An M protein with a single C repeat prevents phagocytosis of Streptococcus pyogenes: use of a temperature-sensitive shuttle vector to deliver homologous sequences to the chromosome of S. pyogenes

Endogenously produced H2O2 is intimately involved in iron metabolism in Streptococcus pneumoniae

IMPORTANCE

Heme degradation provides pathogens with growth essential iron, leveraging on the host heme reservoir. Bacteria typically import and degrade heme enzymatically, and here, we demonstrated a significant deviation from this dogma. We found that Streptococcus pneumoniae liberates iron from met-hemoglobin extracellularly, in a hydrogen peroxide (H2O2)- and cell-dependent manner; this activity serves as a major iron acquisition mechanism for S. pneumoniae. Inhabiting oxygen-rich environments is a major part of pneumococcal biology, and hence, H2O2-mediated heme degradation likely supplies iron during infection. Moreover, H2O2 reaction with ferrous hemoglobin but not with met-hemoglobin is known to result in heme breakdown. Therefore, the ability of pneumococci to degrade heme from met-hemoglobin is a new paradigm. Lastly, this study will inform other research as it demonstrates that extracellular degradation must be considered in the interpretations of experiments in which H2O2-producing bacteria are given heme or hemoproteins as an iron source.

Methylome-dependent transformation of emm1 group A streptococci

Genetic intractability presents a fundamental barrier to the manipulation of bacteria, hindering advancements in microbiological research. Group A Streptococcus (GAS), a lethal human pathogen currently associated with an unprecedented surge of infections worldwide, exhibits poor genetic tractability attributed to the activity of a conserved type 1 restriction modification system (RMS). RMS detect and cleave specific target sequences in foreign DNA that are protected in host DNA by sequence-specific methylation. Overcoming this "restriction barrier" thus presents a major technical challenge. Here, we demonstrate for the first time that different RMS variants expressed by GAS give rise to genotype-specific and methylome-dependent variation in transformation efficiency. Furthermore, we show that the magnitude of impact of methylation on transformation efficiency elicited by RMS variant TRDAG, encoded by all sequenced strains of the dominant and upsurge-associated emm1 genotype, is 100-fold greater than for all other TRD tested and is responsible for the poor transformation efficiency associated with this lineage. In dissecting the underlying mechanism, we developed an improved GAS transformation protocol, whereby the restriction barrier is overcome by the addition of the phage anti-restriction protein Ocr. This protocol is highly effective for TRDAG strains including clinical isolates representing all emm1 lineages and will expedite critical research interrogating the genetics of emm1 GAS, negating the need to work in an RMS-negative background. These findings provide a striking example of the impact of RMS target sequence variation on bacterial transformation and the importance of defining lineage-specific mechanisms of genetic recalcitrance. IMPORTANCE Understanding the mechanisms by which bacterial pathogens are able to cause disease is essential to enable the targeted development of novel therapeutics. A key experimental approach to facilitate this research is the generation of bacterial mutants, through either specific gene deletions or sequence manipulation. This process relies on the ability to transform bacteria with exogenous DNA designed to generate the desired sequence changes. Bacteria have naturally developed protective mechanisms to detect and destroy invading DNA, and these systems severely impede the genetic manipulation of many important pathogens, including the lethal human pathogen group A Streptococcus (GAS). Many GAS lineages exist, of which emm1 is dominant among clinical isolates. Based on new experimental evidence, we identify the mechanism by which transformation is impaired in the emm1 lineage and establish an improved and highly efficient transformation protocol to expedite the generation of mutants.

pBFK, a new thermosensitive shuttle vector for Streptococcus pyogenes gene deletion by homologous recombination

Streptococcus pyogenes, or Group A Streptococcus (GAS), is a major human pathogen for which genetic manipulation remains an ongoing challenge. We created a new temperature-sensitive plasmid pBFK expressing spectinomycin resistance adapted to homologous recombination procedure to perform a complete gene deletion in GAS. Herein the mutagenesis strategy with pBFK was performed in a highly virulent GAS emm3 genotype.

Sequence Variation of Candida albicans Sap2 Enhances Fungal Pathogenicity via Complement Evasion and Macrophage M2-Like Phenotype Induction

Candida albicans (C. albicans) is an opportunistic pathogen increasingly causing candidiasis worldwide. This study aims to investigate the pattern of systemic immune responses triggered by C. albicans with disease associated variation of Sap2, identifying the novel evasion strategies utilized by clinical isolates. Specifically, a variation in clinical isolates is identified at nucleotide position 817 (G to T). This homozygous variation causes the 273rd amino acid exchange from valine to leucine, close to the proteolytic activation center of Sap2. The mutant (Sap2-273L) generated from SC5314 (Sap2-273V) background carrying the V273L variation within Sap2 displays higher pathogenicity. In comparison to mice infected with Sap2-273V strain, mice infected with Sap2-273L exhibit less complement activation indicated by less serum C3a generation and weaker C3b deposition in the kidney. This inhibitory effect is mainly achieved by Sap2_273L -mediated stronger degradation of C3 and C3b. Furthermore, mice infected with Sap2-273L strain exhibit more macrophage phenotype switching from M0 to M2-like and more TGF-β release which further influences T cell responses, generating an immunosuppressed cellular microenvironment characterized by more Tregs and exhausted T cell formation. In summary, the disease-associated sequence variation of Sap2 enhances pathogenicity by complement evasion and M2-like phenotype switching, promoting a more efficient immunosuppressed microenvironment.

A high-throughput sequencing approach identifies immunotherapeutic targets for bacterial meningitis in neonates

BACKGROUND

Worldwide, Escherichia coli is the leading cause of neonatal Gram-negative bacterial meningitis, but full understanding of the pathogenesis of this disease is not yet achieved. Moreover, to date, no vaccine is available against bacterial neonatal meningitis.

METHODS

Here, we used Transposon Sequencing of saturated banks of mutants (TnSeq) to evaluate E. coli K1 genetic fitness in murine neonatal meningitis. We identified E. coli K1 genes encoding for factors important for systemic dissemination and brain infection, and focused on products with a likely outer-membrane or extra-cellular localization, as these are potential vaccine candidates. We used in vitro and in vivo models to study the efficacy of active and passive immunization.

RESULTS

We selected for further study the conserved surface polysaccharide Poly-β-(1-6)-N-Acetyl Glucosamine (PNAG), as a strong candidate for vaccine development. We found that PNAG was a virulence factor in our animal model. We showed that both passive and active immunization successfully prevented and/or treated meningitis caused by E. coli K1 in neonatal mice. We found an excellent opsonophagocytic killing activity of the antibodies to PNAG and in vitro these antibodies were also able to decrease binding, invasion and crossing of E. coli K1 through two blood brain barrier cell lines. Finally, to reinforce the potential of PNAG as a vaccine candidate in bacterial neonatal meningitis, we demonstrated that Group B Streptococcus, the main cause of neonatal meningitis in developed countries, also produced PNAG and that antibodies to PNAG could protect in vitro and in vivo against this major neonatal pathogen.

INTERPRETATION

Altogether, these results indicate the utility of a high-throughput DNA sequencing method to identify potential immunotherapy targets for a pathogen, including in this study a potential broad-spectrum target for prevention of neonatal bacterial infections.

FUNDINGS

ANR Seq-N-Vaq, Charles Hood Foundation, Hearst Foundation, and Groupe Pasteur Mutualité.

Improved genetic transformation by disarmament of type II Restriction-Modification system in Streptococcus zooepidemicus

Streptococcus zooepidemicus, group C Streptococci, is currently used for the industrial production of hyaluronic acid (HA). However, genetic manipulation of S. zooepidemicus is severely limited by its low transformation efficiency, which might be in part due to the Restriction-Modification (R-M) systems. The complete genome sequence of S. zooepidemicus ATCC39920 revealed the presence of two putative R-M systems, type I and type II. The putative type I R-M system is encoded by three closely linked genes: hsdR (SeseC_01315), hsdS, hsdM (SeseC_01318), and the putative type II R-M system consists of two closely linked genes: SeseC_02360 and yhdJ (SeseC_02362). Inactivation of hsdR, encoding the restriction endonuclease (REase) of the type I R-M system, showed no apparent effects on transformation efficiency, implying that disarmament of the type I R-M system alone is not sufficient for increasing transformation efficiency. However, inactivation of SeseC_02360, encoding the REase of the type II R-M system, improved transformation efficiency by 4.97 folds, indicating that type II R-M system is the major barrier that restricts genetic transformation in S. zooepidemicus. Furthermore, S. zooepidemicus strains lacking either of the two R-M systems are phenotypically indistinguishable from the wild-type in terms of cell growth and HA production. In summary, our study revealed that the type II R-M system is the main barrier to genetic transformation in S. zooepidemicus ATCC39920, and that the deletion of the type II R-M system renders S. zooepidemicus more transformable, thus facilitating metabolic engineering of this industrially important microorganism.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-022-03227-x.

Enterococcus faecalis Glucosamine Metabolism Exacerbates Experimental Colitis

BACKGROUND & AIMS

The inflammatory bowel diseases (IBDs), Crohn's disease and ulcerative colitis, are caused in part by aberrant immune responses to resident intestinal bacteria. Certain dietary components, including carbohydrates, are associated with IBDs and alter intestinal bacterial composition. However, the effects of luminal carbohydrates on the composition and colitogenic potential of intestinal bacteria are incompletely understood. We hypothesize that carbohydrate metabolism by resident proinflammatory intestinal bacteria enhances their growth and worsens intestinal inflammation.

METHODS

We colonized germ-free, wild-type, and colitis-susceptible interleukin-10 knockout mice (Il10-/-) with a consortium of resident intestinal bacterial strains and quantified colon inflammation using blinded histologic scoring and spontaneous secretion of IL12/23p40 by colon explants. We measured luminal bacterial composition using real-time 16S polymerase chain reaction, bacterial gene expression using RNA sequencing and real-time polymerase chain reaction, and luminal glucosamine levels using gas chromatography-mass spectrometry.

RESULTS

We show that a consortium of 8 bacterial strains induces severe colitis in Il10-/- mice and up-regulates genes associated with carbohydrate metabolism during colitis. Specifically, Enterococcus faecalis strain OG1RF is proinflammatory and strongly up-regulates OG1RF_11616-11610, an operon that encodes genes of a previously undescribed phosphotransferase system that we show imports glucosamine. Experimental colitis is associated with increased levels of luminal glucosamine and OG1RF_11616 causes worse colitis, not by increasing E faecalis numbers, but rather by mechanisms that require the presence of complex microbiota.

CONCLUSIONS

Further studies of luminal carbohydrate levels and bacterial carbohydrate metabolism during intestinal inflammation will improve our understanding of the pathogenesis of IBDs and may lead to the development of novel therapies for these diseases.

Improved transformation efficiency of group A Streptococcus by inactivation of a type I restriction modification system

Streptococcus pyogenes or group A Streptococcus (GAS) is a leading cause of bacterial pharyngitis, skin and soft tissue infections, life-threatening invasive infections, and the post-infectious autoimmune syndromes of acute rheumatic fever and post-streptococcal glomerulonephritis. Genetic manipulation of this important pathogen is complicated by resistance of the organism to genetic transformation. Very low transformation efficiency is attributed to recognition and degradation of introduced foreign DNA by a type I restriction-modification system encoded by the hsdRSM locus. DNA sequence analysis of this locus in ten GAS strains that had been previously transformed with an unrelated plasmid revealed that six of the ten harbored a spontaneous mutation in hsdR, S, or M. The mutations were all different, and at least five of the six were predicted to result in loss of function of the respective hsd gene product. The unexpected occurrence of such mutations in previously transformed isolates suggested that the process of transformation selects for spontaneous inactivating mutations in the Hsd system. We investigated the possibility of exploiting the increased transformability of hsd mutants by constructing a deletion mutation in hsdM in GAS strain 854, a clinical isolate representative of the globally dominant M1T1 clonal group. Mutant strain 854ΔhsdM exhibited a 5-fold increase in electrotransformation efficiency compared to the wild type parent strain and no obvious change in growth or off-target gene expression. We conclude that genetic transformation of GAS selects for spontaneous mutants in the hsdRSM restriction modification system. We propose that use of a defined hsdM mutant as a parent strain for genetic manipulation of GAS will enhance transformation efficiency and reduce the likelihood of selecting spontaneous hsd mutants with uncharacterized genotypes.

Hemoglobin Induces Early and Robust Biofilm Development in Streptococcus pneumoniae by a Pathway That Involves comC but Not the Cognate comDE Two-Component System

Streptococcus pneumoniae grows in biofilms during both asymptomatic colonization and infection. Pneumococcal biofilms on abiotic surfaces exhibit delayed growth and lower biomass and lack the structures seen on epithelial cells or during nasopharyngeal carriage. We show here that adding hemoglobin to the medium activated unusually early and vigorous biofilm growth in multiple S. pneumoniae serotypes grown in batch cultures on abiotic surfaces. Human blood (but not serum, heme, or iron) also stimulated biofilms, and the pore-forming pneumolysin, ply, was required for this induction. S. pneumoniae transitioning from planktonic into sessile growth in the presence of hemoglobin displayed an extensive transcriptome remodeling within 1 and 2 h. Differentially expressed genes included those involved in the metabolism of carbohydrates, nucleotides, amino acid, and lipids. The switch into adherent states also influenced the expression of several regulatory systems, including the comCDE genes. Inactivation of comC resulted in 67% reduction in biofilm formation, while the deletion of comD or comE had limited or no effect, respectively. These observations suggest a novel route for CSP-1 signaling independent of the cognate ComDE two-component system. Biofilm induction and the associated transcriptome remodeling suggest hemoglobin serves as a signal for host colonization in pneumococcus.

Genetic Manipulation of Group A Streptococcus-Gene Deletion by Allelic Replacement

Genetic manipulation of Streptococcus pyogenes (Group A Streptococcus, GAS) has historically been a challenging process, with considerable variation in efficiency between different strains. Here, we outline an optimized, rapid method for creating markerless isogenic mutations that combines Gibson assembly cloning with a new temperature-sensitive plasmid, pLZts. This method is highly efficient and reduces the time needed to create GAS mutants to ~2-3 weeks, with the ability to prepare multiple mutants simultaneously.

Contribution of glutaminases to glutamine metabolism and acid resistance in Lactobacillus reuteri and other vertebrate host adapted lactobacilli

The bacterial conversion of glutamine to glutamate is catalyzed by glutamine-amidotransferases or glutaminases. Glutamine deamination contributes to the formation of the bioactive metabolites glutamate, γ-aminobutyrate (GABA) and γ-glutamyl peptides, and to acid resistance. This study aimed to investigate the distribution of glutaminase(s) in lactobacilli, and to evaluate their contribution in L. reuteri to amino acid metabolism and acid resistance. Phylogenetic analysis of the glutaminases gls1, gls2 and gls3 in the genus Lactobacillus demonstrated that glutaminase is exclusively present in host-adapted species of lactobacilli. The disruption gls1, gls2 and gls3 in L. reuteri 100-23 had only a limited effect on the conversion of glutamine to glutamate, GABA, or γ-glutamyl peptides in sourdough. The disruption of all glutaminases in L. reuteri 100-23Δgls1Δgls2Δgls3 but not disruption of gls2 and gls3 eliminated the protective effect of glutamine on the survival of the strain at pH 2.5. Glutamine also enhanced acid resistance of L. reuteri 100-23ΔgadB and L. taiwanensis 107q, strains without glutamate decarboxylase activity. Taken together, the study demonstrates that glutaminases of lactobacilli do not contribute substantially to glutamine metabolism but enhance acid resistance. Their exclusive presence in host-adapted lactobacilli provides an additional link between the adaptation of lactobacilli to specific habitats and their functionality when used as probiotics and starter cultures.

RocA Binds CsrS To Modulate CsrRS-Mediated Gene Regulation in Group A Streptococcus

The orphan regulator RocA plays a critical role in the colonization and pathogenesis of the obligate human pathogen group A Streptococcus Despite multiple lines of evidence supporting a role for RocA as an auxiliary regulator of the control of virulence two-component regulatory system CsrRS (or CovRS), the mechanism of action of RocA remains unknown. Using a combination of in vitro and in vivo techniques, we now find that RocA interacts with CsrS in the streptococcal membrane via its N-terminal region, which contains seven transmembrane domains. This interaction is essential for RocA-mediated regulation of CsrRS function. Furthermore, we demonstrate that RocA forms homodimers via its cytoplasmic domain. The serotype-specific RocA truncation in M3 isolates alters this homotypic interaction, resulting in protein aggregation and impairment of RocA-mediated regulation. Taken together, our findings provide insight into the molecular requirements for functional interaction of RocA with CsrS to modulate CsrRS-mediated gene regulation.IMPORTANCE Bacterial two-component regulatory systems, comprising a membrane-bound sensor kinase and cytosolic response regulator, are critical in coordinating the bacterial response to changing environmental conditions. More recently, auxiliary regulators which act to modulate the activity of two-component systems, allowing integration of multiple signals and fine-tuning of bacterial responses, have been identified. RocA is a regulatory protein encoded by all serotypes of the important human pathogen group A Streptococcus Although RocA is known to exert its regulatory activity via the streptococcal two-component regulatory system CsrRS, the mechanism by which it functions was unknown. Based on new experimental evidence, we propose a model whereby RocA interacts with CsrS in the streptococcal cell membrane to enhance CsrS autokinase activity and subsequent phosphotransfer to the response regulator CsrR, which mediates transcriptional repression of target genes.

Enterococcus faecalis Gluconate Phosphotransferase System Accelerates Experimental Colitis and Bacterial Killing by Macrophages

Enterococcus faecalis strains are resident intestinal bacteria associated with invasive infections, inflammatory bowel diseases, and colon cancer. Although factors promoting E. faecalis colonization of intestines are not fully known, one implicated pathway is a phosphotransferase system (PTS) in E. faecalis strain OG1RF that phosphorylates gluconate and contains the genes OG1RF_12399 to OG1RF_12402 (OG1RF_12399-12402). We hypothesize that this PTS permits growth in gluconate, facilitates E. faecalis intestinal colonization, and exacerbates colitis. We generated E. faecalis strains containing deletions/point mutations in this PTS and measured bacterial growth and PTS gene expression in minimal medium supplemented with selected carbohydrates. We show that E. faecalis upregulates OG1RF_12399 transcription specifically in the presence of gluconate and that E. faecalis strains lacking, or harboring a single point mutation in, OG1RF_12399-12402 are unable to grow in minimal medium containing gluconate. We colonized germfree wild-type and colitis-prone interleukin-10-deficient mice with defined bacterial consortia containing the E. faecalis strains and measured inflammation and bacterial abundance in the colon. We infected macrophage and intestinal epithelial cell lines with the E. faecalis strains and measured intracellular bacterial survival and proinflammatory cytokine secretion. The presence of OG1RF_12399-12402 is not required for E. faecalis colonization of the mouse intestine but is associated with an accelerated onset of experimental colitis in interleukin-10-deficient mice, altered bacterial composition in the colon, enhanced E. faecalis survival within macrophages, and increased proinflammatory cytokine secretion by colon tissue and macrophages. Further studies of bacterial carbohydrate metabolism in general, and E. faecalis PTS-gluconate in particular, during inflammation may identify new mechanisms of disease pathogenesis.

DNA methylation from a Type I restriction modification system influences gene expression and virulence in Streptococcus pyogenes

DNA methylation is pervasive across all domains of life. In bacteria, the presence of N6-methyladenosine (m6A) has been detected among diverse species, yet the contribution of m6A to the regulation of gene expression is unclear in many organisms. Here we investigated the impact of DNA methylation on gene expression and virulence within the human pathogen Streptococcus pyogenes, or Group A Streptococcus. Single Molecule Real-Time sequencing and subsequent methylation analysis identified 412 putative m6A sites throughout the 1.8 Mb genome. Deletion of the Restriction, Specificity, and Methylation gene subunits (ΔRSM strain) of a putative Type I restriction modification system lost all detectable m6A at the recognition sites and failed to prevent transformation with foreign-methylated DNA. RNA-sequencing identified 20 genes out of 1,895 predicted coding regions with significantly different gene expression. All of the differentially expressed genes were down regulated in the ΔRSM strain relative to the parent strain. Importantly, we found that the presence of m6A DNA modifications affected expression of Mga, a master transcriptional regulator for multiple virulence genes, surface adhesins, and immune-evasion factors in S. pyogenes. Using a murine subcutaneous infection model, mice infected with the ΔRSM strain exhibited an enhanced host immune response with larger skin lesions and increased levels of pro-inflammatory cytokines compared to mice infected with the parent or complemented mutant strains, suggesting alterations in m6A methylation influence virulence. Further, we found that the ΔRSM strain showed poor survival within human neutrophils and reduced adherence to human epithelial cells. These results demonstrate that, in addition to restriction of foreign DNA, gram-positive bacteria also use restriction modification systems to regulate the expression of gene networks important for virulence.

DNA methylation is common among many bacterial species, yet the contribution of DNA methylation to the regulation of gene expression is unclear outside of a limited number of gram-negative species. We characterized sites of DNA methylation throughout the genome of the gram-positive pathogen Streptococcus pyogenes or Group A Streptococcus. We determined that the gene products of a functional restriction modification system are responsible for genome-wide m6A. The mutant strain lacking DNA methylation showed altered gene expression compared to the parent strain, with several genes important for causing human disease down regulated. Furthermore, we showed that the mutant strain lacking DNA methylation exhibited altered virulence properties compared to the parent strain using various models of pathogenesis. The mutant strain was attenuated for both survival within human neutrophils and adherence to human epithelial cells, and was unable to suppress the host immune response in a murine subcutaneous infection model. Together, these results show that bacterial m6A contributes to differential gene expression and influences the ability of Group A Streptococcus to cause disease. DNA methylation is a conserved feature among bacteria and may represent a potential target for intervention in effort to interfere with the ability of bacteria to cause human disease.

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.

A Sub-population of Group A Streptococcus Elicits a Population-wide Production of Bacteriocins to Establish Dominance in the Host

Bacteria use quorum sensing (QS) to regulate gene expression. We identified a group A Streptococcus (GAS) strain possessing the QS system sil, which produces functional bacteriocins, through a sequential signaling pathway integrating host and bacterial signals. Host cells infected by GAS release asparagine (ASN), which is sensed by the bacteria to alter its gene expression and rate of proliferation. We show that upon ASN sensing, GAS upregulates expression of the QS autoinducer peptide SilCR. Initial SilCR expression activates the autoinduction cycle for further SilCR production. The autoinduction process propagates throughout the GAS population, resulting in bacteriocin production. Subcutaneous co-injection of mice with a bacteriocin-producing strain and the globally disseminated M1T1 GAS clone results in M1T1 killing within soft tissue. Thus, by sensing host signals, a fraction of a bacterial population can trigger an autoinduction mechanism mediated by QS, which acts on the entire bacterial community to outcompete other bacteria within the infection.

Blocking Neuronal Signaling to Immune Cells Treats Streptococcal Invasive Infection

The nervous system, the immune system, and microbial pathogens interact closely at barrier tissues. Here, we find that a bacterial pathogen, Streptococcus pyogenes, hijacks pain and neuronal regulation of the immune response to promote bacterial survival. Necrotizing fasciitis is a life-threatening soft tissue infection in which "pain is out of proportion" to early physical manifestations. We find that S. pyogenes, the leading cause of necrotizing fasciitis, secretes streptolysin S (SLS) to directly activate nociceptor neurons and produce pain during infection. Nociceptors, in turn, release the neuropeptide calcitonin gene-related peptide (CGRP) into infected tissues, which inhibits the recruitment of neutrophils and opsonophagocytic killing of S. pyogenes. Botulinum neurotoxin A and CGRP antagonism block neuron-mediated suppression of host defense, thereby preventing and treating S. pyogenes necrotizing infection. We conclude that targeting the peripheral nervous system and blocking neuro-immune communication is a promising strategy to treat highly invasive bacterial infections. VIDEO ABSTRACT.

SpxA1 and SpxA2 Act Coordinately To Fine-Tune Stress Responses and Virulence in Streptococcus pyogenes

SpxA is a unique transcriptional regulator highly conserved among members of the phylum Firmicutes that binds RNA polymerase and can act as an antiactivator. Why some Firmicutes members have two highly similar SpxA paralogs is not understood. Here, we show that the SpxA paralogs of the pathogen Streptococcus pyogenes, SpxA1 and SpxA2, act coordinately to regulate virulence by fine-tuning toxin expression and stress resistance. Construction and analysis of mutants revealed that SpxA1- mutants were defective for growth under aerobic conditions, while SpxA2- mutants had severely attenuated responses to multiple stresses, including thermal and oxidative stresses. SpxA1- mutants had enhanced resistance to the cationic antimicrobial molecule polymyxin B, while SpxA2- mutants were more sensitive. In a murine model of soft tissue infection, a SpxA1- mutant was highly attenuated. In contrast, the highly stress-sensitive SpxA2- mutant was hypervirulent, exhibiting more extensive tissue damage and a greater bacterial burden than the wild-type strain. SpxA1- attenuation was associated with reduced expression of several toxins, including the SpeB cysteine protease. In contrast, SpxA2- hypervirulence correlated with toxin overexpression and could be suppressed to wild-type levels by deletion of speB These data show that SpxA1 and SpxA2 have opposing roles in virulence and stress resistance, suggesting that they act coordinately to fine-tune toxin expression in response to stress. SpxA2- hypervirulence also shows that stress resistance is not always essential for S. pyogenes pathogenesis in soft tissue.IMPORTANCE For many pathogens, it is generally assumed that stress resistance is essential for pathogenesis. For Streptococcus pyogenes, environmental stress is also used as a signal to alter toxin expression. The amount of stress likely informs the bacterium of the strength of the host's defense response, allowing it to adjust its toxin expression to produce the ideal amount of tissue damage, balancing between too little damage, which will result in its elimination, and too much damage, which will debilitate the host. Here we identify components of a genetic circuit involved in stress resistance and toxin expression that has a fine-tuning function in tissue damage. The circuit consists of two versions of the protein SpxA that regulate transcription and are highly similar but have opposing effects on the severity of soft tissue damage. These results will help us understand how virulence is fine-tuned in other pathogens that have two SpxA proteins.

Zn2+ Uptake in Streptococcus pyogenes: Characterization of adcA and lmb Null Mutants

An effective regulation of metal ion homeostasis is essential for the growth of microorganisms in any environment and in pathogenic bacteria is strongly associated with their ability to invade and colonise their hosts. To gain a better insight into zinc acquisition in Group A Streptococcus (GAS) we characterized null deletion mutants of the adcA and lmb genes of Streptococcus pyogenes strain MGAS5005 encoding the orthologues of AdcA and AdcAII, the two surface lipoproteins with partly redundant roles in zinc homeostasis in Streptococcus pneumoniae. Null adcA and lmb mutants were analysed for their capability to grow in zinc-depleted conditions and were found to be more susceptible to zinc starvation, a phenotype that could be rescued by the addition of Zn²⁺ ions to the growth medium. Expression of AdcA, Lmb and HtpA, the polyhistidine triad protein encoded by the gene adjacent to lmb, during growth under conditions of limited zinc availability was examined by Western blot analysis in wild type and null mutant strains. In the wild type strain, AdcA was always present with little variation in expression levels between conditions of excess or limited zinc availability. In contrast, Lmb and HtpA were expressed at detectable levels only during growth in the presence of low zinc concentrations or in the null adcA mutant, when expression of lmb is required to compensate for the lack of adcA expression. In the latter case, Lmb and HtpA were overexpressed by several fold, thus indicating that also in GAS AdcA is a zinc-specific importer and, although it shares this function with Lmb, the two substrate-binding proteins do not show fully overlapping roles in zinc homeostasis.

Streptococcus pyogenes Sortase Mutants Are Highly Susceptible to Killing by Host Factors Due to Aberrant Envelope Physiology

Cell wall anchored virulence factors are critical for infection and colonization of the host by Gram-positive bacteria. Such proteins have an N-terminal leader sequence and a C-terminal sorting signal, composed of an LPXTG motif, a hydrophobic stretch, and a few positively charged amino acids. The sorting signal halts translocation across the membrane, allowing sortase to cleave the LPXTG motif, leading to surface anchoring. Deletion of sortase prevents the anchoring of virulence factors to the wall; the effects on bacterial physiology however, have not been thoroughly characterized. Here we show that deletion of Streptococcus pyogenes sortase A leads to accumulation of sorting intermediates, particularly at the septum, altering cellular morphology and physiology, and compromising membrane integrity. Such cells are highly sensitive to cathelicidin, and are rapidly killed in blood and plasma. These phenomena are not a loss-of-function effect caused by the absence of anchored surface proteins, but specifically result from the accumulation of sorting intermediates. Reduction in the level of sorting intermediates leads to a return of the sortase mutant to normal morphology, while expression of M protein with an altered LPXTG motif in wild type cells leads to toxicity in the host environment, similar to that observed in the sortase mutant. These unanticipated effects suggest that inhibition of sortase by small-molecule inhibitors could similarly lead to the rapid elimination of pathogens from an infected host, making such inhibitors much better anti-bacterial agents than previously believed.

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.

Sequence variations and protein expression levels of the two immune evasion proteins Gpm1 and Pra1 influence virulence of clinical Candida albicans isolates

Candida albicans, the important human fungal pathogen uses multiple evasion strategies to control, modulate and inhibit host complement and innate immune attack. Clinical C. albicans strains vary in pathogenicity and in serum resistance, in this work we analyzed sequence polymorphisms and variations in the expression levels of two central fungal complement evasion proteins, Gpm1 (phosphoglycerate mutase 1) and Pra1 (pH-regulated antigen 1) in thirteen clinical C. albicans isolates. Four nucleotide (nt) exchanges, all representing synonymous exchanges, were identified within the 747-nt long GPM1 gene. For the 900-nt long PRA1 gene, sixteen nucleotide exchanges were identified, which represented synonymous, as well as non-synonymous exchanges. All thirteen clinical isolates had a homozygous exchange (A to G) at position 73 of the PRA1 gene. Surface levels of Gpm1 varied by 8.2, and Pra1 levels by 3.3 fold in thirteen tested isolates and these differences influenced fungal immune fitness. The high Gpm1/Pra1 expressing candida strains bound the three human immune regulators more efficiently, than the low expression strains. The difference was 44% for Factor H binding, 51% for C4BP binding and 23% for plasminogen binding. This higher Gpm1/Pra1 expressing strains result in enhanced survival upon challenge with complement active, Factor H depleted human serum (difference 40%). In addition adhesion to and infection of human endothelial cells was increased (difference 60%), and C3b surface deposition was less effective (difference 27%). Thus, variable expression levels of central immune evasion protein influences immune fitness of the human fungal pathogen C. albicans and thus contribute to fungal virulence.

Streptococcus agalactiae capsule polymer length and attachment is determined by the proteins CpsABCD

The production of capsular polysaccharides (CPS) or secreted exopolysaccharides is ubiquitous in bacteria, and the Wzy pathway constitutes a prototypical mechanism to produce these structures. Despite the differences in polysaccharide composition among species, a group of proteins involved in this pathway is well conserved. Streptococcus agalactiae (group B Streptococcus; GBS) produces a CPS that represents the main virulence factor of the bacterium and is a prime target in current vaccine development. We used this human pathogen to investigate the roles and potential interdependencies of the conserved proteins CpsABCD encoded in the cps operon, by developing knock-out and functional mutant strains. The mutant strains were examined for CPS quantity, size, and attachment to the cell surface as well as CpsD phosphorylation. We observed that CpsB, -C, and -D compose a phosphoregulatory system where the CpsD autokinase phosphorylates its C-terminal tyrosines in a CpsC-dependent manner. These Tyr residues are also the target of the cognate CpsB phosphatase. An interaction between CpsD and CpsC was observed, and the phosphorylation state of CpsD influenced the subsequent action of CpsC. The CpsC extracellular domain appeared necessary for the production of high molecular weight polysaccharides by influencing CpsA-mediated attachment of the CPS to the bacterial cell surface. In conclusion, although having no impact on cps transcription or the synthesis of the basal repeating unit, we suggest that these proteins are fine-tuning the last steps of CPS biosynthesis (i.e. the balance between polymerization and attachment to the cell wall).

A novel cholesterol-insensitive mode of membrane binding promotes cytolysin-mediated translocation by Streptolysin O

Cytolysin-mediated translocation (CMT), performed by Streptococcus pyogenes, utilizes the cholesterol-dependent cytolysin Streptolysin O (SLO) to translocate the NAD(+) -glycohydrolase (SPN) into the host cell during infection. SLO is required for CMT and can accomplish this activity without pore formation, but the details of SLO's interaction with the membrane preceding SPN translocation are unknown. Analysis of binding domain mutants of SLO and binding domain swaps between SLO and homologous cholesterol-dependent cytolysins revealed that membrane binding by SLO is necessary but not sufficient for CMT, demonstrating a specific requirement for SLO in this process. Despite being the only known receptor for SLO, this membrane interaction does not require cholesterol. Depletion of cholesterol from host membranes and mutation of SLO's cholesterol recognition motif abolished pore formation but did not inhibit membrane binding or CMT. Surprisingly, SLO requires the coexpression and membrane localization of SPN to achieve cholesterol-insensitive membrane binding; in the absence of SPN, SLO's binding is characteristically cholesterol-dependent. SPN's membrane localization also requires SLO, suggesting a co-dependent, cholesterol-insensitive mechanism of membrane binding occurs, resulting in SPN translocation.

Effect of lineage-specific metabolic traits of Lactobacillus reuteri on sourdough microbial ecology

This study determined the effects of specific metabolic traits of Lactobacillus reuteri on its competitiveness in sourdoughs. The competitiveness of lactobacilli in sourdough generally depends on their growth rate; acid resistance additionally contributes to competitiveness in sourdoughs with long fermentation times. Glycerol metabolism via glycerol dehydratase (gupCDE) accelerates growth by the regeneration of reduced cofactors; glutamate metabolism via glutamate decarboxylase (gadB) increases acid resistance by generating a proton motive force. Glycerol and glutamate metabolisms are lineage-specific traits in L. reuteri; therefore, this study employed glycerol dehydratase-positive sourdough isolates of human-adapted L. reuteri lineage I, glutamate decarboxylase-positive strains of rodent-adapted L. reuteri lineage II, as well as mutants with deletions in gadB or gupCDE. The competitivenesses of the strains were quantified by inoculation of wheat and sorghum sourdoughs with defined strains, followed by propagation of doughs with a 10% inoculum and 12-h or 72-h fermentation cycles. Lineage I L. reuteri strains dominated sourdoughs propagated with 12-h fermentation cycles; lineage II L. reuteri strains dominated sourdoughs propagated with 72-h fermentation cycles. L. reuteri 100-23ΔgadB was outcompeted by its wild-type strain in sourdoughs fermented with 72-h fermentation cycles; L. reuteri FUA3400ΔgupCDE was outcompeted by its wild-type strain in sourdoughs fermented with both 12-h and 72-h fermentation cycles. Competition experiments with isogenic pairs of strains resulted in a constant rate of strain displacement of the less competitive mutant strain. In conclusion, lineage-specific traits of L. reuteri determine the competitiveness of this species in sourdough fermentations.

SpyAD, a moonlighting protein of group A Streptococcus contributing to bacterial division and host cell adhesion

Group A streptococcus (GAS) is a human pathogen causing a wide repertoire of mild and severe diseases for which no vaccine is yet available. We recently reported the identification of three protein antigens that in combination conferred wide protection against GAS infection in mice. Here we focused our attention on the characterization of one of these three antigens, Spy0269, a highly conserved, surface-exposed, and immunogenic protein of unknown function. Deletion of the spy0269 gene in a GAS M1 isolate resulted in very long bacterial chains, which is indicative of an impaired capacity of the knockout mutant to properly divide. Confocal microscopy and immunoprecipitation experiments demonstrated that the protein was mainly localized at the cell septum and could interact in vitro with the cell division protein FtsZ, leading us to hypothesize that Spy0269 is a member of the GAS divisome machinery. Predicted structural domains and sequence homologies with known streptococcal adhesins suggested that this antigen could also play a role in mediating GAS interaction with host cells. This hypothesis was confirmed by showing that recombinant Spy0269 could bind to mammalian epithelial cells in vitro and that Lactococcus lactis expressing Spy0269 on its cell surface could adhere to mammalian cells in vitro and to mice nasal mucosa in vivo. On the basis of these data, we believe that Spy0269 is involved both in bacterial cell division and in adhesion to host cells and we propose to rename this multifunctional moonlighting protein as SpyAD (Streptococcus pyogenes Adhesion and Division protein).

Analysis of two-component systems in group B Streptococcus shows that RgfAC and the novel FspSR modulate virulence and bacterial fitness

Group B Streptococcus (GBS), in the transition from commensal organisms to pathogens, will encounter diverse host environments and, thus, require coordinated control of the transcriptional responses to these changes. This work was aimed at better understanding the role of two-component signal transduction systems (TCS) in GBS pathophysiology through a systematic screening procedure. We first performed a complete inventory and sensory mechanism classification of all putative GBS TCS by genomic analysis. Five TCS were further investigated by the generation of knockout strains, and in vitro transcriptome analysis identified genes regulated by these systems, ranging from 0.1% to 3% of the genome. Interestingly, two sugar phosphotransferase systems appeared to be differentially regulated in the TCS-16 knockout strain (TCS loci were numbered in order of their appearance on the chromosome), suggesting an involvement in monitoring carbon source availability. High-throughput analysis of bacterial growth on different carbon sources showed that TCS-16 was necessary for the growth of GBS on fructose-6-phosphate. Additional transcriptional analysis provided further evidence for a stimulus-response circuit where extracellular fructose-6-phosphate leads to autoinduction of TCS-16, with concomitant dramatic upregulation of the adjacent operon, which encodes a phosphotransferase system. The TCS-16-deficient strain exhibited decreased persistence in a model of vaginal colonization. All mutant strains were also characterized in a murine model of systemic infection, and inactivation of TCS-17 (also known as RgfAC) resulted in hypervirulence. Our data suggest a role for the previously unknown TCS-16, here named FspSR, in bacterial fitness and carbon metabolism during host colonization, and the data also provide experimental evidence for TCS-17/RgfAC involvement in virulence.

Two-component systems have been evolved by bacteria to detect environmental changes, and they play key roles in pathogenicity. A comprehensive analysis of TCS in GBS has not been performed previously. In this work, we classify 21 TCS and present evidence for the involvement of two specific TCS in GBS virulence and colonization in vivo. Although pinpointing specific TCS stimuli is notoriously difficult, we used a combination of techniques to identify two systems with different effects on GBS pathogenesis. For one of the systems, we propose that fructose-6-phosphate, a metabolite in glycolysis, is sufficient to induce a regulatory response involving a sugar transport system. Our catalogue and classification of TCS may guide further studies into the role of TCS in GBS pathogenicity and biology.

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.

Novel two-component regulatory systems play a role in biofilm formation of Lactobacillus reuteri rodent isolate 100-23

This study characterized the two-component regulatory systems encoded by bfrKRT and cemAKR, and assessed their influence on biofilm formation by Lactobacillus reuteri 100-23. A method for deletion of multiple genes was employed to disrupt the genetic loci of two-component systems. The operons bfrKRT and cemAKR showed complementary organization. Genes bfrKRT encode a histidine kinase, a response regulator and an ATP-binding cassette-type transporter with a bacteriocin-processing peptidase domain, respectively. Genes cemAKR code for a signal peptide, a histidine kinase and a response regulator, respectively. Deletion of single or multiple genes in the operons bfrKRT and cemAKR did not affect cell morphology, growth or the sensitivity to various stressors. However, gene disruption affected biofilm formation; this effect was dependent on the carbon source. Deletion of bfrK or cemA increased sucrose-dependent biofilm formation in vitro. Glucose-dependent biofilm formation was particularly increased by deletion of cemK. The expression of cemK and cemR was altered by deletion of bfrK, indicating cross-talk between these two regulatory systems. These results may contribute to our understanding of the genetic factors related to the biofilm formation and competitiveness of L. reuteri in intestinal ecosystems.

BsaB, a novel adherence factor of group B Streptococcus

Streptococcus agalactiae (group B Streptococcus [GBS]) is a leading cause of neonatal sepsis and meningitis, peripartum infections in women, and invasive infections in chronically ill or elderly individuals. GBS can be isolated from the gastrointestinal or genital tracts of up to 30% of healthy adults, and infection is thought to arise from invasion from a colonized mucosal site. Accordingly, bacterial surface components that mediate attachment of GBS to host cells or the extracellular matrix represent key factors in the colonization and infection of the human host. We identified a conserved GBS gene of unknown function that was predicted to encode a cell wall-anchored surface protein. Deletion of the gene and a cotranscribed upstream open reading frame (ORF) in GBS strain 515 reduced bacterial adherence to VK2 vaginal epithelial cells in vitro and reduced GBS binding to fibronectin-coated microtiter wells. Expression of the gene product in Lactococcus lactis conferred the ability to adhere to VK2 cells, to fibronectin and laminin, and to fibronectin-coated ME-180 cervical epithelial cells. Expression of the recombinant protein in L. lactis also markedly increased biofilm formation. The adherence function of the protein, named bacterial surface adhesin of GBS (BsaB), depended both on a central BID1 domain found in bacterial intimin-like proteins and on the C-terminal portion of the BsaB protein. Expression of BsaB in GBS, like that of several other adhesins, was regulated by the CsrRS two-component system. We conclude that BsaB represents a newly identified adhesin that participates in GBS attachment to epithelial cells and the extracellular matrix.

Streptococcus pyogenes arginine and citrulline catabolism promotes infection and modulates innate immunity

A bacterium's ability to acquire nutrients from its host during infection is an essential component of pathogenesis. For the Gram-positive pathogen Streptococcus pyogenes, catabolism of the amino acid arginine via the arginine deiminase (ADI) pathway supplements energy production and provides protection against acid stress in vitro. Its expression is enhanced in murine models of infection, suggesting an important role in vivo. To gain insight into the function of the ADI pathway in pathogenesis, the virulence of mutants defective in each of its enzymes was examined. Mutants unable to use arginine (ΔArcA) or citrulline (ΔArcB) were attenuated for carriage in a murine model of asymptomatic mucosal colonization. However, in a murine model of inflammatory infection of cutaneous tissue, the ΔArcA mutant was attenuated but the ΔArcB mutant was hyperattenuated, revealing an unexpected tissue-specific role for citrulline metabolism in pathogenesis. When mice defective for the arginine-dependent production of nitric oxide (iNOS(-/-)) were infected with the ΔArcA mutant, cutaneous virulence was rescued, demonstrating that the ability of S. pyogenes to utilize arginine was dispensable in the absence of nitric oxide-mediated innate immunity. This work demonstrates the importance of arginine and citrulline catabolism and suggests a novel mechanism of virulence by which S. pyogenes uses its metabolism to modulate innate immunity through depletion of an essential host nutrient.

Genetic manipulation of Streptococcus pyogenes (the Group A Streptococcus, GAS)

Streptococcus pyogenes (the Group A Streptococcus, GAS) is a Gram-positive bacterium responsible for a wide spectrum of diseases ranging from mild superficial infections (pharyngitis, impetigo) to severe, often life-threatening invasive diseases (necrotizing fasciitis, streptococcal toxic shock syndrome) in humans. This unit describes molecular techniques for the genetic manipulation of S. pyogenes with detailed protocols for transformation, gene disruption, allelic exchange, transposon mutagenesis, and genetic complementation.

Streptolysin O and its co-toxin NAD-glycohydrolase protect group A Streptococcus from Xenophagic killing

Group A Streptococcus (Streptococcus pyogenes or GAS) causes pharyngitis, severe invasive infections, and the post-infectious syndromes of glomerulonephritis and rheumatic fever. GAS can be internalized and killed by epithelial cells in vitro, a process that may contribute to local innate defense against pharyngeal infection. Secretion of the pore-forming toxin streptolysin O (SLO) by GAS has been reported to stimulate targeted autophagy (xenophagy) upon internalization of the bacteria by epithelial cells. Whereas this process was associated with killing of GAS in HeLa cells, studies in human keratinocytes found SLO production enhanced intracellular survival. To reconcile these conflicting observations, we now report in-depth investigation of xenophagy in response to GAS infection of human oropharyngeal keratinocytes, the predominant cell type of the pharyngeal epithelium. We found that SLO expression was associated with prolonged intracellular survival; unexpectedly, expression of the co-toxin NADase was required for this effect. Enhanced intracellular survival was lost upon deletion of NADase or inactivation of its enzymatic activity. Shortly after internalization of GAS by keratinocytes, SLO-mediated damage to the bacteria-containing vacuole resulted in exposure to the cytosol, ubiquitination of GAS and/or associated vacuolar membrane remnants, and engulfment of GAS in LC3-positive vacuoles. We also found that production of streptolysin S could mediate targeting of GAS to autophagosomes in the absence of SLO, a process accompanied by galectin 8 binding to damaged GAS-containing endosomes. Maturation of GAS-containing autophagosome-like vacuoles to degradative autolysosomes was prevented by SLO pore-formation and by SLO-mediated translocation of enzymatically active NADase into the keratinocyte cytosol. We conclude that SLO stimulates xenophagy in pharyngeal keratinocytes, but the coordinated action of SLO and NADase prevent maturation of GAS-containing autophagosomes, thereby prolonging GAS intracellular survival. This novel activity of NADase to block autophagic killing of GAS in pharyngeal cells may contribute to pharyngitis treatment failure, relapse, and chronic carriage.

Group A Streptococcus (Streptococcus pyogenes or GAS) is the agent of streptococcal pharyngitis (strep throat), invasive infections such as necrotizing fasciitis and streptococcal toxic shock, and post-infectious complications including rheumatic heart disease. Epithelial cells internalize and kill GAS in vitro and may contribute to local innate immune defense in the human pharynx. We now find that production of the secreted pore-forming toxin streptolysin O (SLO) triggered targeted autophagy (termed xenophagy) of GAS in human oropharyngeal keratinocytes, but also enhanced GAS intracellular survival. Increased GAS survival was dependent both on pore-formation by SLO and on SLO-mediated translocation of an enzymatically active co-toxin, NAD-glycohydrolase, into the keratinocyte cytosol. The survival-enhancing effect of both toxins was associated with inhibition of lysosomal fusion with GAS-containing autophagosomes to form functional degradative autolysosomes. These findings reveal a novel coordinated role of two streptococcal toxins in protecting GAS from xenophagic killing and enhancing intracellular survival. Prolonged GAS intracellular survival may contribute to pharyngitis treatment failure, relapse, and chronic carriage.

The two-component response regulator LiaR regulates cell wall stress responses, pili expression and virulence in group B Streptococcus

Group B Streptococcus (GBS) remains the leading cause of early onset sepsis among term infants. Evasion of innate immune defences is critical to neonatal GBS disease pathogenesis. Effectors of innate immunity, as well as numerous antibiotics, frequently target the peptidoglycan layer of the Gram-positive bacterial cell wall. The intramembrane-sensing histidine kinase (IM-HK) class of two-component regulatory systems has been identified as important to the Gram-positive response to cell wall stress. We have characterized the GBS homologue of LiaR, the response regulator component of the Lia system, to determine its role in GBS pathogenesis. LiaR is expressed as part of a three-gene operon (liaFSR) with a promoter located upstream of liaF. A LiaR deletion mutant is more susceptible to cell wall-active antibiotics (vancomycin and bacitracin) as well as antimicrobial peptides (polymixin B, colistin, and nisin) compared to isogenic wild-type GBS. LiaR mutant GBS are significantly attenuated in mouse models of both GBS sepsis and pneumonia. Transcriptional profiling with DNA microarray and Northern blot demonstrated that LiaR regulates expression of genes involved in microbial defence against host antimicrobial systems including genes functioning in cell wall synthesis, pili formation and cell membrane modification. We conclude that the LiaFSR system, the first member of the IM-HK regulatory systems to be studied in GBS, is involved in sensing perturbations in the integrity of the cell wall and activates a transcriptional response that is important to the pathogenesis of GBS infection.

Analysis of polymorphic residues reveals distinct enzymatic and cytotoxic activities of the Streptococcus pyogenes NAD+ glycohydrolase

The Streptococcus pyogenes NAD(+) glycohydrolase (SPN) is secreted from the bacterial cell and translocated into the host cell cytosol where it contributes to cell death. Recent studies suggest that SPN is evolving and has diverged into NAD(+) glycohydrolase-inactive variants that correlate with tissue tropism. However, the role of SPN in both cytotoxicity and niche selection are unknown. To gain insight into the forces driving the adaptation of SPN, a detailed comparison of representative glycohydrolase activity-proficient and -deficient variants was conducted. Of a total 454 amino acids, the activity-deficient variants differed at only nine highly conserved positions. Exchanging residues between variants revealed that no one single residue could account for the inability of the deficient variants to cleave the glycosidic bond of β-NAD(+) into nicotinamide and ADP-ribose; rather, reciprocal changes at 3 specific residues were required to both abolish activity of the proficient version and restore full activity to the deficient variant. Changing any combination of 1 or 2 residues resulted in intermediate activity. However, a change to any 1 residue resulted in a significant decrease in enzyme efficiency. A similar pattern involving multiple residues was observed for comparison with a second highly conserved activity-deficient variant class. Remarkably, despite differences in glycohydrolase activity, all versions of SPN were equally cytotoxic to cultured epithelial cells. These data indicate that the glycohydrolase activity of SPN may not be the only contribution the toxin has to the pathogenesis of S. pyogenes and that both versions of SPN play an important role during infection.

PTS phosphorylation of Mga modulates regulon expression and virulence in the group A streptococcus

The ability of a bacterial pathogen to monitor available carbon sources in host tissues provides a clear fitness advantage. In the group A streptococcus (GAS), the virulence regulator Mga contains homology to phosphotransferase system (PTS) regulatory domains (PRDs) found in sugar operon regulators. Here we show that Mga was phosphorylated in vitro by the PTS components EI/HPr at conserved PRD histidines. A ΔptsI (EI-deficient) GAS mutant exhibited decreased Mga activity. However, PTS-mediated phosphorylation inhibited Mga-dependent transcription of emm in vitro. Using alanine (unphosphorylated) and aspartate (phosphomimetic) mutations of PRD histidines, we establish that a doubly phosphorylated PRD1 phosphomimetic (D/DMga4) is completely inactive in vivo, shutting down expression of the Mga regulon. Although D/DMga4 is still able to bind DNA in vitro, homo-multimerization of Mga is disrupted and the protein is unable to activate transcription. PTS-mediated regulation of Mga activity appears to be important for pathogenesis, as bacteria expressing either non-phosphorylated (A/A) or phosphomimetic (D/D) PRD1 Mga mutants were attenuated in a model of GAS invasive skin disease. Thus, PTS-mediated phosphorylation of Mga may allow the bacteria to modulate virulence gene expression in response to carbohydrate status. Furthermore, PRD-containing virulence regulators (PCVRs) appear to be widespread in Gram-positive pathogens.

Murine vaginal colonization model for investigating asymptomatic mucosal carriage of Streptococcus pyogenes

While many virulence factors promoting Streptococcus pyogenes invasive disease have been described, specific streptococcal factors and host properties influencing asymptomatic mucosal carriage remain uncertain. To address the need for a refined model of prolonged S. pyogenes asymptomatic mucosal colonization, we have adapted a preestrogenized murine vaginal colonization model for S. pyogenes. In this model, derivatives of strains HSC5, SF370, JRS4, NZ131, and MEW123 established a reproducible, asymptomatic colonization of the vaginal mucosa over a period of typically 3 to 4 weeks' duration at a relatively high colonization efficiency. Prior treatment with estradiol prolonged streptococcal colonization and was associated with reduced inflammation in the colonized vaginal epithelium as well as a decreased leukocyte presence in vaginal fluid compared to the levels of inflammation and leukocyte presence in non-estradiol-treated control mice. The utility of our model for investigating S. pyogenes factors contributing to mucosal carriage was verified, as a mutant with a mutation in the transcriptional regulator catabolite control protein A (CcpA) demonstrated significant impairment in vaginal colonization. An assessment of in vivo transcriptional activity in the CcpA(-) strain for several known CcpA-regulated genes identified significantly elevated transcription of lactate oxidase (lctO) correlating with excessive generation of hydrogen peroxide to self-lethal levels. Deletion of lctO did not impair colonization, but deletion of lctO in a CcpA(-) strain prolonged carriage, exceeding even that of the wild-type strain. Thus, while LctO is not essential for vaginal colonization, its dysregulation is deleterious, highlighting the critical role of CcpA in promoting mucosal colonization. The vaginal colonization model should prove effective for future analyses of S. pyogenes mucosal colonization.

Genome-wide identification of genes required for fitness of group A Streptococcus in human blood

The group A streptococcus (GAS) is a strict human pathogen responsible for a wide spectrum of diseases. Although GAS genome sequences are available, functional genomic analyses have been limited. We developed a mariner-based transposon, osKaR, designed to perform Transposon-Site Hybridization (TraSH) in GAS and successfully tested its use in several invasive serotypes. A complex osKaR mutant library in M1T1 GAS strain 5448 was subjected to negative selection in human blood to identify genes important for GAS fitness in this clinically relevant environment. Mutants underrepresented after growth in blood (output pool) compared to growth in rich media (input pool) were identified using DNA microarray hybridization of transposon-specific tags en masse. Using blood from three different donors, we identified 81 genes that met our criteria for reduced fitness in blood from at least two individuals. Genes known to play a role in survival of GAS in blood were found, including those encoding the virulence regulator Mga (mga), the peroxide response regulator PerR (perR), and the RofA-like regulator Ralp-3 (ralp3). We also identified genes previously reported for their contribution to sepsis in other pathogens, such as de novo nucleotide synthesis (purD, purA, pyrB, carA, carB, guaB), sugar metabolism (scrB, fruA), zinc uptake (adcC), and transcriptional regulation (cpsY). To validate our findings, independent mutants with mutations in 10 different genes identified in our screen were confirmed to be defective for survival in blood bactericidal assays. Overall, this work represents the first use of TraSH in GAS to identify potential virulence genes.

Targeted amino acid substitutions impair streptolysin O toxicity and group A Streptococcus virulence

Streptolysin O is a potent pore-forming toxin produced by group A Streptococcus. The aims of the present study were to dissect the relative contributions of different structural domains of the protein to hemolytic activity, to obtain a detoxified form of streptolysin O amenable to human vaccine formulation, and to investigate the role of streptolysin O-specific antibodies in protection against group A Streptococcus infection. On the basis of in silico structural predictions, we introduced two amino acid substitutions, one in the proline-rich domain 1 and the other in the conserved undecapeptide loop in domain 4. The resulting streptolysin O derivative showed no toxicity, was highly impaired in binding to eukaryotic cells, and was unable to form organized oligomeric structures on the cell surface. However, it was fully capable of conferring consistent protection in a murine model of group A Streptococcus infection. When we engineered a streptococcal strain to express the double-mutated streptolysin O, a drastic reduction in virulence as well as a diminished capacity to kill immune cells recruited at the infection site was observed. Furthermore, when mice immunized with the toxoid were challenged with the wild-type and mutant strains, protection only against the wild-type strain, not against the strain expressing the double-mutated streptolysin O, was obtained. We conclude that protection occurs by antibody-mediated neutralization of active toxin.

We present a novel example of structural design of a vaccine antigen optimized for human vaccine use. Having previously demonstrated that immunization of mice with streptolysin O elicits a protective immune response against infection with group A Streptococcus strains of different serotypes, we developed in this study a double-mutated nontoxic derivative that represents a novel tool for the development of protective vaccine formulations against this important human pathogen. Furthermore, the innovative construction of an isogenic strain expressing a functionally inactive toxin and its use in infection and opsonophagocytosis experiments allowed us to investigate the mechanism by which streptolysin O mediates protection against group A Streptococcus. Finally, the ability of this toxin to directly attack and kill host immune cells during infection was studied in an air pouch model, which allowed parallel quantification of cellular recruitment, vitality, and cytokine release at the infection site.

Role of pilus proteins in adherence and invasion of Streptococcus agalactiae to the lung and cervical epithelial cells

Streptococcus agalactiae, or group B Streptococcus (GBS), is an important opportunistic pathogen that causes pneumonia, sepsis, and meningitis in neonates and severe diseases in immunocompromised adults. We have performed comparative genomics of prevalent GBS serotypes of Indian origin (i.e. Ia, III, V, and VII). Pilus-proteins were commonly found up-regulated, and their expression was studied by using antiserum for GBS80 (backbone protein of pilus island-I), GBS67 (ancillary protein of PI-2a), and SAN1518 (backbone protein of PI-2b) by whole cell and Western blot analysis. To check the role of pilus proteins in adherence and invasion, an inhibition assay was performed. Comparative immunoblotting experiments revealed that expression of pili proteins does not differ in geographically different selected serotypes, Ia and V, of India and the United States. In the case of A549 cells, we found that GBS VII invasion and adherence was inhibited by pilus protein-specific antiserum SAN1518 significantly (p < 0.001) by 88.5 and 91%, respectively. We found that mutant strains, deficient in the pilus proteins (Δgbs80 and Δsan1518) exhibit a significant decrease in adherence in the case of type Ia, III, and VII. In the case of type VII, we have found a 95% reduction in invasion when Δsan1518 was used with A549 cells. Because the pilus proteins were identified previously as vaccine candidates against GBS serotypes of developed countries, we also found their role in the attachment and invasion of GBS of Indian origin. Thus, the present work supports the idea of making a more effective pilus protein-based vaccine that can be used universally.

The structure of the CS1 pilus of enterotoxigenic Escherichia coli reveals structural polymorphism

Enterotoxigenic Escherichia coli (ETEC) is a bacterial pathogen that causes diarrhea in children and travelers in developing countries. ETEC adheres to host epithelial cells in the small intestine via a variety of different pili. The CS1 pilus is a prototype for a family of related pili, including the CFA/I pili, present on ETEC and other Gram-negative bacterial pathogens. These pili are assembled by an outer membrane usher protein that catalyzes subunit polymerization via donor strand complementation, in which the N terminus of each incoming pilin subunit fits into a hydrophobic groove in the terminal subunit, completing a β-sheet in the Ig fold. Here we determined a crystal structure of the CS1 major pilin subunit, CooA, to a 1.6-Å resolution. CooA is a globular protein with an Ig fold and is similar in structure to the CFA/I major pilin CfaB. We determined three distinct negative-stain electron microscopic reconstructions of the CS1 pilus and generated pseudoatomic-resolution pilus structures using the CooA crystal structure. CS1 pili adopt multiple structural states with differences in subunit orientations and packing. We propose that the structural perturbations are accommodated by flexibility in the N-terminal donor strand of CooA and by plasticity in interactions between exposed flexible loops on adjacent subunits. Our results suggest that CS1 and other pili of this class are extensible filaments that can be stretched in response to mechanical stress encountered during colonization.

Vitamin D and the human antimicrobial peptide LL-37 enhance group a streptococcus resistance to killing by human cells

The CsrRS two-component regulatory system of group A Streptococcus (GAS; Streptococcus pyogenes) responds to subinhibitory concentrations of the human antimicrobial peptide LL-37. LL-37 signaling through CsrRS results in upregulation of genes that direct synthesis of virulence factors, including the hyaluronic acid capsule and streptolysin O (SLO). Here, we demonstrate that a consequence of this response is augmented GAS resistance to killing by human oropharyngeal keratinocytes, neutrophils, and macrophages. LL-37-induced upregulation of SLO and hyaluronic acid capsule significantly reduced internalization of GAS by keratinocytes and phagocytic killing by neutrophils and macrophages. Because vitamin D induces LL-37 production by macrophages, we tested its effect on macrophage killing of GAS. In contrast to the reported enhancement of macrophage function in relation to other pathogens, treatment of macrophages with 1α,25-dihydroxy-vitamin D3 paradoxically reduced the ability of macrophages to control GAS infection. These observations demonstrate that LL-37 signals through CsrRS to induce a virulence phenotype in GAS characterized by heightened resistance to ingestion and killing by both epithelial cells and phagocytes. By inducing LL-37 production in macrophages, vitamin D may contribute to this paradoxical exacerbation of GAS infection. IMPORTANCE It remains poorly understood why group A Streptococcus (GAS) causes asymptomatic colonization or localized throat inflammation in most individuals but rarely progresses to invasive infection. The human antimicrobial peptide LL-37, which is produced as part of the innate immune response to GAS infection, signals through the GAS CsrRS two-component regulatory system to upregulate expression of multiple virulence factors. This study reports that two CsrRS-regulated GAS virulence factors-streptolysin O and the hyaluronic acid capsule-are critical in LL-37-induced resistance of GAS to killing by human throat epithelial cells and by neutrophils and macrophages. Vitamin D, which increases LL-37 production in macrophages, has the paradoxical effect of increasing GAS resistance to macrophage-mediated killing. In this way, the human innate immune response may promote the transition from GAS colonization to invasive infection.

Attenuation of Streptococcus suis virulence by the alteration of bacterial surface architecture

NeuB, a sialic acid synthase catalyzes the last committed step of the de novo biosynthetic pathway of sialic acid, a major element of bacterial surface structure. Here we report a functional NeuB homologue of Streptococcus suis, a zoonotic agent, and systematically address its molecular and immunological role in bacterial virulence. Disruption of neuB led to thinner capsules and more susceptibility to pH, and cps2B inactivation resulted in complete absence of capsular polysaccharides. These two mutants both exhibited increased adhesion and invasion to Hep-2 cells and improved sensibility to phagocytosis. Not only do they retain the capability of inducing the release of host pro-inflammatory cytokines, but also result in the faster secretion of IL-8. Easier cleaning up of the mutant strains in whole blood is consistent with virulence attenuation seen with experimental infections of both mice and SPF-piglets. Therefore we concluded that altered architecture of S. suis surface attenuates its virulence.

Streptococcus pyogenes SpyCEP influences host-pathogen interactions during infection in a murine air pouch model

Streptococcus pyogenes is a major human pathogen worldwide, responsible for both local and systemic infections. These bacteria express the subtilisin-like protease SpyCEP which cleaves human IL-8 and related chemokines. We show that localization of SpyCEP is growth-phase and strain dependent. Significant shedding was observed only in a strain naturally overexpressing SpyCEP, and shedding was not dependent on SpyCEP autoproteolytic activity. Surface-bound SpyCEP in two different strains was capable of cleaving IL-8. To investigate SpyCEP action in vivo, we adapted the mouse air pouch model of infection for parallel quantification of bacterial growth, host immune cell recruitment and chemokine levels in situ. In response to infection, the predominant cells recruited were neutrophils, monocytes and eosinophils. Concomitantly, the chemokines KC, LIX, and MIP-2 in situ were drastically increased in mice infected with the SpyCEP knockout strain, and growth of this mutant strain was reduced compared to the wild type. SpyCEP has been described as a potential vaccine candidate against S. pyogenes, and we showed that surface-associated SpyCEP was recognized by specific antibodies. In vitro, such antibodies also counteracted the inhibitory effects of SpyCEP on chemokine mediated PMN recruitment. Thus, α-SpyCEP antibodies may benefit the host both directly by enabling opsonophagocytosis, and indirectly, by neutralizing an important virulence factor. The animal model we employed shows promise for broad application in the study of bacterial pathogenesis.

Thermoregulation of capsule production by Streptococcus pyogenes

The capsule of Streptococcus pyogenes serves as an adhesin as well as an anti-phagocytic factor by binding to CD44 on keratinocytes of the pharyngeal mucosa and the skin, the main entry sites of the pathogen. We discovered that S. pyogenes HSC5 and MGAS315 strains are further thermoregulated for capsule production at a post-transcriptional level in addition to the transcriptional regulation by the CovRS two-component regulatory system. When the transcription of the hasABC capsular biosynthetic locus was de-repressed through mutation of the covRS system, the two strains, which have been used for pathogenesis studies in the laboratory, exhibited markedly increased capsule production at sub-body temperature. Employing transposon mutagenesis, we found that CvfA, a previously identified membrane-associated endoribonuclease, is required for the thermoregulation of capsule synthesis. The mutation of the cvfA gene conferred increased capsule production regardless of temperature. However, the amount of the capsule transcript was not changed by the mutation, indicating that a post-transcriptional regulator mediates between CvfA and thermoregulated capsule production. When we tested naturally occurring invasive mucoid strains, a high percentage (11/53, 21%) of the strains exhibited thermoregulated capsule production. As expected, the mucoid phenotype of these strains at sub-body temperature was due to mutations within the chromosomal covRS genes. Capsule thermoregulation that exhibits high capsule production at lower temperatures that occur on the skin or mucosal surface potentially confers better capability of adhesion and invasion when S. pyogenes penetrates the epithelial surface.

Genes important for catalase activity in Enterococcus faecalis

Little in general is known about how heme proteins are assembled from their constituents in cells. The Gram-positive bacterium Enterococcus faecalis cannot synthesize heme and does not depend on it for growth. However, when supplied with heme in the growth medium the cells can synthesize two heme proteins; catalase (KatA) and cytochrome bd (CydAB). To identify novel factors important for catalase biogenesis libraries of E. faecalis gene insertion mutants were generated using two different types of transposons. The libraries of mutants were screened for clones deficient in catalase activity using a colony zymogram staining procedure. Analysis of obtained clones identified, in addition to katA (encoding the catalase enzyme protein), nine genes distributed over five different chromosomal loci. No factors with a dedicated essential role in catalase biogenesis or heme trafficking were revealed, but the results indicate the RNA degradosome (srmB, rnjA), an ABC-type oligopeptide transporter (oppBC), a two-component signal transducer (etaR), and NADH peroxidase (npr) as being important for expression of catalase activity in E. faecalis. It is demonstrated that catalase biogenesis in E. faecalis is independent of the CydABCD proteins and that a conserved proline residue in the N-terminal region of KatA is important for catalase assembly.

Regulation and function of pilus island 1 in group B streptococcus

Group B streptococcus (GBS) pili may enhance colonization and infection by mediating bacterial adhesion to host cells, invasion across endothelial and epithelial barriers, and resistance to bacterial ingestion and killing by host phagocytes. However, it remains unclear how pilus expression is regulated and how modulation of pilus production affects GBS interactions with the human host. We investigated the regulation and function of pilus island 1 (PI-1) pili in GBS strain 2603. We found that PI-1 gene expression was controlled by the CsrRS two-component system, by Ape1, an AraC-type regulator encoded by a divergently transcribed gene immediately upstream of PI-1, and by environmental pH. The response regulator CsrR repressed expression of Ape1, which is an activator of PI-1 gene expression. In addition, CsrR repressed PI-1 gene expression directly, independent of its regulation of Ape1. In vitro assays demonstrated specific binding of both CsrR and Ape1 to chromosomal DNA sequences upstream of PI-1. Pilus gene expression was activated by acidic pH, and this effect was independent of CsrRS and Ape1. Unexpectedly, characterization of PI-1 deletion mutants revealed that PI-1 pili do not mediate adhesion of strain 2603 to A549 respiratory epithelial cells, ME180 cervical cells, or VK2 vaginal cells in vitro. PI-1 pili reduced internalization and intracellular killing of GBS by human monocyte-derived macrophages, by approximately 50%, but did not influence complement-mediated opsonophagocytic killing by human neutrophils. These findings shed new light on the complex nature of pilus regulation and function in modulating GBS interactions with the human host.

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.