Environmental acidification drives S. pyogenes pilus expression and microcolony formation on epithelial cells in a FCT-dependent manner

Different Group A Streptococcus pili lead to varying proinflammatory cytokine responses and virulence

The human pathogen Streptococcus pyogenes, or Group A Streptococcus (GAS), is associated with a variety of diseases ranging from mild skin and soft tissue infections to invasive diseases and immune sequelae such as rheumatic heart disease. We have recently reported that one of the virulence factors of this pathogen, the pilus, has inflammatory properties and strongly stimulates the innate immune system. Here we used a range of nonpathogenic Lactococcus lactis gain-of-function mutants, each expressing one of the major pilus types of GAS, to compare the immune responses generated by various types of fully assembled pili. In vitro assays indicated variability in the inflammatory response induced by different pili, with the fibronectin-binding, collagen-binding, T antigen (FCT)-1-type pilus from GAS serotype M6/T6 inducing significantly stronger cytokine secretion than other pili. Furthermore, we established that the same trend of pili-mediated immune response could be modeled in Galleria mellonella larvae, which possess a similar innate immune system to vertebrates. Counterintuitively, across the panel of pili types examined in this study, we observed a negative correlation between the intensity of the immune response demonstrated in our experiments and the disease severity observed clinically in the GAS strains associated with each pilus type. This observation suggests that pili-mediated inflammation is more likely to promote bacterial clearance instead of causing disruptive damages that intensify pathogenesis. This also indicates that pili may not be the main contributor to the inflammatory symptoms seen in GAS diseases. Rather, the immune-potentiating properties of the pilus components could potentially be exploited as a vaccine adjuvant.

In Vitro Models of Bacterial Biofilms: Innovative Tools to Improve Understanding and Treatment of Infections

Bacterial infections are a growing concern to the health care systems. Bacteria in the human body are often found embedded in a dense 3D structure, the biofilm, which makes their eradication even more challenging. Indeed, bacteria in biofilm are protected from external hazards and are more prone to develop antibiotic resistance. Moreover, biofilms are highly heterogeneous, with properties dependent on the bacteria species, the anatomic localization, and the nutrient/flow conditions. Therefore, antibiotic screening and testing would strongly benefit from reliable in vitro models of bacterial biofilms. This review article summarizes the main features of biofilms, with particular focus on parameters affecting biofilm composition and mechanical properties. Moreover, a thorough overview of the in vitro biofilm models recently developed is presented, focusing on both traditional and advanced approaches. Static, dynamic, and microcosm models are described, and their main features, advantages, and disadvantages are compared and discussed.

The effects of sugar in drinking water on Streptococcus pyogenes colonisation in a murine nasopharyngeal infection model

The number of sugar-sweetened beverages consumed per day has been associated with an increased risk of acute rheumatic fever, an autoimmune disease triggered by superficial Streptococcus pyogenes infection. To explore if there could be a biological basis for this association, we used a mouse model of S. pyogenes nasopharyngeal colonisation combined with a dietary intervention. We observed an increased bacterial load in the nasopharynx of mice receiving sucrose drinking water post-infection, suggesting that high sucrose intake promotes S. pyogenes growth and/or survival. This provides new insight into the potential biological basis behind the association seen in humans.

Streptococcus pyogenes TrxSR Two-Component System Regulates Biofilm Production in Acidic Environments

Bacteria form biofilms for their protection against environmental stress and produce virulence factors within the biofilm. Biofilm formation in acidified environments is regulated by a two-component system, as shown by studies on isogenic mutants of the sensor protein of the two-component regulatory system in Streptococcus pyogenes. In this study, we found that the LiaS histidine kinase sensor mediates biofilm production and pilus expression in an acidified environment through glucose fermentation. The liaS isogenic mutant produced biofilms in a culture acidified by hydrochloric acid but not glucose, suggesting that the acidified environment is sensed by another protein. In addition, the trxS isogenic mutant could not produce biofilms or activate the mga promoter in an acidified environment. Mass spectrometry analysis showed that TrxS regulates M protein, consistent with the transcriptional regulation of emm, which encodes M protein. Our results demonstrate that biofilm production during environmental acidification is directly under the control of TrxS.

Genetics, Structure, and Function of Group A Streptococcal Pili

Streptococcus pyogenes (Group A Streptococcus; GAS) is an exclusively human pathogen. This bacterial species is responsible for a large variety of infections, ranging from purulent but mostly self-limiting oropharynx/skin diseases to streptococcal sequelae, including glomerulonephritis and rheumatic fever, as well as life-threatening streptococcal toxic-shock syndrome. GAS displays a wide array of surface proteins, with antigenicity of the M protein and pili utilized for M- and T-serotyping, respectively. Since the discovery of GAS pili in 2005, their genetic features, including regulation of expression, and structural features, including assembly mechanisms and protein conformation, as well as their functional role in GAS pathogenesis have been intensively examined. Moreover, their potential as vaccine antigens has been studied in detail. Pilus biogenesis-related genes are located in a discrete section of the GAS genome encoding fibronectin and collagen binding proteins and trypsin-resistant antigens (FCT region). Based on the heterogeneity of genetic composition and DNA sequences, this region is currently classified into nine distinguishable forms. Pili and fibronectin-binding proteins encoded in the FCT region are known to be correlated with infection sites, such as the skin and throat, possibly contributing to tissue tropism. As also found for pili of other Gram-positive bacterial pathogens, GAS pilin proteins polymerize via isopeptide bonds, while intramolecular isopeptide bonds present in the pilin provide increased resistance to degradation by proteases. As supported by findings showing that the main subunit is primarily responsible for T-serotyping antigenicity, pilus functions and gene expression modes are divergent. GAS pili serve as adhesins for tonsillar tissues and keratinocyte cell lines. Of note, a minor subunit is considered to have a harpoon function by which covalent thioester bonds with host ligands are formed. Additionally, GAS pili participate in biofilm formation and evasion of the immune system in a serotype/strain-specific manner. These multiple functions highlight crucial roles of pili during the onset of GAS infection. This review summarizes the current state of the art regarding GAS pili, including a new mode of host-GAS interaction mediated by pili, along with insights into pilus expression in terms of tissue tropism.

Singularities of Pyogenic Streptococcal Biofilms - From Formation to Health Implication

Biofilms are generally defined as communities of cells involved in a self-produced extracellular matrix adhered to a surface. In biofilms, the bacteria are less sensitive to host defense mechanisms and antimicrobial agents, due to multiple strategies, that involve modulation of gene expression, controlled metabolic rate, intercellular communication, composition, and 3D architecture of the extracellular matrix. These factors play a key role in streptococci pathogenesis, contributing to therapy failure and promoting persistent infections. The species of the pyogenic group together with Streptococcus pneumoniae are the major pathogens belonging the genus Streptococcus, and its biofilm growth has been investigated, but insights in the genetic origin of biofilm formation are limited. This review summarizes pyogenic streptococci biofilms with details on constitution, formation, and virulence factors associated with formation.

Deciphering Streptococcal Biofilms

Streptococci are a diverse group of bacteria, which are mostly commensals but also cause a considerable proportion of life-threatening infections. They colonize many different host niches such as the oral cavity, the respiratory, gastrointestinal, and urogenital tract. While these host compartments impose different environmental conditions, many streptococci form biofilms on mucosal membranes facilitating their prolonged survival. In response to environmental conditions or stimuli, bacteria experience profound physiologic and metabolic changes during biofilm formation. While investigating bacterial cells under planktonic and biofilm conditions, various genes have been identified that are important for the initial step of biofilm formation. Expression patterns of these genes during the transition from planktonic to biofilm growth suggest a highly regulated and complex process. Biofilms as a bacterial survival strategy allow evasion of host immunity and protection against antibiotic therapy. However, the exact mechanisms by which biofilm-associated bacteria cause disease are poorly understood. Therefore, advanced molecular techniques are employed to identify gene(s) or protein(s) as targets for the development of antibiofilm therapeutic approaches. We review our current understanding of biofilm formation in different streptococci and how biofilm production may alter virulence-associated characteristics of these species. In addition, we have summarized the role of surface proteins especially pili proteins in biofilm formation. This review will provide an overview of strategies which may be exploited for developing novel approaches against biofilm-related streptococcal infections.

In silico characterisation of stand-alone response regulators of Streptococcus pyogenes

Bacterial “stand-alone” response regulators (RRs) are pivotal to the control of gene transcription in response to changing cytosolic and extracellular microenvironments during infection. The genome of group A Streptococcus (GAS) encodes more than 30 stand-alone RRs that orchestrate the expression of virulence factors involved in infecting multiple tissues, so causing an array of potentially lethal human diseases. Here, we analysed the molecular epidemiology and biological associations in the coding sequences (CDSs) and upstream intergenic regions (IGRs) of 35 stand-alone RRs from a collection of global GAS genomes. Of the 944 genomes analysed, 97% encoded 32 or more of the 35 tested RRs. The length of RR CDSs ranged from 297 to 1587 nucleotides with an average nucleotide diversity (π) of 0.012, while the IGRs ranged from 51 to 666 nucleotides with average π of 0.017. We present new evidence of recombination in multiple RRs including mga, leading to mga-2 switching, emm-switching and emm-like gene chimerization, and the first instance of an isolate that encodes both mga-1 and mga-2. Recombination was also evident in rofA/nra and msmR loci with 15 emm-types represented in multiple FCT (fibronectin-binding, collagen-binding, T-antigen)-types, including novel emm-type/FCT-type pairings. Strong associations were observed between concatenated RR allele types, and emm-type, MLST-type, core genome phylogroup, and country of sampling. No strong associations were observed between individual loci and disease outcome. We propose that 11 RRs may form part of future refinement of GAS typing systems that reflect core genome evolutionary associations. This subgenomic analysis revealed allelic traits that were informative to the biological function, GAS strain definition, and regional outbreak detection.

Current Understanding of Group A Streptococcal Biofilms

Background:

It has been proposed that GAS may form biofilms. Biofilms are microbial communities that aggregate on a surface, and exist within a self-produced matrix of extracellular polymeric substances. Biofilms offer bacteria an increased survival advantage, in which bacteria persist, and resist host immunity and antimicrobial treatment. The biofilm phenotype has long been recognized as a virulence mechanism for many Gram-positive and Gram-negative bacteria, however very little is known about the role of biofilms in GAS pathogenesis.

Objective:

This review provides an overview of the current knowledge of biofilms in GAS pathogenesis. This review assesses the evidence of GAS biofilm formation, the role of GAS virulence factors in GAS biofilm formation, modelling GAS biofilms, and discusses the polymicrobial nature of biofilms in the oropharynx in relation to GAS.

Conclusion:

Further study is needed to improve the current understanding of GAS as both a mono-species biofilm, and as a member of a polymicrobial biofilm. Improved modelling of GAS biofilm formation in settings closely mimicking in vivo conditions will ensure that biofilms generated in the lab closely reflect those occurring during clinical infection.

The Arginine Deiminase Pathway Impacts Antibiotic Tolerance during Biofilm-Mediated Streptococcus pyogenes Infections

Bacterial biofilms are responsible for a variety of serious human infections and are notoriously difficult to treat due to their recalcitrance to antibiotics. Further work is necessary to elicit a full understanding of the mechanism of this antibiotic tolerance. The arginine deiminase (ADI) pathway is responsible for bacterial pH maintenance and is highly expressed during biofilm growth in multiple bacterial species. Using the group A Streptococcus (GAS) as a model human pathogen, the ADI pathway was demonstrated to contribute to biofilm growth. The inability of antibiotics to reduce GAS populations when in a biofilm was demonstrated by in vitro studies and a novel animal model of nasopharyngeal infection. However, disruption of the ADI pathway returned GAS biofilms to planktonic levels of antibiotic sensitivity, suggesting the ADI pathway is influential in biofilm-related antibiotic treatment failure and provides a new strategic target for the treatment of biofilm infections in GAS and potentially numerous other bacterial species.IMPORTANCE Biofilm-mediated bacterial infections are a major threat to human health because of their recalcitrance to antibiotic treatment. Through the study of Streptococcus pyogenes, a significant human pathogen that is known to form antibiotic-tolerant biofilms, we demonstrated the role that a bacterial pathway known for responding to acid stress plays in biofilm growth and antibiotic tolerance. This not only provides some insight into antibiotic treatment failure in S. pyogenes infections but also, given the widespread nature of this pathway, provides a potentially broad target for antibiofilm therapies. This discovery has the potential to impact the treatment of many different types of recalcitrant biofilm infections.

Streptococcus pyogenes Capsule Promotes Microcolony-Independent Biofilm Formation

Biofilms play an important role in the pathogenesis of group A streptococcus (GAS), a Gram-positive pathogen responsible for a wide range of infections and with a significant public health impact. Although most GAS serotypes are able to form biofilms, there is a large amount of heterogeneity between individual strains in biofilm formation, as measured by standard crystal violet assays. It is generally accepted that biofilm formation includes the initial adhesion of bacterial cells to a surface followed by microcolony formation, biofilm maturation, and extensive production of extracellular matrix that links together proliferating cells and provides a scaffold for the three-dimensional (3D) biofilm structure. However, our studies show that for GAS strain JS95, microcolony formation is not an essential step in static biofilm formation, and instead, biofilm can be effectively formed from slow-growing or nonreplicating late-exponential- or early-stationary-phase planktonic cells via sedimentation and fixation of GAS chains. In addition, we show that the GAS capsule specifically contributes to the alternative sedimentation-initiated biofilms. Microcolony-independent sedimentation biofilms are similar in morphology and 3D structure to biofilms initiated by actively dividing planktonic bacteria. We conclude that GAS can form biofilms by an alternate noncanonical mechanism that does not require transition from microcolony formation to biofilm maturation and which may be obscured by biofilm phenotypes that arise via the classical biofilm maturation processes.IMPORTANCE The static biofilm assay is a common tool for easy biomass quantification of biofilm-forming bacteria. However, Streptococcus pyogenes biofilm formation as measured by the static assay is strain dependent and yields heterogeneous results for different strains of the same serotype. In this study, we show that two independent mechanisms, for which the protective capsule contributes opposing functions, may contribute to static biofilm formation. We propose that separation of these mechanisms for biofilm formation might uncover previously unappreciated biofilm phenotypes that may otherwise be masked in the classic static assay.

The 5'-nucleotidase S5nA is dispensable for evasion of phagocytosis and biofilm formation in Streptococcus pyogenes

5’-nucleotidases are widespread among all domains of life. The enzymes hydrolyze phosphate residues from nucleotides and nucleotide derivatives. In some pathobiontic bacteria, 5’-nucleotidases contribute to immune evasion by dephosphorylating adenosine mono-, di-, or tri-phosphates, thereby either decreasing the concentration of pro-inflammatory ATP or increasing the concentration of anti-inflammatory adenosine, both acting on purinergic receptors of phagocytic cells. The strict human pathogen Streptococcus pyogenes expresses a surface-associated 5’-nucleotidase (S5nA) under infection conditions that has previously been discussed as a potential virulence factor. Here we show that deletion of the S5nA gene does not significantly affect growth in human blood, evasion of phagocytosis by neutrophils, formation of biofilms and virulence in an infection model with larvae of the greater wax moth Galleria mellonella in S. pyogenes serotypes M6, M18 and M49. Hence, the surface-associated 5’-nucleotidase S5nA seems dispensable for evasion of phagocytosis and biofilm formation in S. pyogenes.

Identification and Characterization of Serotype-Specific Variation in Group A Streptococcus Pilus Expression

Isolates of a given bacterial pathogen often display phenotypic variation, and this can negatively impact public health, for example, by reducing the efficacy of preventative measures. Here, we identify that the human pathogen group A Streptococcus (GAS; Streptococcus pyogenes) expresses pili on its cell surface in a serotype-specific manner. Specifically, we show that serotype M3 GAS isolates, which are nonrandomly associated with causing particularly severe and lethal invasive infections, produce negligible amounts of pili relative to serotype M1 and M49 isolates. Performance of an interserotype transcriptome comparison (serotype M1 versus serotype M3) was instrumental in this discovery. We also identified that the transcriptional regulator Nra positively regulates pilus expression in M3 GAS isolates and that the low level of pilus expression of these isolates correlates with a low level of nra transcription. Finally, we discovered that the phenotypic consequences of low levels of pilus expression by M3 GAS isolates are a reduced ability to adhere to host cells and an increased ability to survive and proliferate in human blood. We propose that an enhanced ability to survive in human blood, in part due to reduced pilus expression, is a contributing factor in the association of serotype M3 isolates with highly invasive infections. In conclusion, our data show that GAS isolates express pili in a serotype-dependent manner and may inform vaccine development, given that pilus proteins are being discussed as possible GAS vaccine antigens.

Biofilm formation of beta-hemolytic group G Streptococcus dysgalactiae subspecies equisimilis isolates and its association with emm polymorphism

Biofilm formation has been well known as a determinant of bacterial virulence. Group G Streptococcus dysgalactiae subspecies equisimilis (SDSE), a relevant pathogen with increasing medical importance, was evaluated for the biofilm-forming potential. Microtiter plate assay was used to assess the most feasible medium for group G SDSE to form a biofilm. Among 246 SDSE isolates examined, 46.7%, 43.5%, 33.3%, and 26.4% of isolates showed moderate or strong biofilm-forming abilities using tryptic soy broth (TSB), brain heart infusion broth (BHI), Todd-Hewitt broth (THB), and C medium with 30 mM glucose (CMG), respectively. The addition of glucose significantly increased the biofilm-forming ability of group G SDSE. FCT (fibronectin-collagen-T-antigen) typing of SDSE was first undertaken and 11 FCT types were found. Positive associations of stG10.0 or negative associations of stG245.0, stG840.0, and stG6.1 with biofilm-forming ability of SDSE were, respectively, found. This was the first investigation demonstrating biofilm-forming potential in clinical group G SDSE isolates; also, some significant associations of biofilm-forming ability with certain emm types were presented.

Investigation of the Fim1 putative pilus locus of Streptococcus equi subspecies equi

The Gram-positive bacterium Streptococcus equi subspecies equi (S. equi) is the causative agent of strangles, among the most frequently diagnosed infectious diseases of horses worldwide. Genome analysis of S. equi strain 4047 (Se4047) identified a putative operon, Fim1, with similarity to the pilus loci of other Gram-positive bacteria. The Fim1 locus was present in all strains of S. equi and its close relative S. equi subspecies zooepidemicus (S. zooepidemicus) that have been studied to date. In this study we provide evidence that the putative structural pilus proteins, SEQ_0936 and CNE, are produced on the cell surface during in vitro growth and in vivo infection. Although the proteins encoded within the Fim1 locus are not essential for attachment or biofilm formation, over-transcription of SEQ_0936 and CNE enhanced attachment to equine tissue in vitro. Our data suggest that whilst the Fim1 locus does not produce a polymerized pilus structure, the products of the Fim1 locus may fulfil an adhesive function. The putative pilus-associated regulator, tetR, which contains a nonsense mutation in S. equi, was able to regulate transcription of the Fim1 locus following repair and over-transcription, confirming its predicted role in the operon.

Pilus biogenesis of Gram-positive bacteria: Roles of sortases and implications for assembly

Successful adherence, colonization, and survival of Gram-positive bacteria require surface proteins, and multiprotein assemblies called pili. These surface appendages are attractive pharmacotherapeutic targets and understanding their assembly mechanisms is essential for identifying a new class of 'anti-infectives' that do not elicit microbial resistance. Molecular details of the Gram-negative pilus assembly are available indepth, but the Gram-positive pilus biogenesis is still an emerging field and investigations continue to reveal novel insights into this process. Pilus biogenesis in Gram-positive bacteria is a biphasic process that requires enzymes called pilus-sortases for assembly and a housekeeping sortase for covalent attachment of the assembled pilus to the peptidoglycan cell wall. Emerging structural and functional data indicate that there are at least two groups of Gram-positive pili, which require either the Class C sortase or Class B sortase in conjunction with LepA/SipA protein for major pilin polymerization. This observation suggests two distinct modes of sortase-mediated pilus biogenesis in Gram-positive bacteria. Here we review the structural and functional biology of the pilus-sortases from select streptococcal pilus systems and their role in Gram-positive pilus assembly.

Increased Pilus Production Conferred by a Naturally Occurring Mutation Alters Host-Pathogen Interaction in Favor of Carriage in Streptococcus pyogenes

Studies of the human pathogen group A Streptococcus (GAS) define the carrier phenotype to be an increased ability to adhere to and persist on epithelial surfaces and a decreased ability to cause disease. We tested the hypothesis that a single amino acid change (Arg135Gly) in a highly conserved sensor kinase (LiaS) of a poorly defined GAS regulatory system contributes to a carrier phenotype through increased pilus production. When introduced into an emm serotype-matched invasive strain, the carrier allele (the gene encoding the LiaS protein with an arginine-to-glycine change at position 135 [liaS^R135G]) recapitulated a carrier phenotype defined by an increased ability to adhere to mucosal surfaces and a decreased ability to cause disease. Gene transcript analyses revealed that the liaS mutation significantly altered transcription of the genes encoding pilus in the presence of bacitracin. Elimination of pilus production in the isogenic carrier mutant decreased its ability to colonize the mouse nasopharynx and to adhere to and be internalized by cultured human epithelial cells and restored the virulence phenotype in a mouse model of necrotizing fasciitis. We also observed significantly reduced survival of the isogenic carrier mutant compared to that of the parental invasive strain after exposure to human neutrophils. Elimination of pilus in the isogenic carrier mutant increased the level of survival after exposure to human neutrophils to that for the parental invasive strain. Together, our data demonstrate that the carrier mutation (liaS^R135G) affects pilus expression. Our data suggest new mechanisms of pilus gene regulation in GAS and that the invasiveness associated with pilus gene regulation in GAS differs from the enhanced invasiveness associated with increased pilus production in other bacterial pathogens.

Modulation of the mechanical properties of bacterial biofilms in response to environmental challenges

In this review, we highlight recent research on the relationship between biofilm matrix composition, biofilm mechanics and environmental stimuli.

Streptococcus pyogenes adhesion and colonization

Streptococcus pyogenes (group A Streptococcus, GAS) is a human-adapted pathogen responsible for a wide spectrum of disease. GAS can cause relatively mild illnesses, such as strep throat or impetigo, and less frequent but severe life-threatening diseases such as necrotizing fasciitis and streptococcal toxic shock syndrome. GAS is an important public health problem causing significant morbidity and mortality worldwide. The main route of GAS transmission between humans is through close or direct physical contact, and particularly via respiratory droplets. The upper respiratory tract and skin are major reservoirs for GAS infections. The ability of GAS to establish an infection in the new host at these anatomical sites primarily results from two distinct physiological processes, namely bacterial adhesion and colonization. These fundamental aspects of pathogenesis rely upon a variety of GAS virulence factors, which are usually under strict transcriptional regulation. Considerable progress has been made in better understanding these initial infection steps. This review summarizes our current knowledge of the molecular mechanisms of GAS adhesion and colonization.

Tissue tropisms in group A Streptococcus: what virulence factors distinguish pharyngitis from impetigo strains?

PURPOSE OF REVIEW

Group A streptococci (GAS) are a common cause of pharyngitis and impetigo, and distinct throat strains and skin strains have been long recognized. This review aims to describe recent advances in molecular differences between throat and skin strains, and the pathogenic mechanisms used by virulence factors that may distinguish between these two groups.

RECENT FINDINGS

Recent findings include a new typing scheme for GAS strains based on sequence clusters of genes encoding the entire surface-exposed portion of M protein; correlations between emm-based typing schemes, clinical disease and surface adhesins; covalent bond formation mediated by GAS pili and other adhesins in binding to host ligands; a key role for superantigens in oropharyngeal infection via binding major histocompatibility complex class II antigen; and migration of GAS-specific Th17 cells from the upper respiratory tract to the brain, which may be relevant to autoimmune sequelae.

SUMMARY

The gap between molecular markers of disease (correlation) and virulence mechanisms (causation) in the establishment of tissue tropisms for GAS infection currently remains wide, but the gap also continues to narrow. Whole genome sequencing combined with mutant construction and improvements in animal models for oropharyngeal infection by GAS may help pave the way for new discoveries.

The YvqE two-component system controls biofilm formation and acid production in Streptococcus pyogenes

In Streptococcus pyogenes, proteins involved in determining virulence are controlled by stand-alone response regulators and by two-component regulatory systems. Previous studies reported that, compared to the parental strain, the yvqE sensor knockout strain showed significantly reduced growth and lower virulence. To determine the function of YvqE, we performed biofilm analysis and pH assays on yvqE mutants, and site-directed mutagenesis of YvqE. The yvqE deletion mutant showed a slower acid production rate, indicating that YvqE regulates acid production from sugar fermentation. The mutant strain, in which the Asp(26) residue in YvqE was replaced with Asn, affected biofilm formation, suggesting that this amino acid senses hydrogen ions produced by fermentative sugar metabolism. Signals received by YvqE were directly or indirectly responsible for inducing pilus expression. This study shows that at low environmental pH, biofilm formation in S. pyogenes is mediated by YvqE and suggests that regulation of pilus expression by environmental acidification could be directly under the control of YvqE.

Artemisia princeps Inhibits Biofilm Formation and Virulence-Factor Expression of Antibiotic-Resistant Bacteria

In this study, we used ethanol extract of A. princeps and investigated its antibacterial effects against MRSA. Ethanol extract of A. princeps significantly inhibited MRSA growth and organic acid production during glucose metabolism at concentrations greater than 1 mg/mL (P < 0.05). MRSA biofilm formation was observed using scanning electron microscopy (SEM) and safranin staining. A. princeps extract was found to inhibit MRSA biofilm formation at concentrations higher than 2 mg/mL significantly (P < 0.05). Bactericidal effects of the A. princeps were observed using confocal laser microscopy, which showed that A. princeps was bactericidal in a dose-dependent manner. Using real-time PCR, expression of mecA, an antibiotic-resistance gene of MRSA, was observed, along with that of sea, agrA, and sarA. A. princeps significantly inhibited mecA, sea, agrA, and sarA, mRNA expression at the concentrations greater than 1 mg/mL (P < 0.05). The phytochemical analysis of A. princeps showed a relatively high content of organic acids and glycosides. The results of this study suggest that the ethanol extract of A. princeps may inhibit proliferation, acid production, biofilm formation, and virulence gene expressions of MRSA, which may be related to organic acids and glycosides, the major components in the extract.

Chamaecyparis obtusa Essential Oil Inhibits Methicillin-Resistant Staphylococcus aureus Biofilm Formation and Expression of Virulence Factors

The emergence of antibiotic-resistant bacteria has caused difficulty in treating infectious diseases. Methicillin-resistant Staphylococcus aureus (MRSA) is one of the most commonly recognized antibiotic-resistant bacteria. Novel antibiotics are urgently required to treat these bacteria. Raw materials derived from natural sources can be used for the development of novel antibiotics, such as Chamaecyparis obtusa (C. obtusa), which has been traditionally used in treating asthmatic disease. In this study, the antibacterial activity of the essential oil (EO) extracted from C. obtusa leaves against MRSA was investigated. MRSA growth and acid production from glucose metabolism were inhibited at concentrations greater than 0.1 mg/mL C. obtusa EO. MRSA biofilm formation was observed using scanning electron microscopy and safranin staining. C. obtusa EO inhibited MRSA biofilm formation at concentrations greater than 0.1 mg/mL. Using real-time polymerase chain reaction, mRNA expression of virulence factor genes, sea, agrA, and sarA, was observed. agrA expression was inhibited with C. obtusa EO concentrations greater than 0.2 mg/mL, whereas inhibition of sea and sarA expression was also observed at a concentration of 0.3 mg/mL. C. obtusa EO was analyzed by gas chromatography (GC) and GC coupled for mass spectrometry, which identified 59 constituents, accounting to 98.99% of the total EO. These findings suggest that C. obtusa EO has antibacterial effects against MRSA, which might be associated with the major components of C. obtusa EO, such as sabinene (19.06%), α-terpinyl acetate (16.99%), bornyl acetate (10.48%), limonene (8.54%), elemol (7.47%), myrcene (5.86%), γ-terpinene (4.04%), and hibaene (3.01%).

Streptococcus pyogenes biofilms-formation, biology, and clinical relevance

Streptococcus pyogenes (group A streptococci, GAS) is an exclusive human bacterial pathogen. The virulence potential of this species is tremendous. Interactions with humans range from asymptomatic carriage over mild and superficial infections of skin and mucosal membranes up to systemic purulent toxic-invasive disease manifestations. Particularly the latter are a severe threat for predisposed patients and lead to significant death tolls worldwide. This places GAS among the most important Gram-positive bacterial pathogens. Many recent reviews have highlighted the GAS repertoire of virulence factors, regulators and regulatory circuits/networks that enable GAS to colonize the host and to deal with all levels of the host immune defense. This covers in vitro and in vivo studies, including animal infection studies based on mice and more relevant, macaque monkeys. It is now appreciated that GAS, like many other bacterial species, do not necessarily exclusively live in a planktonic lifestyle. GAS is capable of microcolony and biofilm formation on host cells and tissues. We are now beginning to understand that this feature significantly contributes to GAS pathogenesis. In this review we will discuss the current knowledge on GAS biofilm formation, the biofilm-phenotype associated virulence factors, regulatory aspects of biofilm formation, the clinical relevance, and finally contemporary treatment regimens and future treatment options.

Biofilm formation by Streptococcus agalactiae: influence of environmental conditions and implicated virulence factors

Streptococcus agalactiae (Group B Streptococcus, GBS) is an important human pathogen that colonizes the urogenital and/or the lower gastro-intestinal tract of up to 40% of healthy women of reproductive age and is a leading cause of sepsis and meningitis in the neonates. GBS can also infect the elderly and immuno-compromised adults, and is responsible for mastitis in bovines. Like other Gram-positive bacteria, GBS can form biofilm-like three-dimensional structures that could enhance its ability to colonize and persist in the host. Biofilm formation by GBS has been investigated in vitro and appears tightly controlled by environmental conditions. Several adhesins have been shown to play a role in the formation of GBS biofilm-like structures, among which are the protein components of pili protruding outside the bacterial surface. Remarkably, antibodies directed against pilus proteins can prevent the formation of biofilms. The implications of biofilm formation in the context of GBS asymptomatic colonization and dissemination to cause invasive disease remain to be investigated in detail.

Actinomyces naeslundii GroEL-dependent initial attachment and biofilm formation in a flow cell system

Actinomyces naeslundii is an early colonizer with important roles in the development of the oral biofilm. The effects of butyric acid, one of short chain fatty acids in A. naeslundii biofilm formation was observed using a flow cell system with Tryptic soy broth without dextrose and with 0.25% sucrose (TSB sucrose). Significant biofilms were established involving live and dead cells in TSB sucrose with 60mM butyric acid but not in concentrations of 6, 30, 40, and 50mM. Biofilm formation failed in 60mM sodium butyrate but biofilm level in 60mM sodium butyrate (pH4.7) adjusted with hydrochloric acid as 60mM butyric media (pH4.7) was similar to biofilm levels in 60mM butyric acid. Therefore, butyric acid and low pH are required for significant biofilm formation in the flow cell. To determine the mechanism of biofilm formation, we investigated initial A. naeslundii colonization in various conditions and effects of anti-GroEL antibody. The initial colonization was observed in the 60mM butyric acid condition and anti-GroEL antibody inhibited the initial colonization. In conclusion, we established a new biofilm formation model in which butyric acid induces GroEL-dependent initial colonization of A. naeslundii resulting in significant biofilm formation in a flow system.

Acidic pH strongly enhances in vitro biofilm formation by a subset of hypervirulent ST-17 Streptococcus agalactiae strains

Streptococcus agalactiae, also known as group B Streptococcus (GBS), is a primary colonizer of the anogenital mucosa of up to 40% of healthy women and an important cause of invasive neonatal infections worldwide. Among the 10 known capsular serotypes, GBS type III accounts for 30 to 76% of the cases of neonatal meningitis. In recent years, the ability of GBS to form biofilm attracted attention for its possible role in fitness and virulence. Here, a new in vitro biofilm formation protocol was developed to guarantee more stringent conditions, to better discriminate between strong-, low-, and non-biofilm-forming strains, and to facilitate interpretation of data. This protocol was used to screen the biofilm-forming abilities of 366 GBS clinical isolates from pregnant women and from neonatal infections of different serotypes in relation to medium composition and pH. The results identified a subset of isolates of serotypes III and V that formed strong biofilms under acidic conditions. Importantly, the best biofilm formers belonged to serotype III hypervirulent clone ST-17. Moreover, the abilities of proteinase K to strongly inhibit biofilm formation and to disaggregate mature biofilms suggested that proteins play an essential role in promoting GBS biofilm initiation and contribute to biofilm structural stability.

Antibacterial Activity of Rhus javanica against Methicillin-Resistant Staphylococcus aureus

In the present study, the leaves of Rhus javanica (R. javanica) were extracted with ethanol, and we investigated the antimicrobial activity of the ethanol extract of R. javanica against methicillin-resistant Staphylococcus aureus (MRSA). Control groups were treated with media containing 0.1% DMSO. The ethanol extract of R. javanica inhibited the growth of MRSA at concentrations ranging from 0.05 to 0.2 mg/mL and inhibited acid production at concentrations higher than 0.1 mg/mL (P < 0.05). MRSA biofilm formation was determined by scanning electron microscopy and safranin staining. The ethanol extract of R. javanica inhibited the formation of MRSA biofilms at concentrations higher than 0.05 mg/mL. In confocal laser scanning microscopy, high concentration (0.4–1.6 mg/mL) of R. javanica extract showed bactericidal effect in a dose-dependent manner. In real-time PCR analysis, R. javanica extract showed the inhibition of the genetic expression of virulence factors such as mecA, sea, agrA, and sarA in MRSA. Preliminary phytochemical analysis revealed the strong presence of phenolics. These results suggest that R. javanica may be a useful medicinal plant for inhibiting MRSA, which may be related to the presence of phenolics in the R. javanica extract.

Could DNA uptake be a side effect of bacterial adhesion and twitching motility?

DNA acquisition promotes the spread of resistance to antibiotics and virulence among bacteria. It is also linked to several natural phenomena including recombination, genome dynamics, adaptation and speciation. Horizontal DNA transfer between bacteria occurs via conjugation, transduction or competence for natural transformation by DNA uptake. Among these, competence is the only mechanism of transformation initiated and entirely controlled by the chromosome of the recipient bacteria. While the molecular mechanisms allowing the uptake of extracellular DNA are increasingly characterized, the function of competence for natural transformation by DNA uptake, the selective advantage maintaining it and the reasons why bacteria take up DNA in the first place are still debated. In this synthesis, I review some of the literature and discuss the four hypotheses on how and why do bacteria take up DNA. I argue that DNA uptake by bacteria is an accidental by-product of bacterial adhesion and twitching motility. Adhesion and motility are generally increased in stressful conditions, which may explain why bacteria increase DNA uptake in these conditions. In addition to its fundamental scientific relevance, the new hypothesis suggested here has significant clinical implications and finds further support from the fact that antibiotics sometimes fail to eliminate the targeted bacterium while inevitably causing stress to others. The widespread misuse of antibiotics may thus not only be selecting for resistant strains, but may also be causing bacteria to take up more DNA with the consequent increase in the chances of acquiring drug resistance and virulence-a scenario in full concordance with the previously reported induction of competence genes by antibiotics in Streptococcus pneumoniae and Legionella pneumophila.

Protective mechanisms of respiratory tract Streptococci against Streptococcus pyogenes biofilm formation and epithelial cell infection

Streptococcus pyogenes (group A streptococci [GAS]) encounter many streptococcal species of the physiological microbial biome when entering the upper respiratory tract of humans, leading to the question how GAS interact with these bacteria in order to establish themselves at this anatomic site and initiate infection. Here we show that S. oralis and S. salivarius in direct contact assays inhibit growth of GAS in a strain-specific manner and that S. salivarius, most likely via bacteriocin secretion, also exerts this effect in transwell experiments. Utilizing scanning electron microscopy documentation, we identified the tested strains as potent biofilm producers except for GAS M49. In mixed-species biofilms, S. salivarius dominated the GAS strains, while S. oralis acted as initial colonizer, building the bottom layer in mixed biofilms and thereby allowing even GAS M49 to form substantial biofilms on top. With the exception of S. oralis, artificial saliva reduced single-species biofilms and allowed GAS to dominate in mixed biofilms, although the overall two-layer structure was unchanged. When covered by S. oralis and S. salivarius biofilms, epithelial cells were protected from GAS adherence, internalization, and cytotoxic effects. Apparently, these species can have probiotic effects. The use of Affymetrix array technology to assess HEp-2 cell transcription levels revealed modest changes after exposure to S. oralis and S. salivarius biofilms which could explain some of the protective effects against GAS attack. In summary, our study revealed a protection effect of respiratory tract bacteria against an important airway pathogen and allowed a first in vitro insight into local environmental processes after GAS enter the respiratory tract.

Pili of gram-positive bacteria: roles in host colonization

In the last decade, pili, which are encoded within pathogenicity islands, have been found in many Gram-positive bacteria, including the major streptococcal and enterococcal pathogens. These long proteinaceous polymers extending from the bacterial surface are constituted of covalently linked pilin subunits, which play major roles in adhesion and host colonization. They are also involved in biofilm formation, a characteristic life-style of the bacteria constituting the oral flora. Pili are highly immunogenic structures that are under the selective pressure of host immune responses. Indeed, pilus expression was found to be heterogeneous in several bacteria with the co-existence of two subpopulations expressing various levels of pili. The molecular mechanisms underlying this complex regulation are poorly characterized except for Streptococcus pneumoniae. In this review, we will discuss the roles of Gram-positive bacteria pili in adhesion to host extracellular matrix proteins, tissue tropism, biofilm formation, modulation of innate immune responses and their contribution to virulence, and in a second part the regulation of their expression. This overview should help to understand the rise of pili as an intensive field of investigation and pinpoints the areas that need further study.

The small regulatory RNA FasX controls pilus expression and adherence in the human bacterial pathogen group A Streptococcus

Bacterial pathogens use cell surface-associated adhesion molecules to promote host attachment and colonization, and the ability to modulate adhesion expression is critical to pathogen success. Here, we show that the human-specific pathogen the group A Streptococcus (GAS) uses a small regulatory RNA (sRNA) to regulate the expression of adhesive pili. The fibronectin/fibrinogen-binding/haemolytic-activity/streptokinase-regulator-X (FasX) sRNA, previously shown to positively regulate expression of the secreted virulence factor streptokinase (SKA), negatively regulates the production of pili on the GAS cell surface. FasX base pairs to the extreme 5' end of mRNA from the pilus biosynthesis operon, and this RNA:RNA interaction reduces the stability of the mRNA, while also inhibiting translation of at least the first gene in the pilus biosynthesis operon (cpa, which encodes a minor pilin protein). The negative regulation of pilus expression by FasX reduces the ability of GAS to adhere to human keratinocytes. Our findings cement FasX sRNA as an important regulator of virulence factor production in GAS and identify that FasX uses at least three distinct mechanisms, positive (ska mRNA) and negative (pilus operon mRNA) regulation of mRNA stability, and negative regulation of mRNA translation (cpa mRNA), to post-transcriptionally regulate target mRNAs during infection.

Identification and functional analysis of the S-layer protein SplA of Paenibacillus larvae, the causative agent of American Foulbrood of honey bees

The Gram-positive, spore-forming bacterium Paenibacillus larvae is the etiological agent of American Foulbrood (AFB), a globally occurring, deathly epizootic of honey bee brood. AFB outbreaks are predominantly caused by two genotypes of P. larvae, ERIC I and ERIC II, with P. larvae ERIC II being the more virulent genotype on larval level. Recently, comparative proteome analyses have revealed that P. larvae ERIC II but not ERIC I might harbour a functional S-layer protein, named SplA. We here determine the genomic sequence of splA in both genotypes and demonstrate by in vitro self-assembly studies of recombinant and purified SplA protein in combination with electron-microscopy that SplA is a true S-layer protein self-assembling into a square 2D lattice. The existence of a functional S-layer protein is novel for this bacterial species. For elucidating the biological function of P. larvae SplA, a genetic system for disruption of gene expression in this important honey bee pathogen was developed. Subsequent analyses of in vivo biological functions of SplA were based on comparing a wild-type strain of P. larvae ERIC II with the newly constructed splA-knockout mutant of this strain. Differences in cell and colony morphology suggest that SplA is a shape-determining factor. Marked differences between P. larvae ERIC II wild-type and mutant cells with regard to (i) adhesion to primary pupal midgut cells and (ii) larval mortality as measured in exposure bioassays corroborate the assumption that the S-layer of P. larvae ERIC II is an important virulence factor. Since SplA is the first functionally proven virulence factor for this species, our data extend the knowledge of the molecular differences between these two genotypes of P. larvae and contribute to explaining the observed differences in virulence. These results present an immense advancement in our understanding of P. larvae pathogenesis.

Paenibacillus larvae is the most devastating bacterial pathogen of honey bees. However, the molecular interactions between infected larvae and P. larvae are poorly understood and little more than speculation exist concerning virulence factors. Recently, a putative S-layer protein has been identified in P. larvae. We here demonstrate that only representatives of P. larvae genotype ERIC II harbor a functional splA-gene and that SplA is a true S-layer protein with self-assembly capability. The presence of a functional S-layer protein is novel for P. larvae. When elucidating the biological function of SplA we broke new ground by establishing primary cell culture for pupal gut cells and by developing a genetic system for disruption of gene expression in this important honey bee pathogen. By using these novel methods we were able to prove that SplA serves as a shape-determining factor, mediates adhesion to host cells, and is a key virulence factor of P. larvae ERIC II. These results present an immense advancement in our understanding of P. larvae pathogenesis. Furthermore, we propose P. larvae as a model system for the analysis of the in vivo functions of S-layer proteins because P. larvae SlpA knockout-mutants retain viability and are thus suitable for functional studies.

The ancillary protein 1 of Streptococcus pyogenes FCT-1 pili mediates cell adhesion and biofilm formation through heterophilic as well as homophilic interactions

Gram-positive pili are known to play a role in bacterial adhesion to epithelial cells and in the formation of biofilm microbial communities. In the present study we undertook the functional characterization of the pilus ancillary protein 1 (AP1_M6) from Streptococcus pyogenes isolates expressing the FCT-1 pilus variant, known to be strong biofilm formers. Cell binding and biofilm formation assays using S. pyogenes in-frame deletion mutants, Lactococcus expressing heterologous FCT-1 pili and purified recombinant AP1_M6, indicated that this pilin is a strong cell adhesin that is also involved in bacterial biofilm formation. Moreover, we show that AP1_M6 establishes homophilic interactions that mediate inter-bacterial contact, possibly promoting bacterial colonization of target epithelial cells in the form of three-dimensional microcolonies. Finally, AP1_M6 knockout mutants were less virulent in mice, indicating that this protein is also implicated in GAS systemic infection.

Common regulators of virulence in streptococci

Streptococcal species are a diverse group of bacteria which can be found in animals and humans. Their interactions with host organisms can vary from commensal to pathogenic. Many of the pathogenic species are causative agents of severe, invasive infections in their hosts, accounting for a high burden of morbidity and mortality, associated with high economic costs in industry and health care. Among them, Streptococcus pyogenes, Streptococcus agalactiae, Streptococcus pneumoniae, and Streptococcus suis are discussed here. An environmentally stimulated and tightly controlled expression of their virulence factors is of utmost importance for their pathogenic potential. Thus, the most universal and widespread regulators from the classes of stand-alone transcriptional regulators, two-component signal transduction systems (TCS), eukaryotic-like serine/threonine kinases, and small noncoding RNAs are the topic of this chapter. The regulatory levels are reviewed with respect to function, activity, and their role in pathogenesis. Understanding of and interfering with transcriptional regulation mechanisms and networks is a promising basis for the development of novel anti-infective therapies.

Involvement of T6 pili in biofilm formation by serotype M6 Streptococcus pyogenes

The group A streptococcus (GAS) Streptococcus pyogenes is known to cause self-limiting purulent infections in humans. The role of GAS pili in host cell adhesion and biofilm formation is likely fundamental in early colonization. Pilus genes are found in the FCT (fibronectin-binding protein, collagen-binding protein, and trypsin-resistant antigen) genomic region, which has been classified into nine subtypes based on the diversity of gene content and nucleotide sequence. Several epidemiological studies have indicated that FCT type 1 strains, including serotype M6, produce large amounts of monospecies biofilm in vitro. We examined the direct involvement of pili in biofilm formation by serotype M6 clinical isolates. In the majority of tested strains, deletion of the tee6 gene encoding pilus shaft protein T6 compromised the ability to form biofilm on an abiotic surface. Deletion of the fctX and srtB genes, which encode pilus ancillary protein and class C pilus-associated sortase, respectively, also decreased biofilm formation by a representative strain. Unexpectedly, these mutant strains showed increased bacterial aggregation compared with that of the wild-type strain. When the entire FCT type 1 pilus region was ectopically expressed in serotype M1 strain SF370, biofilm formation was promoted and autoaggregation was inhibited. These findings indicate that assembled FCT type 1 pili contribute to biofilm formation and also function as attenuators of bacterial aggregation. Taken together, our results show the potential role of FCT type 1 pili in the pathogenesis of GAS infections.

Analysis of two-component sensor proteins involved in the response to acid stimuli in Streptococcus pyogenes

The virulence of Streptococcus pyogenes depends on proteins that are produced by this bacterium. The production of virulence proteins depends on environmental factors, and two-component regulatory systems are considered to be involved in sensing these factors. One of the environmental factors is acid stimuli. We established knockout strains in all speculated two-component regulatory sensor proteins of the M1 clinical strain of S. pyogenes and examined their relevance to acid stimuli. The parental strain and its derived knockout strains were cultured in a medium adjusted to pH 7.6 or 6.0, and their growth in broth was compared. The spy1622 sensor knockout strain showed significant growth reduction compared with the parental strain in broth at pH 6.0, suggesting that the Spy1622 two-component sensor protein is involved in sensing acid stimuli. To further examine the role of the Spy1622 two-component sensor protein in virulence, blood bactericidal assays and mouse infection model experiments were performed. We found that the spy1622 knockout strain was less virulent than the parental strain, which suggests that the Spy1622 two-component sensor protein could play an important role in virulence.