An association between peptidoglycan synthesis and organization of the Streptococcus pyogenes ExPortal

Using fluorescently labeled wheat germ agglutinin to track lipopolysaccharide transport to the outer membrane in Escherichia coli

The cell envelope of gram-negative bacteria consists of two membranes sandwiching the peptidoglycan (PG) cell wall. The outer membrane (OM) contains integrated beta-barrel proteins and has an outer leaflet composed of lipopolysaccharide (LPS). LPS is transported from the inner membrane where it is made to the OM surface by the Lpt system. In the polarly elongating alpha-proteobacterium Brucella abortus, LPS transport has been localized to the polar growth zone and division site. However, LPS transport has not been tracked in live proteobacteria like Escherichia coli that elongate by dispersed incorporation of envelope material along their cell body. Here, we report an investigation into the binding target of fluorescently labeled wheat germ agglutinin (FL-WGA) on E. coli cells that led to the development of a method for visualizing LPS transport. We show that instead of PG or enterobacterial common antigen for which FL-WGA labeling has been used to detect in the past, this probe recognizes LPS modified with a terminal N-acetylglucosamine formed by the defective O-antigen synthesis pathway of laboratory strains of E. coli. This finding enabled the construction of mutants inducible for LPS modification that were used together with FL-WGA labeling to track LPS transport to the cell surface. We show that new LPS is inserted throughout the cell cylinder and at the division site, but not at the cell poles. A similar pattern was observed previously for PG synthesis and OM protein insertion in E. coli, suggesting that LPS transport to the OM is coordinated with these processes.IMPORTANCEGram-negative bacteria like Escherichia coli are surrounded by a multilayered cell envelope that includes an outer membrane (OM) responsible for their high intrinsic resistance to antibiotics. The outer leaflet of this membrane is composed of a glycolipid called lipopolysaccharide (LPS). Here, we report the development of an imaging method to track the transport of LPS to the E. coli outer membrane. The results indicate that transport occurs throughout the cell cylinder and at the division site, but not at the cell poles. A similar pattern was observed previously when cell wall synthesis and the insertion of proteins into the OM were tracked. Our results therefore suggest that LPS transport to the OM is coordinated with other essential processes that underly gram-negative cell envelope biogenesis.

LiaR-dependent gene expression contributes to antimicrobial responses in group A Streptococcus

The ability to sense and respond to host defenses is essential for pathogen survival. Some mechanisms involve two-component systems (TCSs) that respond to host molecules, such as antimicrobial peptides (AMPs), and activate specific gene regulatory pathways to aid in survival. Alongside TCSs, bacteria coordinate cell division proteins, chaperones, cell wall sortases, and secretory translocons at discrete locations within the cytoplasmic membrane, referred to as functional membrane microdomains (FMMs). In group A Streptococcus (GAS), the FMM or "ExPortal" coordinates protein secretion, cell wall synthesis, and sensing of AMP-mediated cell envelope stress via the LiaFSR three-component system. Previously, we showed that GAS exposure to a subset of AMPs (α-defensins) activates the LiaFSR system by disrupting LiaF and LiaS co-localization in the ExPortal, leading to increased LiaR phosphorylation, expression of the transcriptional regulator SpxA2, and altered GAS virulence gene expression. The mechanisms by which LiaFSR integrates cell envelope stress with responses to AMP activity and virulence are not fully elucidated. Here, we show the LiaFSR regulon is comprised of genes encoding SpxA2 and three membrane-associated proteins: a PspC domain-containing protein (PCP), the lipoteichoic acid-modifying protein LafB, and the membrane protein insertase YidC2. Our data support that phosphorylated LiaR induces transcription of these genes via a conserved operator, whose disruption attenuates GAS virulence and increases susceptibility to AMPs in a manner primarily dependent on differential expression of SpxA2. Our work expands our understanding of the LiaFSR regulatory network in GAS and identifies targets for further investigation of mechanisms of cell envelope stress tolerance contributing to GAS pathogenesis.

Expanding the genetic toolbox for the obligate human pathogen Streptococcus pyogenes

Genetic tools form the basis for the study of molecular mechanisms. Despite many recent advances in the field of genetic engineering in bacteria, genetic toolsets remain scarce for non-model organisms, such as the obligatory human pathogen Streptococcus pyogenes. To overcome this limitation and enable the straightforward investigation of gene functions in S. pyogenes, we have developed a comprehensive genetic toolset. By adapting and combining different tools previously applied in other Gram-positive bacteria, we have created new replicative and integrative plasmids for gene expression and genetic manipulation, constitutive and inducible promoters as well as fluorescence reporters for S. pyogenes. The new replicative plasmids feature low- and high-copy replicons combined with different resistance cassettes and a standardized multiple cloning site for rapid cloning procedures. We designed site-specific integrative plasmids and verified their integration by nanopore sequencing. To minimize the effect of plasmid integration on bacterial physiology, we screened publicly available RNA-sequencing datasets for transcriptionally silent sites. We validated this approach by designing the integrative plasmid pSpy0K6 targeting the transcriptionally silent gene SPy_1078. Analysis of the activity of different constitutive promoters indicated a wide variety of strengths, with the lactococcal promoter P 23 showing the strongest activity and the synthetic promoter P xylS2 showing the weakest activity. Further, we assessed the functionality of three inducible regulatory elements including a zinc- and an IPTG-inducible promoter as well as an erythromycin-inducible riboswitch that showed low-to-no background expression and high inducibility. Additionally, we demonstrated the applicability of two codon-optimized fluorescent proteins, mNeongreen and mKate2, as reporters in S. pyogenes. We therefore adapted the chemically defined medium called RPMI4Spy that showed reduced autofluorescence and enabled efficient signal detection in plate reader assays and fluorescence microscopy. Finally, we developed a plasmid-based system for genome engineering in S. pyogenes featuring the counterselection marker pheS*, which enabled the scarless deletion of the sagB gene. This new toolbox simplifies previously laborious genetic manipulation procedures and lays the foundation for new methodologies to study gene functions in S. pyogenes, leading to a better understanding of its virulence mechanisms and physiology.

LiaR-dependent gene expression contributes to antimicrobial responses in group A Streptococcus

The ability to sense and respond to host defenses is essential for pathogen survival. Some mechanisms involve two-component systems (TCS) that respond to host molecules, such as antimicrobial peptides (AMPs) and activate specific gene regulatory pathways to aid in survival. Alongside TCSs, bacteria coordinate cell division proteins, chaperones, cell wall sortases and secretory translocons at discrete locations within the cytoplasmic membrane, referred to as functional membrane microdomains (FMMs). In Group A Streptococcus (GAS), the FMM or "ExPortal" coordinates protein secretion, cell wall synthesis and sensing of AMP-mediated cell envelope stress via the LiaFSR three-component system. Previously we showed GAS exposure to a subset of AMPs (α-defensins) activates the LiaFSR system by disrupting LiaF and LiaS co-localization in the ExPortal, leading to increased LiaR phosphorylation, expression of the transcriptional regulator SpxA2, and altered GAS virulence gene expression. The mechanisms by which LiaFSR integrates cell envelope stress with responses to AMP activity and virulence are not fully elucidated. Here, we show the LiaFSR regulon is comprised of genes encoding SpxA2 and three membrane-associated proteins: a PspC domain-containing protein (PCP), the lipoteichoic acid-modifying protein LafB and the membrane protein insertase YidC2. Our data show phosphorylated LiaR induces transcription of these genes via a conserved operator, whose disruption attenuates GAS virulence and increases susceptibility to AMPs in a manner primarily dependent on differential expression of SpxA2. Our work expands understanding of the LiaFSR regulatory network in GAS and identifies targets for further investigation of mechanisms of cell envelope stress tolerance contributing to GAS pathogenesis.

The Impacts of Sortase A and the 4'-Phosphopantetheinyl Transferase Homolog Sfp on Streptococcus mutans Extracellular Membrane Vesicle Biogenesis

Extracellular membrane vesicles (EMVs) are produced by many Gram-positive organisms, but information regarding vesiculogenesis is incomplete. We used single gene deletions to evaluate the impacts on Streptococcus mutans EMV biogenesis of Sortase A (SrtA), which affects S. mutans EMV composition, and Sfp, a 4'-phosphopantetheinyl transferase that affects Bacillus subtilis EMV stability. ΔsrtA EMVs were notably larger than Δsfp and wild-type (WT) EMVs. EMV proteins identified from all three strains are known to be involved in cell wall biogenesis and cell architecture, bacterial adhesion, biofilm cell density and matrix development, and microbial competition. Notably, the AtlA autolysin was not processed to its mature active form in the ΔsrtA mutant. Proteomic and lipidomic analyses of all three strains revealed multiple dissimilarities between vesicular and corresponding cytoplasmic membranes (CMs). A higher proportion of EMV proteins are predicted substrates of the general secretion pathway (GSP). Accordingly, the GSP component SecA was identified as a prominent EMV-associated protein. In contrast, CMs contained more multi-pass transmembrane (TM) protein substrates of co-translational transport machineries than EMVs. EMVs from the WT, but not the mutant strains, were enriched in cardiolipin compared to CMs, and all EMVs were over-represented in polyketide flavonoids. EMVs and CMs were rich in long-chain saturated, monounsaturated, and polyunsaturated fatty acids, except for Δsfp EMVs that contained exclusively polyunsaturated fatty acids. Lipoproteins were less prevalent in EMVs of all three strains compared to their CMs. This study provides insight into biophysical characteristics of S. mutans EMVs and indicates discrete partitioning of protein and lipid components between EMVs and corresponding CMs of WT, ΔsrtA, and Δsfp strains.

ExPortal and the LiaFSR Regulatory System Coordinate the Response to Cell Membrane Stress in Streptococcus pyogenes

Bacterial two-component systems sense and induce transcriptional changes in response to environmental stressors, including antimicrobials and human antimicrobial peptides. Since the stresses imposed by the host’s defensive responses may act as markers of specific temporal stages of disease progression or host compartments, pathogens often coordinately regulate stress response programs with virulence factor expression. The mechanism by which bacteria recognize these stresses and subsequently induce transcriptional responses remains not well understood. In this study, we showed that LiaFSR senses cell envelope stress through colocalization of LiaF and LiaS with the group A Streptococcus (GAS) ExPortal and is activated in direct response to ExPortal disruption by antimicrobials or human antimicrobial peptides. Our studies shed new light on the sensing of cell envelope stress in Gram-positive bacteria and may contribute to the development of therapies targeting these processes.

LiaFSR is a gene regulatory system important for response to cell membrane stress in Gram-positive bacteria but is minimally studied in the important human pathogen group A Streptococcus (GAS). Using immunofluorescence and immunogold electron microscopy, we discovered that LiaF (a membrane-bound repressor protein) and LiaS (a sensor kinase) reside within the GAS membrane microdomain (ExPortal). Cell envelope stress induced by antimicrobials resulted in ExPortal disruption and activation of the LiaFSR system. The only human antimicrobial peptide whose presence resulted in ExPortal disruption and LiaFSR activation was the alpha-defensin human neutrophil peptide 1 (hNP-1). Elimination of membrane cardiolipin through targeted gene deletion resulted in loss of LiaS colocalization with the GAS ExPortal and activation of LiaFSR, whereas LiaF membrane localization was unaffected. Isogenic mutants lacking either LiaF or LiaS revealed a critical role of LiaF in ExPortal integrity. Thus, LiaF and LiaS colocalize with the GAS ExPortal by distinct mechanisms, further supporting codependence. These are the first data identifying a multicomponent signal system within the ExPortal, thereby providing new insight into bacterial intramembrane signaling in GAS that may serve as a paradigm for Gram-positive bacteria.

Genetics of Group A Streptococci

Streptococcus pyogenes (group A streptococcus) is remarkable in terms of the large number of diseases it can cause in humans and for the large number of streptococcal factors that have been identified as potential virulence determinants for these diseases. A challenge is to link the function of potential virulence factors to the pathogenesis of specific diseases. An exciting advance has been the development of sophisticated genetic systems for the construction of loss-of-function, conditional, hypomorphic, and gain-of-function mutations in targeted S. pyogenes genes that can be used to test specific hypotheses regarding these genes in pathogenesis. This will facilitate a mechanistic understanding of how a specific gene function contributes to the pathogenesis of each streptococcal disease. Since the first S. pyogenes genome was completed in 2001, hundreds of complete and draft genome sequences have been deposited. We now know that the average S. pyogenes genome is approximately 1.85 Mb and encodes ∼1,800 genes and that the function of most of those genes in pathogenesis remains to be elucidated. However, advances in the development of a variety of genetic tools for manipulation of the S. pyogenes genome now provide a platform for the interrogation of gene/phenotype relationships for individual S. pyogenes diseases, which may lead to the development of more sophisticated and targeted therapeutic interventions. This article presents an overview of these genetic tools, including the methods of genetic modification and their applications.

Assessing Membrane Fluidity and Visualizing Fluid Membrane Domains in Bacteria Using Fluorescent Membrane Dyes

Membrane fluidity is a key parameter of bacterial membranes that undergoes quick adaptation in response to environmental challenges and has recently emerged as an important factor in the antibacterial mechanism of membrane-targeting antibiotics. The specific level of membrane fluidity is not uniform across the bacterial cell membrane. Rather, specialized microdomains associated with different cellular functions can exhibit fluidity values that significantly deviate from the average. Assessing changes in the overall membrane fluidity and formation of membrane microdomains is therefore pivotal to understand both the functional organization of the bacterial cell membrane as well as antibiotic mechanisms. Here we describe how two fluorescent membrane dyes, laurdan and DiIC12, can be employed to assess membrane fluidity in living bacteria. We focus on Bacillus subtilis, since this organism has been relatively well-studied with respect to membrane domains. However, we also describe how these assays can be adapted for other bacteria such as Staphylococcus aureus and Streptococcus pneumoniae.

Essential oils from Origanum vulgare and Salvia officinalis exhibit antibacterial and anti-biofilm activities against Streptococcus pyogenes

In the present study, essential oils (EOs) extracted from oregano, sage, cloves, and ginger were evaluated for the phytochemical profile, antibacterial, and anti-biofilm activities against Streptococcus pyogenes. The broth microdilution method was used to determine the minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) of EOs. The minimum biofilm inhibitory concentrations (MBICs) were determined using MTT assay and fixed biofilms were observed through scan electron microscopy. The oregano and sage EOs showed the lowest MIC as well as MBC of 0.25-0.5 mg/mL. Time kill assay results showed that oregano and sage EOs exhibited bactericidal effects within 5 min and 4 h, respectively. Both oregano and sage extracts acts as a potent anti-biofilm agent with dual actions, preventing and eradicating the biofilm. The microscopic visualization of biofilms treated with EOs have shown morphological and density changes compared to the untreated control. Oregano EO was constituted predominantly carvacrol (91.6%) and in sage EO, higher levels of α-thujone (28.5%) and camphor (16.6%) were revealed. EOs of oregano and sage inhibit the growth and biofilm formation of S. pyogenes. Effective concentrations of oregano and sage EOs and their phytochemicals can be used in developing potential plant-derived antimicrobial agents in the management of streptococcal pharyngitis.

Right Place, Right Time: Focalization of Membrane Proteins in Gram-Positive Bacteria

Membrane proteins represent a significant proportion of total bacterial proteins and perform vital cellular functions ranging from exchanging metabolites and genetic material, secretion and sorting, sensing signal molecules, and cell division. Many of these functions are carried out at distinct foci on the bacterial membrane, and this subcellular localization can be coordinated by a number of factors, including lipid microdomains, protein-protein interactions, and membrane curvature. Elucidating the mechanisms behind focal protein localization in bacteria informs not only protein structure-function correlation, but also how to disrupt the protein function to limit virulence. Here we review recent advances describing a functional role for subcellular localization of membrane proteins involved in genetic transfer, secretion and sorting, cell division and growth, and signaling.

The Human Pathogen Streptococcus pyogenes Releases Lipoproteins as Lipoprotein-rich Membrane Vesicles

Bacterial lipoproteins are attractive vaccine candidates because they represent a major class of cell surface-exposed proteins in many bacteria and are considered as potential pathogen-associated molecular patterns sensed by Toll-like receptors with built-in adjuvanticity. Although Gram-negative lipoproteins have been extensively characterized, little is known about Gram-positive lipoproteins. We isolated from Streptococcus pyogenes a large amount of lipoproteins organized in vesicles. These vesicles were obtained by weakening the bacterial cell wall with a sublethal concentration of penicillin. Lipid and proteomic analysis of the vesicles revealed that they were enriched in phosphatidylglycerol and almost exclusively composed of lipoproteins. In association with lipoproteins, a few hypothetical proteins, penicillin-binding proteins, and several members of the ExPortal, a membrane microdomain responsible for the maturation of secreted proteins, were identified. The typical lipidic moiety was apparently not necessary for lipoprotein insertion in the vesicle bilayer because they were also recovered from the isogenic diacylglyceryl transferase deletion mutant. The vesicles were not able to activate specific Toll-like receptor 2, indicating that lipoproteins organized in these vesicular structures do not act as pathogen-associated molecular patterns. In light of these findings, we propose to name these new structures Lipoprotein-rich Membrane Vesicles.

Extracellular Streptomyces lividans vesicles: composition, biogenesis and antimicrobial activity

We selected Streptomyces lividans to elucidate firstly the biogenesis and antimicrobial activities of extracellular vesicles that a filamentous and highly differentiated Gram-positive bacterium produces. Vesicle types range in diameter from 110 to 230 nm and 20 to 60 nm, respectively; they assemble to clusters, and contain lipids and phospholipids allowing their in situ imaging by specific fluorescent dyes. The presence of the identified secondary metabolite undecylprodigiosin provokes red fluorescence of a portion of the heterogeneous vesicle populations facilitating in vivo monitoring. Protuberances containing vesicles generate at tips, and alongside of substrate hyphae, and enumerate during late vegetative growth to droplet-like exudates. Owing to in situ imaging in the presence and absence of a green fluorescent vancomycin derivative, we conclude that protuberances comprising vesicles arise at sites with enhanced levels of peptidoglycan subunits [pentapeptide of lipid II (C55)-linked disaccharides], and reduced levels of polymerized and cross-linked peptidoglycan within hyphae. These sites correlate with enhanced levels of anionic phospholipids and lipids. Vesicles provoke pronounced damages of Aspergillus proliferans, Verticillium dahliae and induced clumping and distortion of Escherichia coli. These harmful effects are likely attributable to the action of the identified vesicular compounds including different enzyme types, components of signal transduction cascades and undecylprodigiosin. Based on our pioneering findings, we highlight novel clues with environmental implications and application potential.

The essential features and modes of bacterial polar growth

Polar growth represents a surprising departure from the canonical dispersed cell growth model. However, we know relatively little of the underlying mechanisms governing polar growth or the requisite suite of factors that direct polar growth. Underscoring how classic doctrine can be turned on its head, the peptidoglycan layer of polar-growing bacteria features unusual crosslinks and in some species the quintessential cell division proteins FtsA and FtsZ are recruited to the growing poles. Remarkably, numerous medically important pathogens utilize polar growth, accentuating the need for intensive research in this area. Here we review models of polar growth in bacteria based on recent research in the Actinomycetales and Rhizobiales, with emphasis on Mycobacterium and Agrobacterium species.

Breaking the bacterial protein targeting and translocation model: oral organisms as a case in point

Insights into the membrane biogenesis of oral and throat bacteria have highlighted key differences in protein localization by the general secretion pathway compared with the well-studied Escherichia coli model system. These intriguing novelties have advanced our understanding of both how these microorganisms have adapted to survive and cause disease in the oral cavity, and the field of protein translocation as a whole. This review focuses on findings that highlight where oral bacteria differ from the E. coli paradigm, why these differences are biologically important, and what questions remain about the differences in pathway function. The majority of insight into protein translocation in microbes of the oral cavity has come from streptococcal species, which will be the main topic of this review. However, other bacteria will be discussed when relevant. An overview of the E. coli model of protein targeting and translocation is provided for comparison.

Streptococcus pyogenes polymyxin B-resistant mutants display enhanced ExPortal integrity

The ExPortal protein secretion organelle in Streptococcus pyogenes is an anionic phospholipid-containing membrane microdomain enriched in Sec translocons and postsecretion protein biogenesis factors. Polymyxin B binds to and disrupts ExPortal integrity, resulting in defective secretion of several toxins. To gain insight into factors that influence ExPortal organization, a genetic screen was conducted to select for spontaneous polymyxin B-resistant mutants displaying enhanced ExPortal integrity. Whole-genome resequencing of 25 resistant mutants revealed from one to four mutations per mutant genome clustered primarily within a core set of 10 gene groups. Construction of mutants with individual deletions or insertions demonstrated that 7 core genes confer resistance and enhanced ExPortal integrity through loss of function, while 3 were likely due to gain of function and/or combinatorial effects. Core resistance genes include a transcriptional regulator of lipid biosynthesis, several genes involved in nutrient acquisition, and a variety of genes involved in stress responses. Two members of the latter class also function as novel regulators of the secreted SpeB cysteine protease. Analysis of the most frequently isolated mutation, a single nucleotide deletion in a track of 9 consecutive adenine residues in pstS, encoding a component of a high-affinity Pi transporter, suggests that this sequence functions as a molecular switch to facilitate stress adaptation. Together, these data suggest the existence of a membrane stress response that promotes enhanced ExPortal integrity and resistance to cationic antimicrobial peptides.