the discussion on evolution of virulence, and the role of horizontal gene transfer in the development of virulence and antibiotic resistance is explored. The knowledge generated from population genetics has the potential to inform strategies to assist in the prevention or treatment of this highly successful human pathogen.

Attenuating Staphylococcus aureus virulence gene regulation: a medicinal chemistry perspective

Virulence gene expression in Staphylococcus aureus is tightly regulated by intricate networks of transcriptional regulators and two-component signal transduction systems. There is now an emerging body of evidence to suggest that the blockade of S. aureus virulence gene expression significantly attenuates infection in experimental models. In this Perspective, we will provide insights into medicinal chemistry strategies for the development of chemical reagents that have the capacity to inhibit staphylococcal virulence expression. These reagents can be broadly grouped into four categories: (1) competitive inhibitors of the accessory gene regulator (agr) quorum sensing system, (2) inhibitors of AgrA–DNA interactions, (3) RNAIII transcription inhibitors, and (4) inhibitors of the SarA family of transcriptional regulators. We discuss the potential of specific examples of antivirulence agents for the management and treatment of staphylococcal infections.

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.

Transcribing virulence in Staphylococcus aureus

Staphylococcus aureus (S. aureus) is an important human pathogen capable of causing a diverse range of infections. Once regarded as an opportunistic pathogen causing primarily nosocomial infections, recent years have seen the emergence of S. aureus strains capable of causing serious infection even in otherwise healthy human hosts. There has been much debate about whether this transition is a function of unique genotypic characteristics or differences in the expression of conserved virulence factors, but irrespective of this debate it is clear that the ability of S. aureus to cause infection in all of its diverse forms is heavily influenced by its ability to modulate gene expression in response to changing conditions within the human host. Indeed, the S. aureus genome encodes more than 100 transcriptional regulators that modulate the production of virulence factors either directly via interactions with cis elements associated with genes encoding virulence factors or indirectly through their complex interactions with each other. The goal of this review is to summarize recent work describing these regulators and their contribution to defining S. aureus as a human pathogen.

Cigarette smoke increases Staphylococcus aureus biofilm formation via oxidative stress

The strong epidemiological association between cigarette smoke (CS) exposure and respiratory tract infections is conventionally attributed to immunosuppressive and irritant effects of CS on human cells. Since pathogenic bacteria such as Staphylococcus aureus are members of the normal microbiota and reside in close proximity to human nasopharyngeal cells, we hypothesized that bioactive components of CS might affect these organisms and potentiate their virulence. Using Staphylococcus aureus as a model organism, we observed that the presence of CS increased both biofilm formation and host cell adherence. Analysis of putative molecular pathways revealed that CS exposure decreased expression of the quorum-sensing agr system, which is involved in biofilm dispersal, and increased transcription of biofilm inducers such as sarA and rbf. CS contains bioactive compounds, including free radicals and reactive oxygen species, and we observed transcriptional induction of bacterial oxidoreductases, including superoxide dismutase, following exposure. Moreover, pretreatment of CS with an antioxidant abrogated CS-mediated enhancement of biofilms. Exposure of bacteria to hydrogen peroxide alone increased biofilm formation. These observations are consistent with the hypothesis that CS induces staphylococcal biofilm formation in an oxidant-dependent manner. CS treatment induced transcription of fnbA (encoding fibronectin binding protein A), leading to increased binding of CS-treated staphylococci to immobilized fibronectin and increased adherence to human cells. These observations indicate that the bioactive effects of CS may extend to the resident microbiota of the nasopharynx, with implications for the pathogenesis of respiratory infection in CS-exposed humans.

Microbial chemical signaling: a current perspective

Communication among microorganisms is mediated through quorum sensing. The latter is defined as cell-density linked, coordinated gene expression in microbial populations as a response to threshold signal concentrations followed by induction of a synchronized population response. This phenomenon is used by a variety of microbes to optimize their survival in a constantly challenging, dynamic milieu, by correlating individual cellular functions to community-based requirements. The synthesis, secretion, and perception of quorum-sensing molecules and their target response play a pivotal role in quorum sensing and are tightly controlled by complex, multilayered and interconnected signal transduction pathways that regulate diverse cellular functions. Quorum sensing exemplifies interactive social behavior innate to the microbial world that controls features such as, virulence, biofilm maturation, antibiotic resistance, swarming motility, and conjugal plasmid transfer. Over the past two decades, studies have been performed to rationalize bacterial cell-to-cell communication mediated by structurally and functionally diverse small molecules. This review describes the theoretical aspects of cellular and quorum-sensing mechanisms that affect microbial physiology and pathobiology.

Inhibitory effects of D-amino acids on Staphylococcus aureus biofilm development

Biofilms are communities of cells held together by a self-produced extracellular matrix typically consisting of protein, exopolysaccharide, and often DNA. A natural signal for biofilm disassembly in Bacillus subtilis is certain D-amino acids, which are incorporated into the peptidoglycan and trigger the release of the protein component of the matrix. D-amino acids also prevent biofilm formation by the related Gram-positive bacterium Staphylococcus aureus. Here we employed fluorescence microscopy and confocal laser scanning microscopy to investigate how D-amino acids prevent biofilm formation by S. aureus. We report that biofilm formation takes place in two stages, initial attachment to surfaces, resulting in small foci, and the subsequent growth of the foci into large aggregates. D-amino acids did not prevent the initial surface attachment of cells but blocked the subsequent growth of the foci into larger assemblies of cells. Using protein- and polysaccharide-specific stains, we have shown that D-amino acids inhibited the accumulation of the protein component of the matrix but had little effect on exopolysaccharide production and localization within the biofilm. We conclude that D-amino acids act in an analogous manner to prevent biofilm development in B. subtilis and S. aureus. Finally, to investigate the potential utility of D-amino acids in preventing device-related infections, we have shown that surfaces impregnated with D-amino acids were effective in preventing biofilm growth.

Golden pigment production and virulence gene expression are affected by metabolisms in Staphylococcus aureus

The pathogenesis of staphylococcal infections is multifactorial. Golden pigment is an eponymous feature of the human pathogen Staphylococcus aureus that shields the microbe from oxidation-based clearance, an innate host immune response to infection. Here, we screened a collection of S. aureus transposon mutants for pigment production variants. A total of 15 previously unidentified genes were discovered. Notably, disrupting metabolic pathways such as the tricarboxylic acid cycle, purine biosynthesis, and oxidative phosphorylation yields mutants with enhanced pigmentation. The dramatic effect on pigment production seems to correlate with altered expression of virulence determinants. Microarray analysis further indicates that purine biosynthesis impacts the expression of approximately 400 genes involved in a broad spectrum of functions including virulence. The purine biosynthesis mutant and oxidative phosphorylation mutant strains exhibit significantly attenuated virulence in a murine abscess model of infection. Inhibition of purine biosynthesis with a known small-molecule inhibitor results in altered virulence gene expression and virulence attenuation during infection. Taken together, these results suggest an intimate link between metabolic processes and virulence gene expression in S. aureus. This study also establishes the importance of purine biosynthesis and oxidative phosphorylation for in vivo survival.

Staphylococcus aureus SarA is a regulatory protein responsive to redox and pH that can support bacteriophage lambda integrase-mediated excision/recombination

Staphylococcus aureus produces a wide array of virulence factors and causes a correspondingly diverse array of infections. Production of these virulence factors is under the control of a complex network of global regulatory elements, one of which is sarA. sarA encodes a DNA binding protein that is considered to function as a transcription factor capable of acting as either a repressor or an activator. Using competitive ELISA assays, we demonstrate that SarA is present at approximately 50 000 copies per cell, which is not characteristic of classical transcription factors. We also demonstrate that SarA is present at all stages of growth in vitro and is capable of binding DNA with high affinity but that its binding affinity and pattern of shifted complexes in electrophoretic mobility shift assays is responsive to the redox state. We also show that SarA binds to the bacteriophage lambda (lambda) attachment site, attL, producing SarA-DNA complexes similar to intasomes, which consist of bacteriophage lambda integrase, Escherichia coli integration host factor and attL DNA. In addition, SarA stimulates intramolecular excision recombination in the absence of lambda excisionase, a DNA binding accessory protein. Taken together, these data suggest that SarA may function as an architectural accessory protein.

Interaction of staphylococci with bone

Staphylococci, in particular Staphylococcus aureus, are the predominant cause of bone infections worldwide. These infections are painful, debilitating and with the rise in antibiotic-resistant forms, increasingly difficult to treat. The growth in the number of prosthetic joint replacement procedures also provides new opportunities for these infections to take hold. Comprehending the mechanisms by which staphylococci interact with and damage bone is critical to the development of new approaches to meet this challenge. This review summarises current understanding of the mechanisms by which staphylococci infect and damage bone. We address the role of the inflammatory response to staphylococcal infection in disrupting the homeostatic balance of bone matrix deposition and resorption and thereby mediating bone destruction. A number of virulence factors that have been shown to contribute to bone infection and pathology are discussed, however no single factor has been defined as being specific to bone infections. Although traditionally considered an extracellular pathogen, there is increasing evidence that staphylococci are able to invade host cells, and that an intracellular lifestyle may facilitate long-term persistence in bone tissue, enabling evasion of antimicrobials and host immune responses. ‘Small colony variant’ strains, with mutations disabling the electron transport pathway appear particularly adept at invading and persisting within host cells, and exhibit enhanced antimicrobial resistance, and may represent a further complication in the treatment and management of staphylococcal bone disease.

Cell-to-cell signalling during pathogenesis

Pathogenic bacteria produce virulence factors only when they sense they are in a location in which the energy required for pathogenesis is warranted. One environmental factor monitored by pathogens is population density, either of its own population or of the population of a host's endogenous flora. To date, four systems have been described which allow pathogens to regulate virulence genes in a population-dependent manner. These systems are found in both Gram-positive and Gram-negative cells and utilize various mechanisms to control gene regulation at a transcriptional level, but they all have one feature in common: they detect autoinducers released by cells belonging to either the pathogen's population or that of the host's flora. This article explores the role of these four signalling systems in bacterial communities and how pathogens use these systems to control genes required during host invasion and infection.

Structure and function predictions of the Msa protein in Staphylococcus aureus

BACKGROUND

Staphylococcus aureus is a human pathogen that causes a wide variety of life-threatening infections using a large number of virulence factors. One of the major global regulators used by S. aureus is the staphylococcal accessory regulator (sarA). We have identified and characterized a new gene (modulator of sarA: msa) that modulates the expression of sarA. Genetic and functional analysis shows that msa has a global effect on gene expression in S. aureus. However, the mechanism of Msa function is still unknown. Function predictions of Msa are complicated by the fact that it does not have a homologous partner in any other organism. This work aims at predicting the structure and function of the Msa protein.

RESULTS

Preliminary sequence analysis showed that Msa is a putative membrane protein. It would therefore be very difficult to purify and crystallize Msa in order to acquire structure information about this protein. We have used several computational tools to predict the physico-chemical properties, secondary structural features, topology, 3D tertiary structure, binding sites, motifs/patterns/domains and cellular location. We have built a consensus that is derived from analysis using different algorithms to predict several structural features. We confirm that Msa is a putative membrane protein with three transmembrane regions. We also predict that Msa has phosphorylation sites and binding sites suggesting functions in signal transduction.

CONCLUSION

Based on our predictions we hypothesise that Msa is a novel signal transducer that might be involved in the interaction of the S. aureus with its environment.

Photothermal flow cytometry in vitro for detection and imaging of individual moving cells

BACKGROUND

Photothermal (PT) cytometry has recently demonstrated great potential for the label-free detection of nonfluorescent cells under static conditions. The goal of our investigation was to expand this technique to the detection of flowing cells in vitro.

METHODS

Cells in flow were irradiated with short, tunable laser pulses (420-2,300 nm, 8 ns), and the absorbed energy was detected by monitoring of the temperature-dependent variations in the refractive index in the cells with a second, collinear probe beam in two modes: (a) PT imaging of single cells with a pulsed probe beam (639 nm, 13 ns) and (b) thermolens monitoring of the integral PT responses from individual cells as whole with a continuous-wave probe beam (633 nm, 2 mW).

RESULTS

PT flow cytometry at the current speed of analysis of 10 cell/s, with the capability to image selected cells of interest flowing at velocities up to 2 m/s, demonstrated the capability for (a) label-free detection of flowing single cells (e.g., blood and cancer cells) on the basis of the differences in their endogenous absorption properties, (b) identification of cells labeled with gold nanoparticles, (c) rapid cell viability testing, (d) aggregation immunoassay, and (e) optimization of selective nanophotothermolysis.

CONCLUSIONS

PT cytometry can be extended to the study of cells in flow. This new technique increases the speed of cell analysis approximately 10(2) times over that of conventional PT technique, with the potential to achieve a rate of 10(4)-10(5) cells/s in specific PT applications, which has previously been realized only with cells under static conditions.

Induction of multiple matrix metalloproteinases in human dermal and synovial fibroblasts by Staphylococcus aureus: implications in the pathogenesis of septic arthritis and other soft tissue infections

Infections of body tissue by Staphylococcus aureus are quickly followed by degradation of connective tissue. Patients with rheumatoid arthritis are more prone to S. aureus-mediated septic arthritis. Various types of collagen form the major structural matrix of different connective tissues of the body. These different collagens are degraded by specific matrix metalloproteinases (MMPs) produced by fibroblasts, other connective tissue cells, and inflammatory cells that are induced by interleukin-1 (IL-1) and tumor necrosis factor (TNF). To determine the host's contribution in the joint destruction of S. aureus-mediated septic arthritis, we analyzed the MMP expression profile in human dermal and synovial fibroblasts upon exposure to culture supernatant and whole cell lysates of S. aureus. Human dermal and synovial fibroblasts treated with cell lysate and filtered culture supernatants had significantly enhanced expression of MMP-1, MMP-2, MMP-3, MMP-7, MMP-10, and MMP-11 compared with the untreated controls (p < 0.05). In the S. aureus culture supernatant, the MMP induction activity was identified to be within the molecular-weight range of 30 to >50 kDa. The MMP expression profile was similar in fibroblasts exposed to a combination of IL-1/TNF. mRNA levels of several genes of the mitogen-activated protein kinase (MAPK) signal transduction pathway were significantly elevated in fibroblasts treated with S. aureus cell lysate and culture supernatant. Also, tyrosine phosphorylation was significantly higher in fibroblasts treated with S. aureus components. Tyrosine phosphorylation and MAPK gene expression patterns were similar in fibroblasts treated with a combination of IL-1/TNF and S. aureus. Mutants lacking staphylococcal accessory regulator (Sar) and accessory gene regulator (Agr), which cause significantly less severe septic arthritis in murine models, were able to induce expression of several MMP mRNA comparable with that of their isogenic parent strain but induced notably higher levels of tissue inhibitors of metalloproteinases (TIMPs). To our knowledge, this is the first report of induction of multiple MMP/TIMP expression from human dermal and synovial fibroblasts upon S. aureus treatment. We propose that host-derived MMPs contribute to the progressive joint destruction observed in S. aureus-mediated septic arthritis.

Pathological fracture in acute osteomyelitis of long bones secondary to community-acquired methicillin-resistant Staphylococcus aureus: two cases and review of the literature

Pathologic fracture is a rare complication of acute bacterial osteomyelitis in adults. Community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) has been increasingly reported in skin and soft tissue and systemic infections in children and adults, including many cases of osteomyelitis. We recently treated two adult patients with acute osteomyelitis of long bones secondary to CA-MRSA complicated by a pathologic fracture. In both patients, the primary source of dissemination to the bone was a skin and soft tissue infection. We speculate that virulence factors specific for CA-MRSA currently circulating in the United States may predispose to a complicated course of acute osteomyelitis.

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.

Identification and characterization of msa (SA1233), a gene involved in expression of SarA and several virulence factors in Staphylococcus aureus

The staphylococcal accessory regulator (sarA) plays a central role in the regulation of virulence in Staphylococcus aureus. To date, studies involving sarA have focused on its activity as a global regulator that modulates transcription of a wide variety of genes (>100) and its role in virulence. However, there is also evidence to suggest the existence of accessory elements that modulate SarA production and/or function. A reporter system was developed to identify such elements, and a new gene, msa (SA1233), mutation of which results in reduced expression of SarA, was identified and characterized. Additionally, it was shown that mutation of msa resulted in altered transcription of the accessory gene regulator (agr) and the genes encoding several virulence factors including alpha toxin (hla) and protein A (spa). However, the impact of mutating msa was different in the laboratory strain RN6390 and the clinical isolate UAMS-1. For instance, mutation of msa caused a decrease in spa and hla transcription in RN6390 but had a different effect in UAMS-1. The strain-dependent effects of the msa mutation were similar to those observed previously, which suggests that msa may modulate the production of specific virulence factors through its impact on sarA. Interestingly, sequence analysis of Msa suggests that it is a putative membrane protein with three membrane-spanning regions, indicating that Msa might interact with the environment. The findings show that msa is involved in the expression of SarA and several virulence factors.

Characterization of the Staphylococcus aureus heat shock, cold shock, stringent, and SOS responses and their effects on log-phase mRNA turnover

Despite its being a leading cause of nosocomal and community-acquired infections, surprisingly little is known about Staphylococcus aureus stress responses. In the current study, Affymetrix S. aureus GeneChips were used to define transcriptome changes in response to cold shock, heat shock, stringent, and SOS response-inducing conditions. Additionally, the RNA turnover properties of each response were measured. Each stress response induced distinct biological processes, subsets of virulence factors, and antibiotic determinants. The results were validated by real-time PCR and stress-mediated changes in antimicrobial agent susceptibility. Collectively, many S. aureus stress-responsive functions are conserved across bacteria, whereas others are unique to the organism. Sets of small stable RNA molecules with no open reading frames were also components of each response. Induction of the stringent, cold shock, and heat shock responses dramatically stabilized most mRNA species. Correlations between mRNA turnover properties and transcript titers suggest that S. aureus stress response-dependent alterations in transcript abundances can, in part, be attributed to alterations in RNA stability. This phenomenon was not observed within SOS-responsive cells.

Microbial interactions in Staphylococcus epidermidis biofilms

Medical device-associated infections, most frequently caused by coagulase-negative staphylococci, especially Staphylococcus epidermidis, are of increasing importance in modern medicine. The formation of adherent, multilayered bacterial biofilms is the most important factor in the pathogenesis of these infections, which regularly fail to respond to appropriate antimicrobial therapy. Progress in elucidating the factors functional in elaboration of S. epidermidis biofilms and the regulation of their expression with a special emphasis on the role of quorum sensing are reviewed. Significant progress has been made in recent years, which provides the rationale for developing better preventive, therapeutic and diagnostic measures.

Characterizing the effect of the Staphylococcus aureus virulence factor regulator, SarA, on log-phase mRNA half-lives

Bacterial pathogens regulate virulence factor expression at both the level of transcription initiation and mRNA processing/turnover. Within Staphylococcus aureus, virulence factor transcript synthesis is regulated by a number of two-component regulatory systems, the DNA binding protein SarA, and the SarA family of homologues. However, little is known about the factors that modulate mRNA stability or influence transcript degradation within the organism. As our entree to characterizing these processes, S. aureus GeneChips were used to simultaneously determine the mRNA half-lives of all transcripts produced during log-phase growth. It was found that the majority of log-phase transcripts (90%) have a short half-life (<5 min), whereas others are more stable, suggesting that cis- and/or trans-acting factors influence S. aureus mRNA stability. In support of this, it was found that two virulence factor transcripts, cna and spa, were stabilized in a sarA-dependent manner. These results were validated by complementation and real-time PCR and suggest that SarA may regulate target gene expression in a previously unrecognized manner by posttranscriptionally modulating mRNA turnover. Additionally, it was found that S. aureus produces a set of stable RNA molecules with no predicted open reading frame. Based on the importance of the S. aureus agr RNA molecule, RNAIII, and small stable RNA molecules within other pathogens, it is possible that these RNA molecules influence biological processes within the organism.