Identification of a novel maturation mechanism and restricted substrate specificity for the SspB cysteine protease of Staphylococcus aureus

Genome sequencing and comparative genomic analysis of bovine mastitis-associated non-aureus staphylococci and mammaliicocci (NASM) strains from India

We describe the whole-genome sequencing and comparative genomic analysis of 22 mastitis-associated NASM strains isolated from India. The mean genome size of the strains was 2.55 Mbp, with an average GC content of 32.2%. We identified 14 different sequence types (STs) among the 22 NASM strains. Of these, ST1 and ST6 of S. chromogenes were exclusively associated with bovine mastitis. Genome-wide SNP-based minimum spanning tree revealed the intricate phylogenetic relationships among NASM strains from India, categorizing them into five major clades. Interestingly, mastitis-associated strains formed separate subclades in all the NASM species studied, indicating distinct host-specific co-evolution. The study identified 32 antimicrobial resistance (AMR) genes and 53 virulence-associated genes, providing insights into the genetic factors that could contribute to the pathogenicity of NASM species. Some virulence and AMR genes were found in the predicted genomic islands, suggesting possible horizontal transfer events.

Staphylococcus aureus Proteases: Orchestrators of Skin Inflammation

Skin homeostasis relies on a delicate balance between host proteases and protease inhibitors along with those secreted from microbial communities, as disruption to this harmony contributes to the pathogenesis of inflammatory skin disorders, including atopic dermatitis and Netherton's syndrome. In addition to being a prominent cause of skin and soft tissue infections, the gram-positive bacterium Staphylococcus aureus is a key player in inflammatory skin conditions due to its array of 10 secreted proteases. Herein we review how S. aureus proteases augment the development of inflammation in skin disorders. These mechanisms include degradation of skin barrier integrity, immune dysregulation and pruritis, and impairment of host defenses. Delineating the diverse roles of S. aureus proteases has the potential to reveal novel therapeutic strategies, such as inhibitors of proteases or their cognate target, as well as neutralizing vaccines to alleviate the burden of inflammatory skin disorders in patients.

Altered quorum sensing and physiology of Staphylococcus aureus during spaceflight detected by multi-omics data analysis

Staphylococcus aureus colonizes the nares of approximately 30% of humans, a risk factor for opportunistic infections. To gain insight into S. aureus virulence potential in the spaceflight environment, we analyzed RNA-Seq, cellular proteomics, and metabolomics data from the "Biological Research in Canisters-23" (BRIC-23) GeneLab spaceflight experiment, a mission designed to measure the response of S. aureus to growth in low earth orbit on the international space station. This experiment used Biological Research in Canisters-Petri Dish Fixation Units (BRIC-PDFUs) to grow asynchronous ground control and spaceflight cultures of S. aureus for 48 h. RNAIII, the effector of the Accessory Gene Regulator (Agr) quorum sensing system, was the most highly upregulated gene transcript in spaceflight relative to ground controls. The agr operon gene transcripts were also highly upregulated during spaceflight, followed by genes encoding phenol-soluble modulins and secreted proteases, which are positively regulated by Agr. Upregulated spaceflight genes/proteins also had functions related to urease activity, type VII-like Ess secretion, and copper transport. We also performed secretome analysis of BRIC-23 culture supernatants, which revealed that spaceflight samples had increased abundance of secreted virulence factors, including Agr-regulated proteases (SspA, SspB), staphylococcal nuclease (Nuc), and EsxA (secreted by the Ess system). These data also indicated that S. aureus metabolism is altered in spaceflight conditions relative to the ground controls. Collectively, these data suggest that S. aureus experiences increased quorum sensing and altered expression of virulence factors in response to the spaceflight environment that may impact its pathogenic potential.

Secretory proteins in the orchestration of microbial virulence: The curious case of Staphylococcus aureus

Microbial virulence showcases an excellent model for adaptive changes that enable an organism to survive and proliferate in a hostile environment and exploit host resources to its own benefit. In Staphylococcus aureus, an opportunistic pathogen of the human host, known for the diversity of the disease conditions it inflicts and the rapid evolution of antibiotic resistance, virulence is a consequence of having a highly plastic genome that is amenable to quick reprogramming and the ability to express a diverse arsenal of virulence factors. Virulence factors that are secreted to the host milieu effectively manipulate the host conditions to favor bacterial survival and growth. They assist in colonization, nutrient acquisition, immune evasion, and systemic spread. The structural and functional characteristics of the secreted virulence proteins have been shaped to assist S. aureus in thriving and disseminating effectively within the host environment and exploiting the host resources to its best benefit. With the aim of highlighting the importance of secreted virulence proteins in bacterial virulence, the present chapter provides a comprehensive account of the role of the major secreted proteins of S. aureus in orchestrating its virulence in the human host.

Comparative secretome analysis of Staphylococcus aureus strains with different within-herd intramammary infection prevalence

Staphylococcus aureus is a major pathogen causing intramammary infection and mastitis in dairy cows. S. aureus genotypes (GT) can differ significantly in their ability to diffuse and persist in the herd; while the association of virulence gene carriage with epidemiological behavior remains unclear, a role for secreted proteins has been postulated. We characterized the secretome of six S. aureus strains belonging to two genotypes with opposite within-herd prevalence, GTB (high) and GTS (low), corresponding to sequence types (ST) 8 and 398, by high-resolution tandem mass spectrometry and differential analysis with Proteome Discoverer. Data are available via ProteomeXchange with identifier PXD029571. Out of 720 identified proteins, 98 were unique or more abundant in GTB/ST8 and 68 in GTS/ST398. GTB/ST8 released more immunoglobulin-binding proteins, complement and antimicrobial peptide inhibitors, enterotoxins, and metabolic enzymes, while GTS/ST398 released more leukocidins, hemolysins, lipases, and peptidases. Furthermore, GTB/ST8 released the von Willebrand factor protein, staphylokinase, and clumping factor B, while GTS released the staphylococcal coagulase and clumping factor A. Hence, GTB/ST8 secretomes indicated a higher propensity for immune evasion and chronicity and GTS/ST398 secretomes for cellular damage and inflammation, consistent with their epidemiological characteristics. Accordingly, GTS/ST398 secretions were significantly more cytotoxic against bovine PBMCs in vitro. Our findings confirm the crucial role of extracellular virulence factors in S. aureus pathogenesis and highlight the need to investigate their differential release adding to gene carriage for a better understanding of the relationship of S. aureus genotypes with epidemiological behavior and, possibly, disease severity.

Intracellular Staphylococcus aureus employs the cysteine protease staphopain A to induce host cell death in epithelial cells

Staphylococcus aureus is a major human pathogen, which can invade and survive in non-professional and professional phagocytes. Uptake by host cells is thought to contribute to pathogenicity and persistence of the bacterium. Upon internalization by epithelial cells, cytotoxic S. aureus strains can escape from the phagosome, replicate in the cytosol and induce host cell death. Here, we identified a staphylococcal cysteine protease to induce cell death after translocation of intracellular S. aureus into the host cell cytoplasm. We demonstrated that loss of staphopain A function leads to delayed onset of host cell death and prolonged intracellular replication of S. aureus in epithelial cells. Overexpression of staphopain A in a non-cytotoxic strain facilitated intracellular killing of the host cell even in the absence of detectable intracellular replication. Moreover, staphopain A contributed to efficient colonization of the lung in a mouse pneumonia model. In phagocytic cells, where intracellular S. aureus is exclusively localized in the phagosome, staphopain A did not contribute to cytotoxicity. Our study suggests that staphopain A is utilized by S. aureus to exit the epithelial host cell and thus contributes to tissue destruction and dissemination of infection.

Staphylococcus aureus is an antibiotic-resistant pathogen that emerges in hospital and community settings and can cause a variety of diseases ranging from skin abscesses to lung inflammation and blood poisoning. The bacterium can asymptomatically colonize the upper respiratory tract and skin of humans and take advantage of opportune conditions, like immunodeficiency or breached barriers, to cause infection. Although S. aureus was not regarded as intracellular bacterium, it can be internalized by human cells and subsequently exit the host cells by induction of cell death, which is considered to cause tissue destruction and spread of infection. The bacterial virulence factors and underlying molecular mechanisms involved in the intracellular lifestyle of S. aureus remain largely unknown. We identified a bacterial cysteine protease to contribute to host cell death of epithelial cells mediated by intracellular S. aureus. Staphopain A induced killing of the host cell after translocation of the pathogen into the cell cytosol, while bacterial proliferation was not required. Further, the protease enhanced survival of the pathogen during lung infection. These findings reveal a novel, intracellular role for the bacterial protease staphopain A.

Unraveling the Impact of Secreted Proteases on Hypervirulence in Staphylococcus aureus

Staphylococcus aureus controls the progression of infection through the coordinated production of extracellular proteases, which selectively modulate virulence determinant stability. This is evidenced by our previous finding that a protease-null strain has a hypervirulent phenotype in a murine model of sepsis, resulting from the unchecked accumulation of virulence factors. Here, we dissect the individual roles of these proteases by constructing and assessing the pathogenic potential of a combinatorial protease mutant library. When strains were constructed bearing increasing numbers of secreted proteases, we observed a variable impact on infectious capacity, where some exhibited hypervirulence, while others phenocopied the wild-type. The common thread for hypervirulent strains was that each lacked both aureolysin and staphopain A. Upon assessment, we found that the combined loss of these two enzymes alone was necessary and sufficient to engender hypervirulence. Using proteomics, we identified a number of important secreted factors, including SPIN, LukA, Sbi, SEK, and PSMα4, as well as an uncharacterized chitinase-related protein (SAUSA300_0964), to be overrepresented in both the aur scpA and the protease-null mutants. When assessing the virulence of aur scpA SAUSA300_0964 and aur scpA lukA mutants, we found that hypervirulence was completely eliminated, whereas aur scpA spn and aur scpA sek strains elicited aggressive infections akin to the protease double mutant. Collectively, our findings shed light on the influence of extracellular proteases in controlling the infectious process and identifies SAUSA300_0964 as an important new component of the S. aureus virulence factor arsenal.IMPORTANCE A key feature of the pathogenic success of S. aureus is the myriad virulence factors encoded within its genome. These are subject to multifactorial control, ensuring their timely production only within an intended infectious niche. A key node in this network of control is the secreted proteases of S. aureus, who specifically and selectively modulate virulence factor stability. In our previous work we demonstrated that deletion of all 10 secreted proteases results in hypervirulence, since virulence factors exist unchecked, leading to overly aggressive infections. Here, using a combinatorial collection of protease mutants, we reveal that deletion of aureolysin and staphopain A is necessary and sufficient to elicit hypervirulence. Using proteomic techniques, we identify the collection of virulence factors that accumulate in hypervirulent protease mutants, and demonstrate that a well-known toxin (LukA) and an entirely novel secreted element (SAUSA300_0964) are the leading contributors to deadly infections observed in protease-lacking strains.

Novel Functions and Signaling Specificity for the GraS Sensor Kinase of Staphylococcus aureus in Response to Acidic pH

Although the GraS sensor kinase of Staphylococcus aureus is known for the sensing of and resistance to cationic antimicrobial peptides (CAMPs), we recently established that it also signals in response to acidic pH, which is encountered on human skin concurrently with CAMPs, antimicrobial unsaturated free fatty acids (uFFA), and calcium. We therefore evaluated how these environmental signals would affect GraS function and resistance to antimicrobial uFFA. Growth at pH 5.5 promoted increased resistance of S. aureus USA300 to linoleic and arachidonic acids but not palmitoleic or sapienic acid. However, enhanced resistance to these C_16:1 uFFA was achieved by supplementing acidic medium with 0.5 mM calcium or subinhibitory CAMPs. Enhanced resistance to uFFA at acidic pH was dependent on GraS and GraS-dependent expression of the lysyl-phosphatidylglycerol synthase enzyme MprF, through a mechanism that did not require the lysyl-transferase function of MprF. In addition to enhanced resistance to antimicrobial uFFA, acidic pH also promoted increased production of secreted proteases in a GraS-dependent manner. During growth at pH 5.5, downstream phenotypes of signaling through GraS, including resistance to uFFA, MprF-dependent addition of positive charge to the cell surface, and increased production of secreted proteases, all occurred independently of acidic amino acids in the extracytoplasmic sensor loop of GraS that were previously found to be required for sensing of CAMPs. Cumulatively, our data indicate that signaling through GraS at acidic pH occurs through a mechanism that is distinct from that described for CAMPs, leading to increased resistance to antimicrobial uFFA and production of secreted proteases.IMPORTANCE Staphylococcus aureus asymptomatically colonizes 30% of humans but is also a leading cause of infectious morbidity and mortality. Since infections are typically initiated by the same strain associated with asymptomatic colonization of the nose or skin, it is important to understand how the microbe can endure exposure to harsh conditions that successfully restrict the growth of other bacteria, including a combination of acidic pH, antimicrobial peptides, and antimicrobial fatty acids. The significance of our research is in showing that acidic pH combined with antimicrobial peptide or environmental calcium can signal through a single membrane sensor protein to promote traits that may aid in survival, including modification of cell surface properties, increased resistance to antimicrobial fatty acids, and enhanced production of secreted proteases.

β,γ-Diaryl α-methylene-γ-butyrolactones as potent antibacterials against methicillin-resistant Staphylococcus aureus

A selected series of racemic α-methylene-γ-butyrolactones (AMGBL) synthesized via allylboration or allylindation reactions were screened against methicillin-resistant Staphylococcus aureus (MRSA) USA300. Unlike natural AMGBLs, such as parthenolide, synthetic analogs bearing aryl moieties at the β- and γ-positions are potent against MRSA. The most potent molecules were comparable to vancomycin and linezolid, the drugs of the last resort for MRSA infections, in their effectiveness with minimum inhibitory concentrations (MICs) ranging from 3.0 to 5.2 μM. These lactones also exhibited potent antibacterial activity against other clinically important multidrug-resistant Gram-positive bacteria (except enterococci), while also showing high tolerability to mammalian cells. Several of these molecules surpassed vancomycin in their rapid killing of the high MRSA inoculum (2 h vs 12 h) in a standard time-kill kinetics assay, with compounds 1l and 1m significantly reducing the intracellular burden of MRSA by about 98-99%, at low concentrations. Additionally, the compounds surpassed vancomycin in inhibiting staphylococcal protease production, indicating that synthetic methylene lactones warrant further investigations as promising anti-MRSA candidates.

Staphylococcus aureus Fatty Acid Kinase FakA Modulates Pathogenesis during Skin Infection via Proteases

Staphylococcus aureus fatty acid kinase FakA is necessary for the incorporation of exogenous fatty acids into the lipid membrane. We previously demonstrated that the inactivation of fakA leads to decreased α-hemolysin (Hla) production but increased expression of the proteases SspAB and aureolysin in vitro, and that the ΔfakA mutant causes larger lesions than the wild type (WT) during murine skin infection. As expected, necrosis is Hla dependent in the presence or absence of FakA, as both hla and hla ΔfakA mutants are unable to cause necrosis of the skin. At day 4 postinfection, while the ΔfakA mutant maintains larger and more necrotic abscesses, bacterial numbers are similar to those of the WT, indicating the enhanced tissue damage of mice infected with the ΔfakA mutant is not due to an increase in bacterial burden. At this early stage of infection, skin infected with the ΔfakA mutant has decreased levels of proinflammatory cytokines, such as interleukin-17A (IL-17A) and IL-1α, compared to those of WT-infected skin. At a later stage of infection (day 7), abscess resolution and bacterial clearance are hindered in ΔfakA mutant-infected mice. The paradoxical findings of decreased Hla in vitro but increased necrosis in vivo led us to investigate the role of the proteases regulated by FakA. Utilizing Δaur and ΔsspAB mutants in both the WT and fakA mutant backgrounds, we found that the absence of these proteases in a fakA mutant reduced dermonecrosis to levels similar to those of the WT strain. These studies suggest that the overproduction of proteases is one factor contributing to the enhanced pathogenesis of the ΔfakA mutant during skin infection.

Structural insights into the role of the N-terminus in the activation and function of extracellular serine protease from Staphylococcus epidermidis

Extracellular serine protease (Esp) from Staphylococcus epidermidis is a glutamyl endopeptidase that inhibits the growth and formation of S. aureus biofilms. Previously, crystal structures of the matured and active Esp have been determined. Interestingly, many of the staphylococcal glutamyl endopeptidase zymogens, including V8 from Staphylococcus aureus and Esp from S. epidermidis, contain unusually long pro-peptide segments; however, their function is not known. With the aim of elucidating the function of these pro-peptide segments, crystal structures of the Esp zymogen (Pro-Esp) and its variants were determined. It was observed that the N-terminus of the Pro-Esp crystal structure is flexible and is not associated with the main body of the enzyme, unlike in the known active Esp structure. In addition, the loops that border the putative substrate-binding pocket of Pro-Esp are flexible and disordered; the structural components that are responsible for enzyme specificity and efficiency in serine proteases are disordered in Pro-Esp. However, the N-terminal locked Pro-Esp variants exhibit a rigid substrate-binding pocket similar to the active Esp structure and regain activity. These structural studies highlight the role of the N-terminus in stabilizing the structural components responsible for the activity and specificity of staphylococcal glutamyl endopeptidases.

Mapping the Global Network of Extracellular Protease Regulation in Staphylococcus aureus

The complex regulatory role of the proteases necessitates very tight coordination and control of their expression. While this process has been well studied, a major oversight has been the consideration of proteases as a single entity rather than as 10 enzymes produced from four different promoters. As such, in this study, we comprehensively characterized the regulation of each protease promoter, discovering vast differences in the way each protease operon is controlled. Additionally, we broaden the picture of protease regulation using a global screen to identify novel loci controlling protease activity, uncovering a cadre of new effectors of protease expression. The impact of these elements on the activity of proteases and known regulators was characterized by producing a comprehensive regulatory circuit that emphasizes the complexity of protease regulation in Staphylococcus aureus.

A primary function of the extracellular proteases of Staphylococcus aureus is to control the progression of infection by selectively modulating the stability of virulence factors. Consequently, a regulatory network exists to titrate protease abundance/activity to influence the accumulation, or lack thereof, of individual virulence factors. Herein, we comprehensively map this system, exploring the regulation of the four protease loci by known and novel factors. In so doing, we determined that seven major elements (SarS, SarR, Rot, MgrA, CodY, SaeR, and SarA) form the primary network of control, with the latter three being the most powerful. We note that expression of aureolysin is largely repressed by these factors, while the spl operon is subject to the strongest upregulation of any protease loci, particularly by SarR and SaeR. Furthermore, when exploring scpA expression, we find it to be profoundly influenced in opposing fashions by SarA (repressor) and SarR (activator). We also present the screening of >100 regulator mutants of S. aureus, identifying 7 additional factors (ArgR2, AtlR, MntR, Rex, XdrA, Rbf, and SarU) that form a secondary circuit of protease control. Primarily, these elements serve as activators, although we reveal XdrA as a new repressor of protease expression. With the exception or ArgR2, each of the new effectors appears to work through the primary network of regulation to influence protease production. Collectively, we present a comprehensive regulatory circuit that emphasizes the complexity of protease regulation and suggest that its existence speaks to the importance of these enzymes to S. aureus physiology and pathogenic potential.

IMPORTANCE The complex regulatory role of the proteases necessitates very tight coordination and control of their expression. While this process has been well studied, a major oversight has been the consideration of proteases as a single entity rather than as 10 enzymes produced from four different promoters. As such, in this study, we comprehensively characterized the regulation of each protease promoter, discovering vast differences in the way each protease operon is controlled. Additionally, we broaden the picture of protease regulation using a global screen to identify novel loci controlling protease activity, uncovering a cadre of new effectors of protease expression. The impact of these elements on the activity of proteases and known regulators was characterized by producing a comprehensive regulatory circuit that emphasizes the complexity of protease regulation in Staphylococcus aureus.

Interaction of host and Staphylococcus aureus protease-system regulates virulence and pathogenicity

Staphylococcus aureus causes various health care- and community-associated infections as well as certain chronic TH2 driven inflammatory diseases. It is a potent pathogen with serious virulence and associated high morbidity. Severe pathogenicity is accredited to the S. aureus secreted virulence factors such as proteases and host protease modulators. These virulence factors promote adhesion and invasion of bacteria through damage of tight junction barrier and keratinocytes. They inhibit activation and transmigration of various immune cells such as neutrophils (and neutrophil proteases) to evade opsono-phagocytosis and intracellular bacterial killing. Additionally, they protect the bacteria from extracellular killing by disrupting integrity of extracellular matrix. Platelet activation and agglutination is also impaired by these factors. They also block the classical as well as alternative pathways of complement activation and assist in spread of infection through blood and tissue. As these factors are exquisite factors of S. aureus mediated disease development, we have focused on review of diversification of various protease-system associated virulence factors, their structural building, diverse role in disease development and available therapeutic counter measures. This review summarises the role of protease-associated virulence factors during invasion and progression of disease.

Within-host evolution of bovine Staphylococcus aureus selects for a SigB-deficient pathotype characterized by reduced virulence but enhanced proteolytic activity and biofilm formation

Staphylococcus aureus is a major cause of bovine mastitis, commonly leading to long-lasting, persistent and recurrent infections. Thereby, S. aureus constantly refines and permanently adapts to the bovine udder environment. In this work, we followed S. aureus within-host adaptation over the course of three months in a naturally infected dairy cattle with chronic, subclinical mastitis. Whole genome sequence analysis revealed a complete replacement of the initial predominant variant by another isogenic variant. We report for the first time within-host evolution towards a sigma factor SigB-deficient pathotype in S. aureus bovine mastitis, associated with a single nucleotide polymorphism in rsbU (G368A → G122D), a contributor to SigB-functionality. The emerged SigB-deficient pathotype exhibits a substantial shift to new phenotypic traits comprising strong proteolytic activity and poly-N-acetylglucosamine (PNAG)-based biofilm production. This possibly unlocks new nutritional resources and promotes immune evasion, presumably facilitating extracellular persistence within the host. Moreover, we observed an adaptation towards attenuated virulence using a mouse infection model. This study extends the role of sigma factor SigB in S. aureus pathogenesis, so far described to be required for intracellular persistence during chronic infections. Our findings suggest that S. aureus SigB-deficiency is an alternative mechanism for persistence and underpin the clinical relevance of staphylococcal SigB-deficient variants which are consistently isolated during human chronic infections.

Contribution of YjbIH to Virulence Factor Expression and Host Colonization in Staphylococcus aureus

To persist within the host and cause disease, Staphylococcus aureus relies on its ability to precisely fine-tune virulence factor expression in response to rapidly changing environments. During an unbiased transposon mutant screen, we observed that disruption of a two-gene operon, yjbIH, resulted in decreased levels of pigmentation and aureolysin (Aur) activity relative to the wild-type strain. Further analyses revealed that YjbH, a predicted thioredoxin-like oxidoreductase, is predominantly responsible for the observed yjbIH mutant phenotypes, though a minor role exists for the putative truncated hemoglobin YjbI. These differences were due to significantly decreased expression of crtOPQMN and aur Previous studies found that YjbH targets the disulfide- and oxidative stress-responsive regulator Spx for degradation by ClpXP. The absence of yjbH or yjbI resulted in altered sensitivities to nitrosative and oxidative stress and iron deprivation. Additionally, aconitase activity was altered in the yjbH and yjbI mutant strains. Decreased levels of pigmentation and aureolysin (Aur) activity in the yjbH mutant were found to be Spx dependent. Lastly, we used a murine sepsis model to determine the effect of the yjbIH deletion on pathogenesis and found that the mutant was better able to colonize the kidneys and spleens during an acute infection than the wild-type strain. These studies identified changes in pigmentation and protease activity in response to YjbIH and are the first to have shown a role for these proteins during infection.

Protease-Mediated Growth of Staphylococcus aureus on Host Proteins Is opp3 Dependent

Staphylococcus aureus has the ability to cause infections in a variety of niches, suggesting a robust metabolic capacity facilitating proliferation under various nutrient conditions. The mature skin abscess is glucose depleted, indicating that peptides and free amino acids are important sources of nutrients for S. aureus. Our studies have found that mutations in both pyruvate carboxykinase and glutamate dehydrogenase, enzymes that function in essential gluconeogenesis reactions when amino acids serve as the major carbon source, reduce bacterial burden in a murine skin abscess model. Moreover, peptides liberated from collagen by host protease MMP-9 as well as the staphylococcal protease aureolysin support S. aureus growth in an Opp3-dependent manner under nutrient-limited conditions. Additionally, the presence of peptides induces aureolysin expression. Overall, these studies define one pathway by which S. aureus senses a nutrient-limiting environment and induces factors that function to acquire and utilize carbon from host-derived sources.

Staphylococcus aureus has the ability to cause infections in multiple organ systems, suggesting an ability to rapidly adapt to changing carbon and nitrogen sources. Although there is little information about the nutrients available at specific sites of infection, a mature skin abscess has been characterized as glucose depleted, indicating that peptides and free amino acids are an important source of nutrients for the bacteria. Our studies have found that mutations in enzymes necessary for growth on amino acids, including pyruvate carboxykinase (ΔpckA) and glutamate dehydrogenase (ΔgudB), reduced the ability of the bacteria to proliferate within a skin abscess, suggesting that peptides and free amino acids are important for S. aureus growth. Furthermore, we found that collagen, an abundant host protein that is present throughout a skin abscess, serves as a reservoir of peptides. To liberate peptides from the collagen, we identified that the host protease, MMP-9, as well as the staphylococcal proteases aureolysin and staphopain B function to cleave collagen into peptide fragments that can support S. aureus growth under nutrient-limited conditions. Moreover, the oligopeptide transporter Opp3 is the primary staphylococcal transporter responsible for peptide acquisition. Lastly, we observed that the presence of peptides (3-mer to 7-mer) induces the expression of aureolysin, suggesting that S. aureus has the ability to detect peptides in the environment.

Staphylococcus aureus Secreted Toxins and Extracellular Enzymes

Staphylococcus aureus is a formidable pathogen capable of causing infections in different sites of the body in a variety of vertebrate animals, including humans and livestock. A major contribution to the success of S. aureus as a pathogen is the plethora of virulence factors that manipulate the host's innate and adaptive immune responses. Many of these immune modulating virulence factors are secreted toxins, cofactors for activating host zymogens, and exoenzymes. Secreted toxins such as pore-forming toxins and superantigens are highly inflammatory and can cause leukocyte cell death by cytolysis and clonal deletion, respectively. Coagulases and staphylokinases are cofactors that hijack the host's coagulation system. Exoenzymes, including nucleases and proteases, cleave and inactivate various immune defense and surveillance molecules, such as complement factors, antimicrobial peptides, and surface receptors that are important for leukocyte chemotaxis. Additionally, some of these secreted toxins and exoenzymes can cause disruption of endothelial and epithelial barriers through cell lysis and cleavage of junction proteins. A unique feature when examining the repertoire of S. aureus secreted virulence factors is the apparent functional redundancy exhibited by the majority of the toxins and exoenzymes. However, closer examination of each virulence factor revealed that each has unique properties that have important functional consequences. This chapter provides a brief overview of our current understanding of the major secreted virulence factors critical for S. aureus pathogenesis.

Staphylococcus aureus Manipulates Innate Immunity through Own and Host-Expressed Proteases

Neutrophils, complement system and skin collectively represent the main elements of the innate immune system, the first line of defense of the host against many common microorganisms. Bacterial pathogens have evolved strategies to counteract all these defense activities. Specifically, Staphylococcus aureus, a major human pathogen, secretes a variety of immune evasion molecules including proteases, which cleave components of the innate immune system or disrupt the integrity of extracellular matrix and intercellular connections of tissues. Additionally, S. aureus secretes proteins that can activate host zymogens which, in turn, target specific defense components. Secreted proteins can also inhibit the anti-bacterial function of neutrophils or complement system proteases, potentiating S. aureus chances of survival. Here, we review the current understanding of these proteases and modulators of host proteases in the functioning of innate immunity and describe the importance of these mechanisms in the pathology of staphylococcal diseases.

Extracellular Bacterial Proteases in Chronic Wounds: A Potential Therapeutic Target?

Significance: Bacterial biofilms are considered to be responsible for over 80% of persistent infections, including chronic lung infections, osteomyelitis, periodontitis, endocarditis, and chronic wounds. Over 60% of chronic wounds are colonized with bacteria that reside within a biofilm. The exaggerated proteolytic environment of chronic wounds, more specifically elevated matrix metalloproteinases, is thought to be one of the possible reasons as to why chronic wounds fail to heal. However, the role of bacterial proteases within chronic wounds is not fully understood. Recent Advances: Recent research has shown that bacterial proteases can enable colonization and facilitate bacterial immune evasion. The inhibition of bacterial proteases such as Pseudomonas aeruginosa elastase B (LasB) has resulted in the disruption of the bacterial biofilm in vitro. P. aeruginosa is thought to be a key pathogen in chronic wound infection, and therefore, the disruption of these biofilms, potentially through the targeting of P. aeruginosa bacterial proteases, is an attractive therapeutic endeavor. Critical Issues: Disrupting biofilm formation through the inhibition of bacterial proteases may lead to the dissemination of bacteria from the biofilm, allowing planktonic cells to colonize new sites within the wound. Future Directions: Despite a plethora of evidence supporting the role of bacterial proteases as virulence factors in infection, there remains a distinct lack of research into the effect of bacterial proteases in chronic wounds. To assess the viability of targeting bacterial proteases, future research should aim to understand the role of these proteases in a variety of chronic wound subtypes.

Microbial inhibitors of cysteine proteases

Cysteine proteases are one of the major classes of proteolytic enzymes involved in a number of physiological and pathological processes in plants, animals and microorganisms. When their synthesis, activity and localization in mammalian cells are altered, they may contribute to the development of many diseases, including rheumatoid arthritis, osteoporosis and cancer. Therefore, cysteine proteases have become promising drug targets for the medical treatment of these disorders. Inhibitors of cysteine proteases are also produced by almost every group of living organisms, being responsible for the control of intracellular proteolytic activity. Microorganisms synthesize cysteine protease inhibitors not only to regulate the activity of endogenous, often virulent enzymes, but also to hinder the host's proteolytic defense system and evade its immune responses against infections. Present work describes known to date microbial inhibitors of cysteine proteases in terms of their structure, enzyme binding mechanism, specificity and pathophysiological roles. The overview of both proteinaceous and small-molecule inhibitors produced by all groups of microorganisms (bacteria, archaea, fungi, protists) and viruses is provided. Subsequently, possible applications of microbial inhibitors in science, medicine and biotechnology are also highlighted.

Autocatalytic activation of a thermostable glutamyl endopeptidase capable of hydrolyzing proteins at high temperatures

Glutamyl endopeptidases (GSEs) specifically hydrolyze peptide bonds formed by α-carboxyl groups of Glu and Asp residues. We cloned the gene for a thermophilic GSE (designated TS-GSE) from Thermoactinomyces sp. CDF. A proform of TS-GSE that contained a 61-amino acid N-terminal propeptide and a 218-amino acid mature domain was produced in Escherichia coli. We found that the proform possessed two processing sites and was capable of autocatalytic activation via multiple pathways. The N-terminal propeptide could be autoprocessed at the Glu^-1-Ser¹ bond to directly generate the mature enzyme. It could also be autoprocessed at the Glu^-12-Lys^-11 bond to yield an intermediate, which was then converted into the mature form after removal of the remaining part of the propeptide. The segment surrounding the two processing sites was flexible, which allowed the proform and the intermediate form to be trans-processed into the mature form by either active TS-GSE or heterogeneous proteases. Deletion analysis revealed that the N-terminal propeptide is important for the correct folding and maturation of TS-GSE. The propeptide, even its last 11-amino acid peptide segment, could inhibit the activity of its cognate mature domain. The mature TS-GSE displayed a temperature optimum of 85 °C and retained approximately 90 % of its original activity after incubation at 70 °C for 6 h, representing the most thermostable GSE reported to date. Mutational analysis suggested that the disulfide bonds Cys³²-Cys⁴⁸ and Cys¹⁸⁰-Cys¹⁸³ cumulatively contributed to the thermostability of TS-GSE.

Purification and characterization of naturally occurring HIV-1 (South African subtype C) protease mutants from inclusion bodies

Human immunodeficiency virus (HIV) infections in sub-Saharan Africa represent about 56% of global infections. Many studies have targeted HIV-1 protease for the development of drugs against AIDS. Recombinant HIV-1 protease is used to screen new drugs from synthetic compounds or natural substances. Along with the wild type (C-SA) we also over-expressed and characterized two mutant forms from patients that had shown resistance to protease inhibitors. Using recombinant DNA technology, we constructed three recombinant plasmids in pGEX-6P-1 and expressed them containing a sequence encoding wild type HIV protease and two mutants (I36T↑T contains 100 amino acids and L38L↑N↑L contains 101 amino acids). These recombinant proteins were isolated from inclusion bodies by using QFF anion exchange and GST trap columns. In SDS-PAGE, we obtained these HIV proteases as single bands of approximately 11.5, 11.6 and 11.7 kDa for the wild type, I36T↑Tand L38L↑N↑L mutants, respectively. The enzyme was recovered efficiently (0.25 mg protein/L of Escherichia coli culture) and had high specific activity of 2.02, 2.20 and 1.33 μmol min(-1) mg(-1) at an optimal pH of 5 and temperature of 37 °C for the wild type, I36T↑T and L38L↑N↑L, respectively. The method employed here provides an easy and rapid purification of the HIV-1(C-SA) protease from the inclusion bodies, with high yield and high specific activities.

Identification of the Staphylococcus aureus vfrAB operon, a novel virulence factor regulatory locus

During a screen of the Nebraska Transposon Mutant Library, we identified 71 mutations in the Staphylococcus aureus genome that altered hemolysis on blood agar medium. Although many of these mutations disrupted genes known to affect the production of alpha-hemolysin, two of them were associated with an apparent operon, designated vfrAB, that had not been characterized previously. Interestingly, a ΔvfrB mutant exhibited only minor effects on the transcription of the hla gene, encoding alpha-hemolysin, when grown in broth, as well as on RNAIII, a posttranscriptional regulatory RNA important for alpha-hemolysin translation, suggesting that VfrB may function at the posttranscriptional level. Indeed, a ΔvfrB mutant had increased aur and sspAB protease expression under these conditions. However, disruption of the known secreted proteases in the ΔvfrB mutant did not restore hemolytic activity in the ΔvfrB mutant on blood agar. Further analysis revealed that, in contrast to the minor effects of VfrB on hla transcription when strains were cultured in liquid media, the level of hla transcription was decreased 50-fold in the absence of VfrB on solid media. These results demonstrate that while VfrB represses protease expression when strains are grown in broth, hla regulation is highly responsive to factors associated with growth on solid media. Intriguingly, the ΔvfrB mutant displayed increased pathogenesis in a model of S. aureus dermonecrosis, further highlighting the complexity of VfrB-dependent virulence regulation. The results of this study describe a phenotype associated with a class of highly conserved yet uncharacterized proteins found in Gram-positive bacteria, and they shed new light on the regulation of virulence factors necessary for S. aureus pathogenesis.

Bacterial proteases: targets for diagnostics and therapy

Proteases are essential for the proliferation and growth of bacteria, and are also known to contribute to bacterial virulence. This makes them interesting candidates as diagnostic and therapeutic targets for infectious diseases. In this review, the authors discuss the most recent developments and potential applications for bacterial proteases in the diagnosis and treatment of bacterial infections. Current and future bacterial protease targets are described and their limitations outlined.

Enteric bacterial proteases in inflammatory bowel disease- pathophysiology and clinical implications

Numerous reports have identified a dysbiosis in the intestinal microbiota in patients suffering from inflammatory bowel diseases (IBD), yet the mechanism(s) in which this complex microbial community initiates or perpetuates inflammation remains unclear. The purpose of this review is to present evidence for one such mechanism that implicates enteric microbial derived proteases in the pathogenesis of IBD. We highlight and discuss studies demonstrating that proteases and protease receptors are abundant in the digestive system. Additionally, we investigate studies demonstrating an association between increased luminal protease activity and activation of protease receptors, ultimately resulting in increased intestinal permeability and exacerbation of colitis in animal models as well as in human IBD. Proteases are essential for the normal functioning of bacteria and in some cases can serve as virulence factors for pathogenic bacteria. Although not classified as traditional virulence factors, proteases originating from commensal enteric bacteria also have a potential association with intestinal inflammation via increased enteric permeability. Reports of increased protease activity in stools from IBD patients support a possible mechanism for a dysbiotic enteric microbiota in IBD. A better understanding of these pathways and characterization of the enteric bacteria involved, their proteases, and protease receptors may pave the way for new therapeutic approaches for these diseases.

Staphopains modulate Staphylococcus aureus biofilm integrity

Staphylococcus aureus is a known cause of chronic biofilm infections that can reside on medical implants or host tissue. Recent studies have demonstrated an important role for proteinaceous material in the biofilm structure. The S. aureus genome encodes many secreted proteases, and there is growing evidence that these enzymes have self-cleavage properties that alter biofilm integrity. However, the specific contribution of each protease and mechanism of biofilm modulation is not clear. To address this issue, we utilized a sigma factor B (ΔsigB) mutant where protease activity results in a biofilm-negative phenotype, thereby creating a condition where the protease(s) responsible for the phenotype could be identified. Using a plasma-coated microtiter assay, biofilm formation was restored to the ΔsigB mutant through the addition of the cysteine protease inhibitor E-64 or by using Staphostatin inhibitors that specifically target the extracellular cysteine proteases SspB and ScpA (called Staphopains). Through construction of gene deletion mutants, we determined that an sspB scpA double mutant restored ΔsigB biofilm formation, and this recovery could be replicated in plasma-coated flow cell biofilms. Staphopain levels were also found to be decreased under biofilm-forming conditions, possibly allowing biofilm establishment. The treatment of S. aureus biofilms with purified SspB or ScpA enzyme inhibited their formation, and ScpA was also able to disperse an established biofilm. The antibiofilm properties of ScpA were conserved across S. aureus strain lineages. These findings suggest an underappreciated role of the SspB and ScpA cysteine proteases in modulating S. aureus biofilm architecture.

Inactivation of the serine proteinase operon (proMCD) of Staphylococcus warneri M: serine proteinase and cysteine proteases are involved in the autolysis

Unlike other members of coagulase negative staphylococci (CNS), strain warneri has proMCD operon, a homologue of sspABC proteinase operon of S. aureus. The proM and proC encode serine glutamyl endopeptidase and cysteine protease respectively, whereas proD directs homologue of SspC, putative cytoplasmic inhibitor which protects the host bacterium from premature activation of SspB. We determined whole nucleotide sequence of proMCD operon of S. warneri M, succeeded in expression of these genes, and investigated their functions by gene inactivation and complementation experiments. In gelatin zymography of the culture supernatant, a 20-kDa band corresponding to PROC cysteine protease was detected. By Western blotting, PROD was also confirmed in the cytoplasmic protein fraction. PROC and PROD showed significant similarity to SspB and SspC of S. aureus (73% and 58%, respectively). Inactivation mutants of proMCD, proCD and proD genes were established, separately. In the proMCD mutant, degradation/processing of extracellular proteins was drastically reduced, suggesting that PROM was responsible for the cleavage of extracellular proteins. By the proD mutation, the growth profile was not affected, and secretion of PROC was retained. Extracellular protein profiles of the proCD and proD mutants were not so different each other, but autolysin profiles were slightly dissimilar, around 39-48 kDa and 20kDa bands in zymogram. Experiments in buffer systems showed that autolysis was significantly diminished in proMCD mutant, and was promoted by addition of purified PROM. The proC gene was cloned into a multicopy plasmid, and introduced into the proMCD mutant. Compared with the wild type, autolysis of the proC-complemented strain was definitely enhanced by addition of purified PROM. These results suggested that PROM and PROC affected the coccal autolysis, through processing of the autolysin.

Bacterial proteases and virulence

Bacterial pathogens rely on proteolysis for variety of purposes during the infection process. In the cytosol, the main proteolytic players are the conserved Clp and Lon proteases that directly contribute to virulence through the timely degradation of virulence regulators and indirectly by providing tolerance to adverse conditions such as those experienced in the host. In the membrane, HtrA performs similar functions whereas the extracellular proteases, in close contact with host components, pave the way for spreading infections by degrading host matrix components or interfering with host cell signalling to short-circuit host cell processes. Common to both intra- and extracellular proteases is the tight control of their proteolytic activities. In general, substrate recognition by the intracellular proteases is highly selective which is, in part, attributed to the chaperone activity associated with the proteases either encoded within the same polypeptide or on separate subunits. In contrast, substrate recognition by extracellular proteases is less selective and therefore these enzymes are generally expressed as zymogens to prevent premature proteolytic activity that would be detrimental to the cell. These extracellular proteases are activated in complex cascades involving auto-processing and proteolytic maturation. Thus, proteolysis has been adopted by bacterial pathogens at multiple levels to ensure the success of the pathogen in contact with the human host.

Collagen-based wound dressings for the treatment of diabetes-related foot ulcers: a systematic review

BACKGROUND

Diabetic foot ulcers are a major source of morbidity, limb loss, and mortality. A prolonged inflammatory response, extracellular matrix degradation irregularities, and increased bacteria presence have all been hypothesized as major contributing factors in the delayed healing of diabetic wounds. Collagen components such as fibroblast and keratinocytes are fundamental to the process of wound healing and skin formation. Wound dressings that contain collagen products create a biological scaffold matrix that supports the regulation of extracellular components and promotes wound healing.

METHODS

A systematic review of studies reporting collagen wound dressings used in the treatment of Diabetic foot ulcers was conducted. Comprehensive searches were run in Ovid MEDLINE, PubMed, EMBASE, and ISI Web of Science to capture citations pertaining to the use of collagen wound dressings in the treatment of diabetic foot ulcers. The searches were limited to human studies reported in English.

RESULTS

Using our search strategy, 26 papers were discussed, and included 13 randomized designs, twelve prospective cohorts, and one retrospective cohort, representing 2386 patients with diabetic foot ulcers. Our design was not a formal meta-analysis. In those studies where complete epithelialization, 58% of collagen-treated wounds completely healed (weighted mean 67%). Only 23% of studies reported control group healing with 29% healing (weighted mean 11%) described for controls.

CONCLUSION

Collagen-based wound dressings can be an effective tool in the healing of diabetic foot wounds. The current studies show an overall increase in healing rates despite limitations in study designs. This study suggests that future works focus on biofilms and extracellular regulation, and include high risk patients.

Extracellular proteases are key mediators of Staphylococcus aureus virulence via the global modulation of virulence-determinant stability

Staphylococcus aureus is a highly virulent and successful pathogen that causes a diverse array of diseases. Recently, an increase of severe infections in healthy subjects has been observed, caused by community-associated methicillin-resistant S. aureus (CA-MRSA). The reason for enhanced CA-MRSA virulence is unclear; however, work suggests that it results from hypersecretion of agr-regulated toxins, including secreted proteases. In this study, we explore the contribution of exo-proteases to CA-MRSA pathogenesis using a mutant lacking all 10 enzymes. We show that they are required for growth in peptide-rich environments, serum, in the presence of antimicrobial peptides (AMPs), and in human blood. We also reveal that extracellular proteases are important for resisting phagocytosis by human leukocytes. Using murine infection models, we reveal contrasting roles for the proteases in morbidity and mortality. Upon exo-protease deletion, we observed decreases in abscess formation, and impairment during organ invasion. In contrast, we observed hypervirulence of the protease-null strain in the context of mortality. This dichotomy is explained by proteomic analyses, which demonstrates exo-proteases to be key mediators of virulence-determinant stability. Specifically, increased abundance of both secreted (e.g. α-toxin, Psms, LukAB, LukE, PVL, Sbi, γ-hemolysin) and surface-associated (e.g. ClfA+B, FnbA+B, IsdA, Spa) proteins was observed upon protease deletion. Collectively, our findings provide a unique insight into the progression of CA-MRSA infections, and the role of secreted proteolytic enzymes.

Induction of the staphylococcal proteolytic cascade by antimicrobial fatty acids in community acquired methicillin resistant Staphylococcus aureus

Community acquired methicillin resistant Staphylococcus aureus (CA-MRSA), and the USA300 strain of CA-MRSA in particular, are known for their rapid community transmission, and propensity to cause aggressive skin and soft tissue infections. To assess factors that contribute to these hallmark traits of CA-MRSA, we evaluated how growth of USA300 and production of secreted virulence factors was influenced on exposure to physiologic levels of unsaturated free fatty acids that would be encountered on the skin or anterior nares, which represent the first sites of contact with healthy human hosts. There was a sharp threshold between sub-inhibitory and inhibitory concentrations, such that 100 µM sapienic acid (C16∶1) and linoleic acid (C18∶1) were sufficient to prevent growth after 24 h incubation, while 25 µM allowed unrestricted growth, and 50 µM caused an approximate 10–12 h lag, followed by unimpeded exponential growth. Conversely, saturated palmitic or stearic acids did not affect growth at 100 µM. Although growth was not affected by 25 µM sapienic or linoleic acid, these and other unsaturated C16 and C18 fatty acids, but not their saturated counterparts, promoted robust production of secreted proteases comprising the Staphylococcal proteolytic cascade. This trait was also manifested to varying degrees in other CA-MRSA, and in genetically diverse methicillin susceptible S. aureus strains. Therefore, induction of the Staphylococcal proteolytic cascade by unsaturated fatty acids is another feature that should now be evaluated as a potential contributing factor in the aggressive nature of skin and soft tissue infections caused by USA300, and as a general virulence mechanism of S. aureus.

Papain-like proteases of Staphylococcus aureus

Staphylococcus aureus remains one of the major humanpathogens, causing a number of diverse infections. the growing antibiotic resistance, including vancomycin and methicilin-resistant strains raises the special interest in virulence mechanism of this pathogen. among a number of extracellular virulence factors, S. aureus secretes several proteases of three catalytic classes-metallo, serine and papain-like cysteine proteases. the expression of proteolytic enzymes is strictly controlled by global regulators of virulence factors expression agr and sar and proteases take a role in a phenotype change in postlogarithmic phase of growth. the staphylococcal proteases are secreted as proenzymes and undergo activation in a cascade manner. Staphopains, two cysteine, papain-like proteases of S. aureus are both approximately 20 kDa proteins that have almost identical three-dimensional structures, despite sharing limited primary sequence identity. although staphopain a displays activity similar to cathepsins, recognising hydrophobic residues at P2 position and large charged residues at P1, staphopain B differs significantly, showing significant preference towards β-branched residues at P2 and accepting only small, neutral residues at the P1 position. there is limited data available on the virulence potential of staphopains in in vivo models. However, in vitro experiments have demonstrated a very broad activity of these enzymes, including destruction of connective tissue, disturbance of clotting and kinin systems and direct interaction with host immune cells. Staphopain genes in various staphylococci species are regularly followed by a gene encoding an extremely specific inhibitor of the respective staphopain. This pattern is conserved across species and it is believed that inhibitors (staphostatins) protect the cytoplasm of the cell from premature activation of staphopains during protein folding. Notably, production and activity of staphopains is controlled on each level, from gene expression, through presence of specific inhibitors in cytoplasm, to the cascade-like activation in extracellular environment. Since these systems are highly conserved, this points to the importance of these proteases in the survival and/or pathogenicity of S. aureus.

Inhibition of Staphylococcus aureus cysteine proteases by human serpin potentially limits staphylococcal virulence

Bacterial proteases are considered virulence factors and it is presumed that by abrogating their activity, host endogenous protease inhibitors play a role in host defense against invading pathogens. Here we present data showing that Staphylococcus aureus cysteine proteases (staphopains) are efficiently inhibited by Squamous Cell Carcinoma Antigen 1 (SCCA1), an epithelial-derived serpin. The high association rate constant (k(ass)) for inhibitory complex formation (1.9×10(4) m/s and 5.8×10(4) m/s for staphopain A and staphopain B interaction with SCCA1, respectively), strongly suggests that SCCA1 can regulate staphopain activity in vivo at epithelial surfaces infected/colonized by S. aureus. The mechanism of staphopain inhibition by SCCA1 is apparently the same for serpin interaction with target serine proteases whereby the formation of a covalent complex result in cleavage of the inhibitory reactive site peptide bond and associated release of the C-terminal serpin fragment. Interestingly, the SCCA1 reactive site closely resembles a motif in the reactive site loop of native S. aureus-derived inhibitors of the staphopains (staphostatins). Given that S. aureus is a major pathogen of epithelial surfaces, we suggest that SCCA1 functions to temper the virulence of this bacterium by inhibiting the staphopains.

Substrate specificity of Staphylococcus aureus cysteine proteases--Staphopains A, B and C

Human strains of Staphylococcus aureus secrete two papain-like proteases, staphopain A and B. Avian strains produce another homologous enzyme, staphopain C. Animal studies suggest that staphopains B and C contribute to bacterial virulence, in contrast to staphopain A, which seems to have a virulence unrelated function. Here we present a detailed study of substrate preferences of all three proteases. The specificity of staphopain A, B and C substrate-binding subsites was mapped using different synthetic substrate libraries, inhibitor libraries and a protein substrate combinatorial library. The analysis demonstrated that the most efficiently hydrolyzed sites, using Schechter and Berger nomenclature, comprise a P2-Gly↓Ala(Ser) sequence motif, where P2 distinguishes the specificity of staphopain A (Leu) from that of both staphopains B and C (Phe/Tyr). However, we show that at the same time the overall specificity of staphopains is relaxed, insofar as multiple substrates that diverge from the sequences described above are also efficiently hydrolyzed.

From transcription to activation: how group A streptococcus, the flesh-eating pathogen, regulates SpeB cysteine protease production

Streptococcal pyrogenic exotoxin B (SpeB) is a protease secreted by group A streptococci and known to degrade a wide range of host and GAS proteins in vitro. Although the role of SpeB in GAS infection is debated, recent evidence has conclusively demonstrated that SpeB is critical for the pathogenesis of severe invasive disease caused by GAS. Genetic inactivation of the speB gene results in significantly decreased virulence in a necrotizing fasciitis model of infection. Production of fully active SpeB by GAS is extremely complex. Following transcription and translation the SpeB protein is secreted as an inactive zymogen, which is autocatalytically processed through a series of intermediates to form an active protease. Each step from transcription to protease activation is tightly controlled and regulated by the bacterial cell reflecting the critical role played by this virulence factor in GAS infection. Here we review the molecular aspects of SpeB production by GAS from transcription to activation and the multiple layers of control involved.

Staphylococcus aureus seroproteomes discriminate ruminant isolates causing mild or severe mastitis

Staphylococcus aureus is a major cause of mastitis in ruminants. In ewe mastitis, symptoms range from subclinical to gangrenous mastitis. S. aureus factors or host-factors contributing to the different outcomes are not completely elucidated. In this study, experimental mastitis was induced on primiparous ewes using two S. aureus strains, isolated from gangrenous (strain O11) or subclinical (strain O46) mastitis. Strains induced drastically distinct clinical symptoms when tested in ewe and mice experimental mastitis. Notably, they reproduced mild (O46) or severe (O11) mastitis in ewes. Ewe sera were used to identify staphylococcal immunoreactive proteins commonly or differentially produced during infections of variable severity and to define core and accessory seroproteomes. Such SERological Proteome Analysis (SERPA) allowed the identification of 89 immunoreactive proteins, of which only 52 (58.4%) were previously identified as immunogenic proteins in other staphylococcal infections. Among the 89 proteins identified, 74 appear to constitute the core seroproteome. Among the 15 remaining proteins defining the accessory seroproteome, 12 were specific for strain O11, 3 were specific for O46. Distribution of one protein specific for each mastitis severity was investigated in ten other strains isolated from subclinical or clinical mastitis. We report here for the first time the identification of staphylococcal immunogenic proteins common or specific to S. aureus strains responsible for mild or severe mastitis. These findings open avenues in S. aureus mastitis studies as some of these proteins, expressed in vivo, are likely to account for the success of S. aureus as a pathogen of the ruminant mammary gland.

Human SCCA serpins inhibit staphylococcal cysteine proteases by forming classic "serpin-like" covalent complexes

Proteolytic enzymes secreted by Staphylococcus aureus are considered important virulence factors. Here, we present data showing that staphylococci-derived cysteine proteases (staphopains) are efficiently inhibited by squamous cell carcinoma antigen 1 (SCCA1), a serpin abundant on the epithelial surfaces. The high association rate constant (k(ass)) for inhibitory complex formation (1.9 × 10⁴ and 5.8 ×10⁴ M⁻¹ s⁻¹ for staphopain A and staphopain B interaction with SCCA1, respectively) argues that SCCA1 can restrain staphopain activity in vivo at epithelial sites colonized by S. aureus. The mechanism of staphopain inhibition by SCCA1 is apparently the same as for serpin interaction with target serine proteases. The formation of a covalent complex results in cleavage of the SCCA1 reactive site peptide bond, and it is associated with the release of the C-terminal peptide of 37 amino acid residues from the serpin. Significantly, the SCCA1 reactive site closely resembles a motif in the reactive site loop of natural S. aureus-derived inhibitors of the staphopains (staphostatins). Taking into account that SCCA1 is predominantly expressed in epithelial tissues, including respiratory pathways, hair follicles and skin [Kato, H. (1996). Expression and function of squamous cell carcinoma antigen. Anticancer Res.16, 2149-2153.], all of which are regularly colonized by S. aureus, the physiological relevance of SCCA1-staphopain B interaction as a defense mechanism seems to be very well substantiated.

Degradation of fibrinogen and collagen by staphopains, cysteine proteases released from Staphylococcus aureus

Staphylococcus aureus is the most frequently isolated pathogen in gram-positive sepsis often complicated by a blood clotting disorder, and is the leading cause of infective endocarditis induced by bacterial destruction of endocardial tissues. The bacterium secretes cysteine proteases referred to as staphopain A (ScpA) and staphopain B (SspB). To investigate virulence activities of staphopains pertinent to clotting disorders and tissue destruction, we examined their effects on collagen, one of the major tissue components, and on plasma clotting. Both staphopains prolonged the partial thromboplastin time of plasma in a dose- and activity-dependent manner, with SspB being threefold more potent than ScpA. Staphopains also prolonged the thrombin time of both plasma and fibrinogen, indicating that these enzymes can cause impaired plasma clotting through fibrinogen degradation. Whereas SspB cleaved the fibrinogen Aα-chain at the C-terminal region very efficiently, ScpA degraded it rather slowly. This explains the superior ability of the former enzyme to impair fibrinogen clottability. Enzymically active staphopains, at concentrations as low as 10 nM, degraded collagen with comparable efficiency. These results show novel virulence activities of staphopains in degrading fibrinogen and collagen, and suggest an involvement of staphopains in the clotting impairment and tissue destruction caused by staphylococcal infection.

Biological relevance of natural alpha-toxin fragments from Staphylococcus aureus

Serine proteases represent an essential part of cellular homeostasis by generating biologically active peptides. In bacteria, proteolysis serves two different roles: a major housekeeping function and the destruction of foreign or target cell proteins, thereby promoting bacterial invasion. In the process, other virulence factors such as exotoxins become affected. In Staphylococcus aureus culture supernatant, the pore-forming alpha-toxin is cleaved by the coexpressed V8 protease and aureolysin. The oligomerizing and pore-forming abilities of five such spontaneously occurring N- and C-terminal alpha-toxin fragments were studied. (3)H-marked alpha-toxin fragments bound to rabbit erythrocyte membranes but only fragments with intact C termini, missing 8, 12 and 71 amino acids from their N-terminal, formed stable oligomers. All isolated fragments induced intoxication of mouse adrenocortical Y1 cells in vitro, though the nature of membrane damage for a fragment, degraded at its C terminus, remained obscure. Only one fragment, missing the first eight N-terminal amino acids, induced irreversible intoxication of Y1 cells in the same manner as the intact toxin. Four of the isolated fragments caused swelling, indicating altered channel formation. Fragments missing 12 and 71 amino acids from the N terminus occupied the same binding sites on Y1 cell membranes, though they inhibited membrane damage caused by intact toxin. In conclusion, N-terminal deletions up to 71 amino acids are tolerated, though the kinetics of channel formation and the channel's properties are altered. In contrast, digestion at the C terminus results in nonfunctional species.

Elafin is specifically inactivated by RgpB from Porphyromonas gingivalis by distinct proteolytic cleavage

Abstract Porphyromonas gingivalis, the major causative bacterium of periodontitis, contributes significantly to elevated proteolytic activity at periodontal pockets owing to the presence of both bacteria and host, predominantly neutrophil-derived, serine proteases. Normally the activity of the latter enzymes is tightly regulated by endogenous proteins, including elafin, a potent neutrophil elastase and proteinase 3 inhibitor released from epithelial cells at sites of inflammation. Here, we report that all three gingipains (HRgpA, RgpB, and Kgp) have the ability to degrade elafin, with RgpB being far more efficient than other gingipains. RgpB efficiently inactivates the inhibitory activity of elafin at subnanomolar concentrations through proteolysis limited to the Arg22-Cys23 peptide bond within the surface loop harboring the inhibitor active site. Notably, elafin resists inactivation by several Staphylococcus aureus-derived serine and cysteine proteases, confirming the high stability of this protein against proteolytic degradation. Therefore, we conclude that elafin inactivation by RgpB represents a specific pathogenic adaptation of P. gingivalis to disturb the protease-protease inhibitor balance in the infected gingival tissue. This contributes to enhanced degradation of host proteins and generation of a pool of peptides serving as nutrients for this asaccharolytic pathogen.

Recent human-to-poultry host jump, adaptation, and pandemic spread of Staphylococcus aureus

The impact of globalization on the emergence and spread of pathogens is an important veterinary and public health issue. Staphylococcus aureus is a notorious human pathogen associated with serious nosocomial and community-acquired infections. In addition, S. aureus is a major cause of animal diseases including skeletal infections of poultry, which are a large economic burden on the global broiler chicken industry. Here, we provide evidence that the majority of S. aureus isolates from broiler chickens are the descendants of a single human-to-poultry host jump that occurred approximately 38 years ago (range, 30 to 63 years ago) by a subtype of the worldwide human ST5 clonal lineage unique to Poland. In contrast to human subtypes of the ST5 radiation, which demonstrate strong geographic clustering, the poultry ST5 clade was distributed in different continents, consistent with wide dissemination via the global poultry industry distribution network. The poultry ST5 clade has undergone genetic diversification from its human progenitor strain by acquisition of novel mobile genetic elements from an avian-specific accessory gene pool, and by the inactivation of several proteins important for human disease pathogenesis. These genetic events have resulted in enhanced resistance to killing by chicken heterophils, reflecting avian host-adaptive evolution. Taken together, we have determined the evolutionary history of a major new animal pathogen that has undergone rapid avian host adaptation and intercontinental dissemination. These data provide a new paradigm for the impact of human activities on the emergence of animal pathogens.

Extracellular proteases inhibit protein-dependent biofilm formation in Staphylococcus aureus

In Staphylococcus aureus, biofilm formation can be associated with the production of surface-anchored proteins, including Bap, SasG, FnBPs or Spa. By mutational analysis, and using a model strain in which biofilm formation was Bap-dependent, we found that sigma(B) was essential for protein-dependent biofilm development. Non-polar mutations of sigma(B) in genetically unrelated S. aureus strains lowered the Bap expression and completely impaired biofilm development. Although Northern blot analysis showed a slight reduction in bap transcription, we demonstrated that Aur and SspA, two proteases that are overexpressed in the sigB mutant strain and are capable of degrading Bap, inhibit biofilm formation. Interestingly, a double sigB-agr mutant, which showed a diminished capacity to express extracellular proteases, was able to restore biofilm formation. Since the vast majority of the S. aureus global regulators control the expression of the extracellular proteases, the results of this work demonstrate the existence of a new pathway controlling protein-mediated biofilm formation in which different global regulators modulate biofilm formation by controlling the expression of S. aureus extracellular proteases.

Properties of a non collagen-degrading Bacillus subtilis keratinase

Bacillus subtilis S14 produces a keratinase (KerS14) with non collagen-degrading activity. Indeed, this is the first keratinase described so far that does not have any detectable effect on collagen, which is a crucial property for an enzyme intended to be used in skin dehairing. Because of its importance as an industrial tanning enzyme, we report the biochemical characterization of KerS14. This protein exhibited an apparent molecular mass of 27 kDa, a pI of 6.5, and an optimum pH in the range of 8.0-9.0. The enzyme's activity was stimulated by Mn2+ (7.7-fold), Ca2+ (6.1-fold), Mg2+ (4.9-fold), and Co2+ (4.0-fold) but was inhibited by Cu2+ and Zn2+. Using p-nitroanilide and methylcoumarine derivatized peptides, we observed that KerS14 prefered Arg at subsite P1, small amino acid residues at subsite P2, and Gln or Glu at subsite P3. KerS14 presented higher keratin degradation specificity than other commercial proteases. Its high keratinolytic activity and the absence of virtually any activity against collagen remark the biotechnological potential of this enzyme to be used at larger scales in tannery dehairing processes.

Agr-mediated dispersal of Staphylococcus aureus biofilms

The agr quorum-sensing system of Staphylococcus aureus modulates the expression of virulence factors in response to autoinducing peptides (AIPs). Recent studies have suggested a role for the agr system in S. aureus biofilm development, as agr mutants exhibit a high propensity to form biofilms, and cells dispersing from a biofilm have been observed displaying an active agr system. Here, we report that repression of agr is necessary to form a biofilm and that reactivation of agr in established biofilms through AIP addition or glucose depletion triggers detachment. Inhibitory AIP molecules did not induce detachment and an agr mutant was non-responsive, indicating a dependence on a functional, active agr system for dispersal. Biofilm detachment occurred in multiple S. aureus strains possessing divergent agr systems, suggesting it is a general S. aureus phenomenon. Importantly, detachment also restored sensitivity of the dispersed cells to the antibiotic rifampicin. Proteinase K inhibited biofilm formation and dispersed established biofilms, suggesting agr-mediated detachment occurred in an ica-independent manner. Consistent with a protease-mediated mechanism, increased levels of serine proteases were detected in detaching biofilm effluents, and the serine protease inhibitor PMSF reduced the degree of agr-mediated detachment. Through genetic analysis, a double mutant in the agr-regulated Aur metalloprotease and the SplABCDEF serine proteases displayed minimal extracellular protease activity, improved biofilm formation, and a strongly attenuated detachment phenotype. These findings indicate that induction of the agr system in established S. aureus biofilms detaches cells and demonstrate that the dispersal mechanism requires extracellular protease activity.

A biofilm is a surface-attached community of cells bound together by an extracellular matrix. In a bacterial infection, biofilm-encased cells are protected from antibiotic therapy and host immune response, and these encased cells can develop into a chronic infection. Staphylococcus aureus is a prominent bacterial pathogen known to form biofilms on many medical implants and host tissues. In this report, we demonstrate that repression of the S. aureus quorum-sensing system is required to form a biofilm, and quorum-sensing reactivation in established biofilms disperses the cells. Genetic and molecular analysis demonstrates that quorum-sensing is activated before and required for the detachment mechanism. Detachment is protease-mediated, as established biofilms are sensitive to a non-specific protease and quorum-sensing activation increases the production of extracellular proteases. Using mutations in the protease genes, we show that these secreted enzymes are required for the detachment mechanism. These findings denote that S. aureus quorum-sensing can function as a dispersal mechanism to colonize new sites, and our results suggest this mechanism could be modulated to treat recalcitrant biofilms.