High-frequency intracellular invasion of epithelial cells by serotype M1 group A streptococci: M1 protein-mediated invasion and cytoskeletal rearrangements

Interplay between group A Streptococcus and host innate immune responses

SUMMARYGroup A Streptococcus (GAS), also known as Streptococcus pyogenes, is a clinically well-adapted human pathogen that harbors rich virulence determinants contributing to a broad spectrum of diseases. GAS is capable of invading epithelial, endothelial, and professional phagocytic cells while evading host innate immune responses, including phagocytosis, selective autophagy, light chain 3-associated phagocytosis, and inflammation. However, without a more complete understanding of the different ways invasive GAS infections develop, it is difficult to appreciate how GAS survives and multiplies in host cells that have interactive immune networks. This review article attempts to provide an overview of the behaviors and mechanisms that allow pathogenic GAS to invade cells, along with the strategies that host cells practice to constrain GAS infection. We highlight the counteractions taken by GAS to apply virulence factors such as streptolysin O, nicotinamide-adenine dinucleotidase, and streptococcal pyrogenic exotoxin B as a hindrance to host innate immune responses.

Insights gained from sequencing Australian non-invasive and invasive Streptococcus pyogenes isolates

Epidemiological data have indicated that invasive infections caused by the Gram-positive cocci Streptococcus pyogenes (group A streptococcus, GAS) have increased in many Australian states over the past two decades. In July 2022, invasive GAS (iGAS) infections became nationally notifiable in Australia via public-health agencies. Surveillance for S. pyogenes infections has been sporadic within the state of New South Wales (NSW). This has led to a lack of genetic data on GAS strains in circulation, particularly for non-invasive infections, which are the leading cause of GAS's burden on the Australian healthcare system. To address this gap, we used whole-genome sequencing to analyse the genomes of 318 S. pyogenes isolates collected within two geographical regions of NSW. Invasive isolates were collected in 2007-2017, whilst non-invasive isolates were collected in 2019-2021. We found that at least 66 different emm-types were associated with clinical disease within NSW. There was no evidence of any Australian-specific clones in circulation. The M1UK variant of the emm1 global pandemic clone (M1global) has been detected in our isolates from 2013 onwards. We detected antimicrobial-resistance genes (mainly tetM, ermA or ermB genes) in less than 10 % of our 318 isolates, which were more commonly associated with non-invasive infections. Superantigen virulence gene carriage was reasonably proportionate between non-invasive and invasive infection isolates. Our study adds rich data on the genetic makeup of historical S. pyogenes infections within Australia. Ongoing surveillance of invasive and non-invasive GAS infections within NSW by whole-genome sequencing is warranted to inform on outbreaks, antimicrobial resistance and vaccine coverage.

An Overview of Contemporary and Future Therapeutic Strategies for Scalp Psoriasis

Scalp psoriasis is a common manifestation of psoriasis that significantly impacts a patient's quality of life. About 80% of cases of psoriasis involve the scalp, making it the most frequently affected area of the body. The treatment of scalp psoriasis is particularly crucial because of its hard-to-treat nature and substantial adverse impacts on overall well-being. Along with the physical symptoms of discomfort and itching, psoriasis, especially when it affects the scalp, can cause severe psychological damage. Treating scalp psoriasis can be challenging due to its location and associated symptoms, such as scaling and pruritus, which is why various drugs have become widely used for refractory cases. Topical treatments like corticosteroids and vitamin D analogs manage scalp psoriasis by reducing inflammation and regulating skin cell growth. Tar-based shampoos, salicylic acid solutions, and moisturizers control scaling. Phototherapy with UVB light reduces inflammation. Severe cases may require systemic medications such as oral retinoids and immunosuppressants. While various therapies are accessible for scalp psoriasis, concerns arise due to their limited advantages and the absence of controlled studies assessing their effectiveness. Considering these challenges, there is a clear demand for innovative approaches to address this condition effectively. Recent advancements in topical therapies, phototherapy, systemic agents, and complementary therapies have shown promising results in managing scalp psoriasis. Also, the advent of biologics, specifically anti-IL-17 and anti-IL-23 drugs for scalp psoriasis, has seen significant improvements. The review highlights the lack of well-tolerated and effective treatments for scalp psoriasis and underscores the importance of further research in this area. The objective of this review is to clarify the different treatment options currently available or being investigated in clinical trials for managing scalp psoriasis.

Streptococcus pyogenes ("Group A Streptococcus"), a Highly Adapted Human Pathogen-Potential Implications of Its Virulence Regulation for Epidemiology and Disease Management

Streptococcus pyogenes (group A streptococci; GAS) is an exclusively human pathogen. It causes a variety of suppurative and non-suppurative diseases in people of all ages worldwide. Not all can be successfully treated with antibiotics. A licensed vaccine, in spite of its global importance, is not yet available. GAS express an arsenal of virulence factors responsible for pathological immune reactions. The transcription of all these virulence factors is under the control of three types of virulence-related regulators: (i) two-component systems (TCS), (ii) stand-alone regulators, and (iii) non-coding RNAs. This review summarizes major TCS and stand-alone transcriptional regulatory systems, which are directly associated with virulence control. It is suggested that this treasure of knowledge on the genetics of virulence regulation should be better harnessed for new therapies and prevention methods for GAS infections, thereby changing its global epidemiology for the better.

Induction of cyclophilin A by influenza A virus infection facilitates group A Streptococcus coinfection

During influenza A epidemics, bacterial coinfection is a major cause of increased morbidity and mortality. However, the roles of host factors in regulating influenza A virus (IAV)-triggered bacterial coinfection remain elusive. Cyclophilin A (CypA) is an important regulator of infection and immunity. Here, we show that IAV-induced CypA expression facilitates group A Streptococcus (GAS) coinfection both in vitro and in vivo. Upon IAV infection, CypA interacts with focal adhesion kinase (FAK) and inhibited E3 ligase cCbl-mediated, K48-linked ubiquitination of FAK, which positively regulates integrin α5 expression and actin rearrangement via the FAK/Akt signaling pathway to facilitate GAS colonization and invasion. Notably, CypA deficiency or inhibition by cyclosporine A significantly inhibits IAV-triggered GAS coinfection in mice. Collectively, these findings reveal that CypA is critical for GAS infection, and induction of CypA expression is another way for IAV to promote bacterial coinfection, suggesting that CypA is a promising therapeutic target for the secondary bacterial infection.

Streptococcal protein SIC activates monocytes and induces inflammation

Streptococcus pyogenes is a major bacterial pathogen in the human population and isolates of the clinically important M1 serotype secrete protein Streptococcal inhibitor of complement (SIC) known to interfere with human innate immunity. Here we find that SIC from M1 bacteria interacts with TLR2 and CD14 on monocytes leading to the activation of the NF-κB and p38 MAPK pathways and the release of several pro-inflammatory cytokines (e.g. TNFα and INFγ). In human plasma, SIC binds clusterin and histidine-rich glycoprotein, and whole plasma, and these two purified plasma proteins enhanced the activation of monocytes by SIC. Isolates of the M55 serotype secrete an SIC homolog, but this protein did not activate monocytes. M1 isolates are common in cases of invasive S. pyogenes infections characterized by massive inflammation, and the results of this study indicate that the pro-inflammatory property of SIC contributes to the pathology of these severe clinical conditions.

Pathogenic Mechanisms of Streptococcal Necrotizing Soft Tissue Infections

Necrotizing skin and soft tissue infections (NSTIs) are severe life-threatening and rapidly progressing infections. Beta-hemolytic streptococci, particularly S. pyogenes (group A streptococci (GAS)) but also S. dysgalactiae subsp. equisimilis (SDSE, most group G and C streptococcus), are the main causative agents of monomicrobial NSTIs and certain types, such as emm1 and emm3, are over-represented in NSTI cases. An arsenal of bacterial virulence factors contribute to disease pathogenesis, which is a complex and multifactorial process. In this chapter, we summarize data that have provided mechanistic and immuno-pathologic insight into host-pathogens interactions that contribute to tissue pathology in streptococcal NSTIs. The role of streptococcal surface associated and secreted factors contributing to the hyper-inflammatory state and immune evasion, bacterial load in the tissue and persistence strategies, including intracellular survival and biofilm formation, as well as strategies to mimic NSTIs in vitro are discussed.

Intracellular Invasion Ability and Associated Microbiological Characteristics of Streptococcus canis in Isolates from Japan

This study evaluated the cell invasion ability (CIA) of Streptococcus canis isolates, and clarified the relationship between high-frequency CIA and its microbiological features. Of the companion animal-origin isolates (n = 117) that were obtained in 2017, 40 isolates were randomly selected with the host information, with two human blood-origin isolates included. CIA was measured using human colon carcinoma epithelium and the hemolytic activity (HA) using sheep blood, along with S. canis M-like protein (SCM) allele typing, sequence type (ST) determination, and antimicrobial resistance (AMR) phenotyping/genotyping. CIA measurements revealed that 19 and 24 isolates had high- and low-frequencies, respectively. HA assessment revealed that 24 and 19 isolates were categorized as high- and low- level, respectively. No difference was observed in the high-/low-level HA between the high- /low-frequency CIA populations. A significant difference was found in the high-/low-frequency CIA between the SCM group I/II populations. Additionally, a significantly higher CIA was found in the SCM allele type 10/type 11 than in the others. A significant association was observed between high-frequency CIA and the ST21/ST41 populations. No difference was found in the high-/low-frequency CIA between the presence and absence of the AMR phenotype/genotype. These observations suggest a relationship between high-frequency CIA and its microbiological characteristics (SCM allele type 10/type 11 or ST21/ST41).

Intracellular Invasion by Streptococcus pyogenes : Invasins, Host Receptors, and Relevance to Human Disease

The human oral-nasal mucosa is the primary reservoir for Streptococcus pyogenes infections. Although the most common infection of consequence in temperate climates is pharyngitis, the past 25 years have witnessed a dramatic increase in invasive disease in many regions of the world. Historically, S. pyogenes has been associated with sepsis and fulminate systemic infections, but the mechanism by which these streptococci traverse mucosal or epidermal barriers is not understood. The discovery that S. pyogenes can be internalized by mammalian epithelial cells at high frequencies ( 1 – 3 ) and/or open tight junctions to pass between cells ( 4 ) provides potential explanations for changes in epidemiology and the ability of this species to breach such barriers. In this article, the invasins and pathways that S. pyogenes uses to reach the intracellular state are reviewed, and the relationship between intracellular invasion and human disease is discussed.

Group A Streptococcus-Mediated Host Cell Signaling

In the past decade, the field of the cellular microbiology of group A Streptococcus (S. pyogenes) infection has made tremendous advances and touched upon several important aspects of pathogenesis, including receptor biology, invasive and evasive phenomena, inflammasome activation, strain-specific autophagic bacterial killing, and virulence factor-mediated programmed cell death. The noteworthy aspect of S. pyogenes-mediated cell signaling is the recognition of the role of M protein in a variety of signaling events, starting with the targeting of specific receptors on the cell surface and on through the induction and evasion of NETosis, inflammasome, and autophagy/xenophagy to pyroptosis and apoptosis. Variations in reports on S. pyogenes-mediated signaling events highlight the complex mechanism of pathogenesis and underscore the importance of the host cell and S. pyogenes strain specificity, as well as in vitro/in vivo experimental parameters. The severity of S. pyogenes infection is, therefore, dependent on the virulence gene expression repertoire in the host environment and on host-specific dynamic signaling events in response to infection. Commonly known as an extracellular pathogen, S. pyogenes finds host macrophages as safe havens wherein it survives and even multiplies. The fact that endothelial cells are inherently deficient in autophagic machinery compared to epithelial cells and macrophages underscores the invasive nature of S. pyogenes and its ability to cause severe systemic diseases. S. pyogenes is still one of the top 10 causes of infectious mortality. Understanding the orchestration of dynamic host signaling networks will provide a better understanding of the increasingly complex mechanism of S. pyogenes diseases and novel ways of therapeutically intervening to thwart severe and often fatal infections.

Intracellular Streptococcal Uptake and Persistence: A Potential Cause of Erysipelas Recurrence

Erysipelas is a severe streptococcal infection of the skin primarily spreading through the lymphatic vessels. Penicillin is the treatment of choice. The most common complication consists in relapses which occur in up to 40% or more of patients despite appropriate antibiotic treatment. They cause lymphatic damage resulting in irreversible lymphedema and ultimately elephantiasis nostras and lead to major health restrictions and high socio-medical costs. Prevention of relapses is an unmet need, because even long-term prophylactic penicillin application does eventually not reduce the risk of recurrence. In this article we assess risk factors and causes of erysipelas recurrence. A systematic literature search for clinical studies addressing potential causes and measures for prevention of erysipelas recurrence was combined with a review of experimental and clinical data assessing the ability and clinical relevance of streptococci for intracellular uptake and persistence. The literature review found that venous insufficiency, lymphedema, and intertrigo from fungal infections are considered to be major risk factors for recurrence of erysipelas but cannot adequately explain the high recurrence rate. As hitherto unrecognized likely cause of erysipelas relapses we identify the ability of streptococci for intracellular uptake into and persistence within epithelial and endothelial cells and macrophages. This creates intracellular streptococcal reservoirs out of reach of penicillins which do not reach sufficient bactericidal intracellular concentrations. Incomplete streptococcal elimination due to intracellular streptococcal persistence has been observed in various deep tissue infections and is considered as cause of relapsing streptococcal pharyngitis despite proper antibiotic treatment. It may also serves as endogenous infectious source of erysipelas relapses. We conclude that the current antibiotic treatment strategies and elimination of conventional risk factors employed in erysipelas management are insufficient to prevent erysipelas recurrence. The reactivation of streptococcal infection from intracellular reservoirs represents a plausible explanation for the frequent occurrence erysipelas relapses. Prevention of erysipelas relapses therefore demands for novel antibiotic strategies capable of eradicating intracellular streptococcal persistence.

Streptococcus equi subsp. zooepidemicus Invades and Survives in Epithelial Cells

Streptococcus equi subsp. zooepidemicus (S. zooepidemicus) is an opportunistic pathogen of several species including humans. S. zooepidemicus is found on mucus membranes of healthy horses, but can cause acute and chronic endometritis. Recently S. zooepidemicus was found able to reside in the endometrium for prolonged periods of time. Thus, we hypothesized that an intracellular phase may be part of the S. zooepidemicus pathogenesis and investigated if S. zooepidemicus was able to invade and survive inside epithelial cells. HEp-2 and HeLa cell lines were co-cultured with two S. zooepidemicus strains (1-4a and S31A1) both originating from the uterus of mares suffering from endometritis. Cells were fixed at different time points during the 23 h infection assay and field emission scanning electron microscopy (FESEM) was used to characterize adhesion and invasion mechanisms. The FESEM images showed three morphologically different types of invasion for both bacterial strains. The main port of entry was through large invaginations in the epithelial cell membrane. Pili-like bacterial appendages were observed when the S. zooepidemicus cells were in close proximity to the epithelial cells indicating that attachment and invasion were active processes. Adherent and intracellular S. zooepidemicus, and bacteria in association with lysosomes was determined by immunofluorescence staining techniques and fluorescence microscopy. Quantification of intracellular bacteria was determined in penicillin protection assays. Both S. zooepidemicus strains investigated were able to invade epithelial cells although at different magnitudes. The immunofluorescence data showed significantly higher adhesion and invasion rates for strain 1-4a when compared to strain S31A1. S. zooepidemicus was able to survive intracellularly, but the survival rate decreased over time in the cell culture system. Phagosome-like compartments containing S. zooepidemicus at some stages fused with lysosomes to form a phagolysosome. The results indicate that an intracellular phase may be one way S. zooepidemicus survives in the host, and could in part explain how S. zooepidemicus can cause recurrent/persistent infections. Future studies should reveal the ability of S. zooepidemicus to internalize and survive in primary equine endometrial cells and during in vivo conditions.

Enzyme-linked immunosorbent assay for group A Streptococcal anti-DNase B in human sera, using recombinant proteins - Comparison to the DNA methyl green micromethod

Among the four known Streptococcal nucleases comprising of DNase A, B, C and D; DNase B is the most common, and determination of the levels of antibody to DNase B (ADB) is often used to confirm a clinical diagnosis of Streptococcus pyogenes/group A Streptococcal (GAS) infection. The commonly used assays for antibodies that neutralize DNase B or streptolysin O activity use partially purified antigens that often fail to detect antibody changes subsequent to culture documented infections. Therefore, an enzyme-linked immunosorbent assay (ELISA) was developed employing his-tagged recombinant DNase B as plate antigen for comparison to the commonly used DNA methyl green micromethod (DMGM). DNAs from various Streptococcal species were screened for presence of dnaseB gene by PCR. Measurements of ADB in sera collected from subjects belonging to different ages, and ethnic groups were used to compare the two methods. dnaseB was not detected by PCR in DNA samples isolated from different strains of group B (GBS), C (GCS) and G (GGS) Streptococci. The ADB based ELISA proved to be highly sensitive and more responsive to changes in antibody concentration than DMGM. Use of recombinant DNase B eliminates the variability associated with the enzyme, partially purified from Streptococcal culture supernatants from various commercial sources and may provide a more reliable source of antigen to a wider group of laboratories concerned with GAS diagnosis.

Caveolin 1 restricts Group A Streptococcus invasion of nonphagocytic host cells

Caveolae are composed of 2 major proteins, caveolin 1 (CAV1) and cavin 1 or polymerase transcript release factor I (CAVIN1). Here, we demonstrate that CAV1 levels modulate invasion of Group A Streptococcus (GAS) into nonphagocytic mammalian cells. GAS showed enhanced internalisation into CAV1-knockout mouse embryonic fibroblasts and CAV1 knockdown human epithelial HEp-2 cells, whereas overexpression of CAV1 in HEp-2 cells reduced GAS invasion. This effect was not dependent on the expression of the GAS fibronectin binding protein SfbI, which had previously been implicated in caveolae-mediated uptake. Nor was this effect dependent on CAVIN1, as knockout of CAVIN1 in mouse embryonic fibroblasts resulted in reduced GAS internalisation. Although CAV1 restricted GAS invasion into host cells, we observed only minimal association of invading GAS (strain M1T1⁵⁴⁴⁸ ) with CAV1 by immunofluorescence and very low association of invading M1T1⁵⁴⁴⁸ with caveolae by transmission electron microscopy. These observations suggest that physical interaction with caveolae is not needed for CAV1 restriction of invading GAS. An indirect mechanism of action is also consistent with the finding that changing membrane fluidity reverses the increased invasion observed in CAV1-null cells. Together, these results suggest that CAV1 protects host cells against GAS invasion by a caveola-independent mechanism.

Galectin-3 Inhibits Galectin-8/Parkin-Mediated Ubiquitination of Group A Streptococcus

Group A streptococcus (GAS) is an important human pathogen that causes a wide variety of cutaneous and systemic infections. Although originally thought to be an extracellular bacterium, numerous studies have demonstrated that GAS can trigger internalization into nonimmune cells to escape from immune surveillance or antibiotic-mediated killing. Epithelial cells possess a defense mechanism involving autophagy-mediated targeting and killing of GAS within lysosome-fused autophagosomes. In endothelial cells, in contrast, we previously showed that autophagy is not sufficient for GAS killing. In the present study, we showed higher galectin-3 (Gal-3) expression and lower Gal-8 expression in endothelial cells than in epithelial cells. The recruitment of Gal-3 to GAS is higher and the recruitment of Gal-8 to GAS is lower in endothelial cells than in epithelial cells. We further showed that Gal-3 promotes GAS replication and diminishes the recruitment of Gal-8 and ubiquitin, the latter of which is a critical protein for autophagy sequestration. After knockdown of Gal-3 in endothelial cells, the colocalization of Gal-8, parkin, and ubiquitin-decorated GAS is significantly increased, as is the interaction of Gal-8 and parkin, an E3 ligase. Furthermore, inhibition of Gal-8 in epithelial cells attenuates recruitment of parkin; both Gal-8 and parkin contribute to ubiquitin recruitment and GAS elimination. Animal studies confirmed that Gal-3-knockout mice develop less-severe skin damage and that GAS replication can be detected only in the air pouch and not in organs and endothelial cells. These results demonstrate that Gal-3 inhibits ubiquitin recruitment by blocking Gal-8 and parkin recruitment, resulting in GAS replication in endothelial cells.

In epithelial cells, GAS can be efficiently killed within the lysosome-fused autophaosome compartment. However, we previously showed that, in spite of LC-3 recruitment, the autophagic machinery is not sufficient for GAS killing in endothelial cells. In this report, we provide the first evidence that Gal-3, highly expressed in endothelial cells, blocks the tagging of ubiquitin to GAS by inhibiting recruitment of Gal-8 and parkin, leading to an enhancement of GAS replication. We also provide the first demonstration that Gal-8 can interact with parkin, the critical E3 ligase, for resistance to intracellular bacteria by facilitating the decoration of bacteria with ubiquitin chains. Our findings reveal that differential levels of Gal-3 and Gal-8 expression and recruitment to GAS between epithelial cells and endothelial cells may contribute to the different outcomes of GAS elimination or survival and growth of GAS in these two types of cells.

Systematic review of factors contributing to penicillin treatment failure in Streptococcus pyogenes pharyngitis

Objective

Review the evidence for various explanations for microbiologic treatment failure following use of penicillin in group A streptococcal (GAS) tonsillopharyngitis.

DATA SOURCE

Systematic review of the literature based on Medline and EMBASE searches, and review of reference lists of included studies.

RESULTS

The explanations for penicillin treatment failure in GAS tonsillopharyngitis include 1) carrier state, 2) lack of compliance, 3) recurrent exposure, 4) in vivo copathogenicity of β-lactamase–producing normal pharyngeal flora, 5) in vivo bacterial coaggregation, 6) poor antibiotic penetration to tonsillopharyngeal tissue, 7) in vivo eradication of normal protective flora, 8) early initiation of antibiotic therapy resulting in suppression of an adequate host immune response, 9) intracellular localization of GAS, 10) GAS tolerance to penicillin, 11) contaminated toothbrushes or orthodontic appliances, and 12) transmission from the family pet. There is very little type I or II evidence to support any of the above-cited explanations for treatment failure in GAS tonsillopharyngitis; available studies are mostly observational (in patients) or laboratory-based without clinical confirmation.

CONCLUSION

Multiple explanations have been offered by investigators to explain penicillin treatment failures in GAS tonsillopharyngitis, but the evidence base to support the proposed explanations is generally weak by current standards. Further research is needed to better understand the mechanism(s) of penicillin treatment failure in GAS tonsillopharyngitis.

DiiA is a novel dimorphic cell wall protein of Streptococcus pneumoniae involved in invasive disease

OBJECTIVES

Many outer multidomain proteins play fundamental virulent roles in an allele-dependent manner. We aimed to investigate the influence of the outer SP1992 protein, here renamed DiiA (Dimorphic invasion-involved A), in pneumococcal disease.

METHODS

The presence and type of diiA allele was screened by PCR in 560 clinical isolates. Isogenic mutants carrying progressive diiA deletions were constructed and checked in mouse models of infection. DiiA binding to human molecules was carried out by surface plasmon resonance.

RESULTS

The diiA gene is exclusive of Streptococcus pneumoniae and included in the core genome. DiiA variants contain one or two imperfect repeats (R1 and R2), an unstructured region and a cell-wall anchor domain. Clonal complexes carrying both repeats were associated with invasive disease, while those carrying R2 preferentially caused non-invasive syndromes in patients with underlying risk factors. Mutants lacking both repeats were less efficient in nasopharyngeal colonization and dissemination from lungs. Moreover, the ΔdiiA defective strain suffered a severe impairment in bacterial proliferation in blood. Purified DiiA bound to collagen and lactoferrin with high affinity.

CONCLUSIONS

DiiA is a distinctive pneumococcal virulence factor contributing to colonization and long-term invasion in this pathogen.

Insufficient Acidification of Autophagosomes Facilitates Group A Streptococcus Survival and Growth in Endothelial Cells

Group A streptococcus (GAS) is an important human pathogen, and its invasion via blood vessels is critically important in serious events such as bacteremia or multiorgan failure. Although GAS was identified as an extracellular bacterium, the internalization of GAS into nonphagocytic cells may provide a strategy to escape from immune surveillance and antibiotic killing. However, GAS has also been reported to induce autophagy and is efficiently killed within lysosome-fused autophagosomes in epithelial cells. In this study, we show that GAS can replicate in endothelial cells and that streptolysin O is required for GAS growth. Bacterial replication can be suppressed by altering GAS gene expression in an acidic medium before internalization into endothelial cells. The inhibitory effect on GAS replication can be reversed by treatment with bafilomycin A1, a specific inhibitor of vacuolar-type H⁺-ATPase. Compared with epithelial cells in which acidification causes autophagy-mediated clearance of GAS, there was a defect in acidification of GAS-containing vesicles in endothelial cells. Consequently, endothelial cells fail to maintain low pH in GAS-containing autophagosomes, thereby permitting GAS replication inside LAMP-1- and LC3-positive vesicles. Furthermore, treatment of epithelial cells with bafilomycin A1 resulted in defective GAS clearance by autophagy, with subsequent bacterial growth intracellularly. Therefore, low pH is a key factor for autophagy-mediated suppression of GAS growth inside epithelial cells, while defective acidification of GAS-containing vesicles results in bacterial growth in endothelial cells.

Previous reports showed that GAS can induce autophagy and is efficiently killed within lysosome-fused autophagosomes in epithelial cells. In endothelial cells, in contrast, induction of autophagy is not sufficient for GAS killing. In this study, we provide the first evidence that low pH is required to prevent intracellular growth of GAS in epithelial cells and that this mechanism is defective in endothelial cells. Treatment of GAS with low pH altered GAS growth rate and gene expression of virulence factors and resulted in enhanced susceptibility of GAS to intracellular lysosomal killing. Our findings reveal the existence of different mechanisms of host defense against GAS invasion between epithelial and endothelial cells.

In Situ Peptide-MHC-II Tetramer Staining of Antigen-Specific CD4+ T Cells in Tissues

The invention of peptide-MHC-tetramer technology to label antigen-specific T cells has led to an enhanced understanding of T lymphocyte biology. Here we describe the development of an in situ pMHC-II tetramer staining method to visualize antigen-specific CD4⁺ T cells in tissues. This method complements other methods developed that similarly use MHC class II reagents to stain antigen-specific CD4⁺ T cells in situ. In this study, we used group A streptococcus (GAS) expressing a surrogate peptide (2W) to inoculate C57BL/6 mice, and used fresh nasal-associated lymphoid tissues (NALT) in optimizing the in situ staining of 2W:I-A^b specific CD4⁺ T cells. The results showed 2W:I-A^b tetramer-binding CD4⁺ T cells in GAS-2W but not GAS infected mice. This method holds promise to be broadly applicable to study the localization, abundance, and phenotype of antigen-specific CD4⁺ T cells in undisrupted tissues.

FbsC, a novel fibrinogen-binding protein, promotes Streptococcus agalactiae-host cell interactions

Streptococcus agalactiae (group B Streptococcus or GBS) is a common cause of invasive infections in newborn infants and adults. The ability of GBS to bind human fibrinogen is of crucial importance in promoting colonization and invasion of host barriers. We characterized here a novel fibrinogen-binding protein of GBS, designated FbsC (Gbs0791), which is encoded by the prototype GBS strain NEM316. FbsC, which bears two bacterial immunoglobulin-like tandem repeat domains and a C-terminal cell wall-anchoring motif (LPXTG), was found to be covalently linked to the cell wall by the housekeeping sortase A. Studies using recombinant FbsC indicated that it binds fibrinogen in a dose-dependent and saturable manner, and with moderate affinity. Expression of FbsC was detected in all clinical GBS isolates, except those belonging to the hypervirulent lineage ST17. Deletion of fbsC decreases NEM316 abilities to adhere to and invade human epithelial and endothelial cells, and to form biofilm in vitro. Notably, bacterial adhesion to fibrinogen and fibrinogen binding to bacterial cells were abolished following fbsC deletion in NEM316. Moreover, the virulence of the fbsC deletion mutant and its ability to colonize the brain were impaired in murine models of infection. Finally, immunization with recombinant FbsC significantly protected mice from lethal GBS challenge. In conclusion, FbsC is a novel fibrinogen-binding protein expressed by most GBS isolates that functions as a virulence factor by promoting invasion of epithelial and endothelial barriers. In addition, the protein has significant immunoprotective activity and may be a useful component of an anti-GBS vaccine.

Contribution of the highly conserved EaeH surface protein to enterotoxigenic Escherichia coli pathogenesis

Enterotoxigenic Escherichia coli (ETEC) strains are among the most common causes of diarrheal illness worldwide. These pathogens disproportionately afflict children in developing countries, where they cause substantial morbidity and are responsible for hundreds of thousands of deaths each year. Although these organisms are important targets for enteric vaccines, most development efforts to date have centered on a subset of plasmid-encoded fimbrial adhesins known as colonization factors and heat-labile toxin (LT). Emerging data suggest that ETEC undergoes considerable changes in its surface architecture, sequentially deploying a number of putative adhesins during its interactions with the host. We demonstrate here that one putative highly conserved, chromosomally encoded adhesin, EaeH, engages the surfaces of intestinal epithelial cells and contributes to bacterial adhesion, LT delivery, and colonization of the small intestine.

Differences in virulence repertoire and cell invasive potential of group A Streptococcus emm1-2 in comparison to emm1 genotype

Group A streptococcus (GAS, Streptococcus pyogenes) type emm1 is widely associated with streptococcal invasive disease. This type is prevalent worldwide but is rare in India. Instead, emm1-2 type which is closely related to emm1 but is a distinct type is more prevalent. Although emm1 has been well characterized, information available on emm1-2 is rare. In this study we present a comparative study of both types. DNA microarray analysis showed segregation of emm1 and emm1-2 isolates into two distinct clusters. Out of 229 arrayed genes, 83-87% were present, 6-9% absent and 4-8% genes were ambiguous in emm1 isolates. emm1-2 strains harboured only 68-77%, 11-13% were absent and 10-20% ambiguous genes. Fourteen genes, present in all emm1, were completely absent in the emm1-2 isolates. sfb1 is a gene which encodes for Streptococcal fibronectin binding adhesin and invasin which has restricted distribution among different emm types of GAS. A variant of sfb1 (sfb1-2) was the only gene which was present in all emm1-2 isolates, but absent from all emm1 strains. Sfb1 and Sfb1-2 differ in sequences in the aromatic domain and the proline rich repeat region, whereas the fibronectin binding region was conserved and exhibited similar fibronectin binding activity. The presence of Sfb1-2 in emm1-2 strains was concomitant with significantly higher fibronectin-binding and invasion efficiency of HEp-2 cells when compared to emm1 isolates. The role of Sfb1-2 in invasion was confirmed by latex bead assay. emm1-2 isolates follow membrane ruffling mechanism during invasion and intracellularly follow classical endocytic pathway. Further studies are required to understand the correlation between the presence of emm1-2 isolates and the disease pattern in North India.

Disease manifestations and pathogenic mechanisms of Group A Streptococcus

Streptococcus pyogenes, also known as group A Streptococcus (GAS), causes mild human infections such as pharyngitis and impetigo and serious infections such as necrotizing fasciitis and streptococcal toxic shock syndrome. Furthermore, repeated GAS infections may trigger autoimmune diseases, including acute poststreptococcal glomerulonephritis, acute rheumatic fever, and rheumatic heart disease. Combined, these diseases account for over half a million deaths per year globally. Genomic and molecular analyses have now characterized a large number of GAS virulence determinants, many of which exhibit overlap and redundancy in the processes of adhesion and colonization, innate immune resistance, and the capacity to facilitate tissue barrier degradation and spread within the human host. This improved understanding of the contribution of individual virulence determinants to the disease process has led to the formulation of models of GAS disease progression, which may lead to better treatment and intervention strategies. While GAS remains sensitive to all penicillins and cephalosporins, rising resistance to other antibiotics used in disease treatment is an increasing worldwide concern. Several GAS vaccine formulations that elicit protective immunity in animal models have shown promise in nonhuman primate and early-stage human trials. The development of a safe and efficacious commercial human vaccine for the prophylaxis of GAS disease remains a high priority.

Disease manifestations and pathogenic mechanisms of Group A Streptococcus

Streptococcus pyogenes, also known as group A Streptococcus (GAS), causes mild human infections such as pharyngitis and impetigo and serious infections such as necrotizing fasciitis and streptococcal toxic shock syndrome. Furthermore, repeated GAS infections may trigger autoimmune diseases, including acute poststreptococcal glomerulonephritis, acute rheumatic fever, and rheumatic heart disease. Combined, these diseases account for over half a million deaths per year globally. Genomic and molecular analyses have now characterized a large number of GAS virulence determinants, many of which exhibit overlap and redundancy in the processes of adhesion and colonization, innate immune resistance, and the capacity to facilitate tissue barrier degradation and spread within the human host. This improved understanding of the contribution of individual virulence determinants to the disease process has led to the formulation of models of GAS disease progression, which may lead to better treatment and intervention strategies. While GAS remains sensitive to all penicillins and cephalosporins, rising resistance to other antibiotics used in disease treatment is an increasing worldwide concern. Several GAS vaccine formulations that elicit protective immunity in animal models have shown promise in nonhuman primate and early-stage human trials. The development of a safe and efficacious commercial human vaccine for the prophylaxis of GAS disease remains a high priority.

The M1 protein of Streptococcus pyogenes triggers an innate uptake mechanism into polarized human endothelial cells

Serotype M1 Streptococcus pyogenes is a major human pathogen associated with severe invasive diseases causing high morbidity and mortality. In a substantial number of cases, invasive disease develops in previously healthy individuals with no obvious port of entry. This has led to the hypothesis that the source of streptococci in these cases is a transient bacteraemia. This study focuses on the analysis of interaction of tissue-invasive serotype M1 S. pyogenes with human endothelial cells (EC) of the vascular system. We identify the M1 surface protein of S. pyogenes as the EC invasin which is recognised by polarized human blood EC, thereby triggering rapid, phagocytosis-like uptake of streptococci into polarized EC layers. Upon internalization, the M1 S. pyogenes serotype is incorporated into phagosomes which traffic via the endosomal/lysosomal pathway. However, some of the streptococci successfully evade this innate killing process and hereby mediate their escape into the cytoplasm of the host cell. The results of this study demonstrate that blood EC possess an efficient uptake mechanism for serotype M1 S. pyogenes. Despite efficient phagocytosis, streptococcal survival within EC constitutes one potential mechanism which favours intracellular persistence and thus facilitates continuous infection and dissemination from the primary side of infection into deep tissue.

Adherence and invasion of streptococci to eukaryotic cells and their role in disease pathogenesis

Streptococcal adhesion, invasion, intracellular trafficking, dissemination, and persistence in eukaryotic cells have a variety of implications in the infection pathogenesis. While cell adhesion establishes the initial host contact, adhering bacteria exploit the host cell for their own benefit. Internalization into the host cell is an essential step for bacterial survival and subsequent dissemination and persistence, thus playing a key role in the course of infection. This chapter summarizes the current knowledge about the diverse mechanisms of streptococcal adhesion to and invasion into different eukaryotic cells and the impact on dissemination and persistence which is reflected by consequences for the pathogenesis of streptococcal infections.

Robust antigen specific th17 T cell response to group A Streptococcus is dependent on IL-6 and intranasal route of infection

Group A streptococcus (GAS, Streptococcus pyogenes) is the cause of a variety of clinical conditions, ranging from pharyngitis to autoimmune disease. Peptide-major histocompatibility complex class II (pMHCII) tetramers have recently emerged as a highly sensitive means to quantify pMHCII-specific CD4⁺ helper T cells and evaluate their contribution to both protective immunity and autoimmune complications induced by specific bacterial pathogens. In lieu of identifying an immunodominant peptide expressed by GAS, a surrogate peptide (2W) was fused to the highly expressed M1 protein on the surface of GAS to allow in-depth analysis of the CD4⁺ helper T cell response in C57BL/6 mice that express the I-A^b MHCII molecule. Following intranasal inoculation with GAS-2W, antigen-experienced 2W:I-A^b-specific CD4⁺ T cells were identified in the nasal-associated lymphoid tissue (NALT) that produced IL-17A or IL-17A and IFN-γ if infection was recurrent. The dominant Th17 response was also dependent on the intranasal route of inoculation; intravenous or subcutaneous inoculations produced primarily IFN-γ⁺ 2W:I-A^b+ CD4⁺ T cells. The acquisition of IL-17A production by 2W:I-A^b-specific T cells and the capacity of mice to survive infection depended on the innate cytokine IL-6. IL-6-deficient mice that survived infection became long-term carriers despite the presence of abundant IFN-γ-producing 2W:I-A^b-specific CD4⁺ T cells. Our results suggest that an imbalance between IL-17- and IFN-γ-producing CD4⁺ T cells could contribute to GAS carriage in humans.

Group A streptococcus (GAS) causes many different conditions, ranging from strep throat, flesh eating disease to post infectious complications involving the heart. Here, we used a novel technique to study the CD4⁺ T cell immune response against GAS infection in a mouse model. We first generated a recombinant GAS strain that expresses a specific epitope (2W) - M protein fusion and used this to intranasally inoculate mice. Peptide specific CD4⁺ T cells were concentrated and analyzed using 2W-MHC-II tetramers. This technology allowed us to probe the antigen specific CD4⁺ T cell response to new depths and certainty. Infection induced a robust 2W-specific Th17 cell response, which was dependent on the route of infection, IL-6, and was independent of superantigens. IL-6^-/- mice were exquisitely susceptible to intranasal infection. However, those that survived became immune carriers, unable to clear streptococci from NALT. Further, multiple infections generated an IL-17⁺ IFN-γ⁺ double positive population of CD4⁺ T cells that are known to be associated with autoimmune disease in humans and directly responsible for autoimmune pathology in rodent models. Our results provide a new direction for understanding two important consequences of streptococcal pharyngitis, the very common immune carrier state, and the rarer state involving autoimmune complications.

The FbaB-type fibronectin-binding protein of Streptococcus pyogenes promotes specific invasion into endothelial cells

Invasive serotype M3 Streptococcus pyogenes are among the most frequently isolated organisms from patients suffering from invasive streptococcal disease and have the potential to invade primary human endothelial cells (EC) via a rapid and efficient mechanism. FbaB protein, the fibronectin-binding protein expressed by M3 S. pyogenes, was herein identified as a potent invasin for EC. By combining heterologous gene expression with allelic replacement, we demonstrate that FbaB is essential and sufficient to trigger EC invasion via a Rac1-dependent phagocytosis-like uptake. FbaB-mediated uptake follows the classical endocytic pathway with lysosomal destination. FbaB is demonstrated to be a streptococcal invasin exhibiting EC tropism. FbaB thus initiates a process that may contribute to the deep tissue tropism and spread of invasive S. pyogenes isolates into the vascular EC lining.

Staphylococcus aureus keratinocyte invasion is dependent upon multiple high-affinity fibronectin-binding repeats within FnBPA

Staphylococcus aureus is a commensal organism and a frequent cause of skin and soft tissue infections, which can progress to serious invasive disease. This bacterium uses its fibronectin binding proteins (FnBPs) to invade host cells and it has been hypothesised that this provides a protected niche from host antimicrobial defences, allows access to deeper tissues and provides a reservoir for persistent or recurring infections. FnBPs contain multiple tandem fibronectin-binding repeats (FnBRs) which bind fibronectin with varying affinity but it is unclear what selects for this configuration. Since both colonisation and skin infection are dependent upon the interaction of S. aureus with keratinocytes we hypothesised that this might select for FnBP function and thus composition of the FnBR region. Initial experiments revealed that S. aureus attachment to keratinocytes is rapid but does not require FnBRs. By contrast, invasion of keratinocytes was dependent upon the FnBR region and occurred via similar cellular processes to those described for endothelial cells. Despite this, keratinocyte invasion was relatively inefficient and appeared to include a lag phase, most likely due to very weak expression of α₅β₁ integrins. Molecular dissection of the role of the FnBR region revealed that efficient invasion of keratinocytes was dependent on the presence of at least three high-affinity (but not low-affinity) FnBRs. Over-expression of a single high-affinity or three low-affinity repeats promoted invasion but not to the same levels as S. aureus expressing an FnBPA variant containing three high-affinity repeats. In summary, invasion of keratinocytes by S. aureus requires multiple high-affinity FnBRs within FnBPA, and given the importance of the interaction between these cell types and S. aureus for both colonisation and infection, may have provided the selective pressure for the multiple binding repeats within FnBPA.

Augmented adherence and internalization of group A Streptococcus pyogenes to influenza A virus infected MDCK cells

Respiratory tract infections are one of the leading causes of morbidity and mortality. There is considerable epidemiologic evidence that infection with respiratory viruses increases the incidence and severity of secondary bacterial complications. However, very limited number of studies were concerned with the mechanism behind such synergy. In this context, our study aimed to explore the interaction between Group A Streptococcus pyogenes (GAS) and Influenza A virus (IAV). Our results revealed that the GAS adherence and internalization into Madin-Darby canine kidney (MDCK) cells markedly increased after IAV infection. When M6 protein defective mutant of GAS was used, the virus enhanced adherence and internalization was nearly abolished indicating the involvement of M protein binding sites on the MDCK cell surface. Interestingly, the modulation of some O-linked glycolproteins as well as sialic acid, mucin and fibrinogen-like residues on the surface of MDCK cells contributed to augmented bacterial adherence and/or internalization. In the same way, qRT-PCR experiments showed an overexpression of the membrane associated mucin (MUC1) on the surface of the MDCK cells after IAV infection. Altogether, the present study revealed that IAV infection augments the adherence and internalization of GAS to MDCK cells via modulation of membrane associated O-linked glycoproteins, fibrinogen, sialic acid residues and the mucin, MUC1 on the surface of MDCK cell.

Streptolysin O inhibits clathrin-dependent internalization of group A Streptococcus

Group A Streptococcus (GAS) can be internalized by epithelial cells, including keratinocytes from human skin or pharyngeal epithelium. Internalization of GAS by epithelial cells has been postulated both to play a role in host defense and to provide a sanctuary site for GAS survival. The cholesterol-binding cytolysin streptolysin O (SLO) appears to enhance virulence in part by inhibiting GAS internalization by human keratinocytes and by disrupting the lysosomal degradation of internalized GAS. We now report that low-level production of SLO by an inducible expression system reduced GAS internalization by keratinocytes. Induced SLO expression also prevented lysosomal colocalization with intracellular bacteria and acidification of GAS-containing vacuoles. Exogenous recombinant SLO mimicked the inhibitory effect of SLO secretion on GAS entry but not that on colocalization with the lysosomal marker LAMP-1, implying that disruption of lysosomal degradation requires intracellular secretion of SLO. The internalization of SLO-negative GAS was blocked by the depletion of host cell cholesterol and by the inhibition or knocking down of the expression of clathrin or dynamin. SLO also inhibited the cellular uptake of other cargos that are internalized by clathrin-mediated uptake or by macropinocytosis. We conclude that SLO interferes with the internalization of GAS through local perturbation of the keratinocyte cell membrane and disruption of a clathrin-dependent uptake pathway.

Differences in the aromatic domain of homologous streptococcal fibronectin-binding proteins trigger different cell invasion mechanisms and survival rates

Group A streptococci (GAS, Streptococcus pyogenes) and Group G streptococci (GGS, Streptococcus dysgalactiae ssp. equisimilis) adhere to and invade host cells by binding to fibronectin. The fibronectin-binding protein SfbI from GAS acts as an invasin by using a caveolae-mediated mechanism. In the present study we have identified a fibronectin-binding protein, GfbA, from GGS, which functions as an adhesin and invasin. Although there is a high degree of similarity in the C-terminal sequence of SfbI and GfbA, the invasion mechanisms are different. Unlike caveolae-mediated invasion by SfbI-expressing GAS, the GfbA-expressing GGS isolate trigger cytoskeleton rearrangements. Heterologous expression of GfbA on the surface of a commensal Streptococcus gordonii and purified recombinant protein also triggered actin rearrangements. Expression of a truncated GfbA (lacking the aromatic domain) and chimeric GfbA/SfbI protein (replacing the aromatic domain of SfbI with the GfbA aromatic domain) on S. gordonii or recombinant proteins alone showed that the aromatic domain of GfbA is responsible for different invasion mechanisms. This is the first evidence for a biological function of the aromatic domain of fibronectin-binding proteins. Furthermore, we show that streptococci invading via cytoskeleton rearrangements and intracellular trafficking along the classical endocytic pathway are less persistence than streptococci entering via caveolae.

The structure and function of serum opacity factor: a unique streptococcal virulence determinant that targets high-density lipoproteins

Serum opacity factor (SOF) is a virulence determinant expressed by a variety of streptococcal and staphylococcal species including both human and animal pathogens. SOF derives its name from its ability to opacify serum where it targets and disrupts the structure of high-density lipoproteins resulting in formation of large lipid vesicles that cause the serum to become cloudy. SOF is a multifunctional protein and in addition to its opacification activity, it binds to a number of host proteins that mediate adhesion of streptococci to host cells, and it plays a role in resistance to phagocytosis in human blood. This article will provide an overview of the structure and function of SOF, its role in the pathogenesis of streptococcal infections, its vaccine potential, its prevalence and distribution in bacteria, and the molecular mechanism whereby SOF opacifies serum and how an understanding of this mechanism may lead to therapies for reducing high-cholesterol concentrations in blood, a major risk factor for cardiovascular disease.

Streptococcal M proteins and their role as virulence determinants

Group A streptococci (GAS, Streptococcus pyogenes) are exclusive human pathogens that have been extensively studied for many decades. The spectrum of diseases caused by these bacteria ranges from uncomplicated and superficial to severe and invasive infections. In order to give rise to these complications, GAS have evolved a number of surface-bound and secreted virulence factors, of which the M proteins are probably the best characterized. Evidence has emerged that M proteins are multifunctional pathogenic determinants, and over the years many interactions between M proteins and the human host have been reported. The present review article aims to present a state-of-the-art overview of the most important virulence mechanisms employed by M proteins to trigger disease.

Two unusual cases of severe soft tissue infection caused by Streptococcus dysgalactiae subsp. equisimilis

We present two cases of invasive infection caused by Streptococcus dysgalactiae subsp. equisimilis, one that showed rapidly developing necrotizing fasciitis in a previously healthy man and one that showed severe cellulitis and septic shock even though the bacterium possessed a mutated emm gene, predicted to encode a truncated M protein.

Streptococcus adherence and colonization

Streptococci readily colonize mucosal tissues in the nasopharynx; the respiratory, gastrointestinal, and genitourinary tracts; and the skin. Each ecological niche presents a series of challenges to successful colonization with which streptococci have to contend. Some species exist in equilibrium with their host, neither stimulating nor submitting to immune defenses mounted against them. Most are either opportunistic or true pathogens responsible for diseases such as pharyngitis, tooth decay, necrotizing fasciitis, infective endocarditis, and meningitis. Part of the success of streptococci as colonizers is attributable to the spectrum of proteins expressed on their surfaces. Adhesins enable interactions with salivary, serum, and extracellular matrix components; host cells; and other microbes. This is the essential first step to colonization, the development of complex communities, and possible invasion of host tissues. The majority of streptococcal adhesins are anchored to the cell wall via a C-terminal LPxTz motif. Other proteins may be surface anchored through N-terminal lipid modifications, while the mechanism of cell wall associations for others remains unclear. Collectively, these surface-bound proteins provide Streptococcus species with a "coat of many colors," enabling multiple intimate contacts and interplays between the bacterial cell and the host. In vitro and in vivo studies have demonstrated direct roles for many streptococcal adhesins as colonization or virulence factors, making them attractive targets for therapeutic and preventive strategies against streptococcal infections. There is, therefore, much focus on applying increasingly advanced molecular techniques to determine the precise structures and functions of these proteins, and their regulatory pathways, so that more targeted approaches can be developed.

Invasion of endothelial cells by tissue-invasive M3 type group A streptococci requires Src kinase and activation of Rac1 by a phosphatidylinositol 3-kinase-independent mechanism

Streptococcus pyogenes can cause invasive diseases in humans, such as sepsis or necrotizing fasciitis. Among the various M serotypes of group A streptococci (GAS), M3 GAS lacks the major epithelial invasins SfbI/PrtF1 and M1 protein but has a high potential to cause invasive disease. We examined the uptake of M3 GAS into human endothelial cells and identified host signaling factors required to initiate streptococcal uptake. Bacterial uptake is accompanied by local F-actin accumulation and formation of membrane protrusions at the entry site. We found that Src kinases and Rac1 but not phosphatidylinositol 3-kinases (PI3Ks) are essential to mediate S. pyogenes internalization. Pharmacological inhibition of Src activity reduced bacterial uptake and abolished the formation of membrane protrusions and actin accumulation in the vicinity of adherent streptococci. We found that Src kinases are activated in a time-dependent manner in response to M3 GAS. We also demonstrated that PI3K is dispensable for internalization of M3 streptococci and the formation of F-actin accumulations at the entry site. Furthermore, Rac1 was activated in infected cells and accumulated with F-actin in a PI3K-independent manner at bacterial entry sites. Genetic interference with Rac1 function inhibited streptococcal internalization, demonstrating an essential role of Rac1 for the uptake process of streptococci into endothelial cells. In addition, we demonstrated for the first time accumulation of the actin nucleation complex Arp2/3 at the entry port of invading M3 streptococci.

Immune evasion by Staphylococcus aureus conferred by iron-regulated surface determinant protein IsdH

The ability of Staphylococcus aureus to avoid innate immune responses including neutrophil-mediated phagocytosis is crucial for the organism to cause infection. This multifactorial process involves several secreted and cell-surface-associated proteins. In this paper we report a novel mechanism of combating neutrophils that involves iron-regulated surface determinant protein H (IsdH). The IsdH protein is part of a complex that is only expressed under iron-restricted conditions in order to bind haemoglobin and extract and transport haem into the cytoplasm. A null mutant defective in expression of IsdH, and mutants expressing variants of IsdH with substitutions in residues predicted to be involved in ligand binding, were generated from S. aureus 8325-4. The IsdH-defective mutants were shown by several measures to have reduced virulence compared with the wild-type. The mutant was engulfed more rapidly by human neutrophils in the presence of serum opsonins, survived poorly in fresh whole human blood and was less virulent in a mouse model of sepsis. The protective mechanism seems to stem from an accelerated degradation of the serum opsonin C3b.

Comparative study of 5-day cefcapene-pivoxil and 10-day amoxicillin or cefcapene-pivoxil for treatment of group A streptococcal pharyngitis in children

In order to compare the bacteriological and clinical efficacy and safety of cefcapene-pivoxil (CFPN-PI) for 5 days, CFPN-PI for 10 days, and amoxicillin (AMPC) for 10 days for the treatment of pharyngitis due to group A beta-hemolytic streptococcus (GAS) in children, a prospective multicenter randomized open-label comparative study was performed with 12 pediatric clinics in Asahikawa between June 2006 and February 2007. Two hundred and fifty children (age range 6 months to 12 years) with signs and symptoms of acute pharyngitis were enrolled. All had a positive throat culture for GAS and were fully evaluable. Eighty-two patients received CFPN-PI 9-10 mg/kg/day three times a day for 5 days, 88 received CFPN-PI three times a day for 10 days, and 80 received AMPC three times a day for 10 days. The CFPN-PI for 5 days regimen, the CFPN-PI for 10 days regimen, and the AMPC for 10 days regimen produced bacteriological eradication at the end of treatment in 93.8%, 96.2%, and 91.7% of the patients, respectively. The clinical cure rate observed at the end of therapy was 100% of the patients in the three groups. Relapse rates were 1.3% in CFPN-PI for 5 days, 4.0% in CFPN-PI for 10 days, and 2.9% in AMPC for 10 days. There were no significant differences in eradication rate, clinical cure rate, and relapse rate between the three treatment groups. The only adverse effects were infrequent diarrhea in all three groups, and a rash which occurred in 6 patients (8.0%) of the AMPC treatment group. Five days of treatment with CFPN-PI was as efficacious in bacteriological eradication and clinical response as 10 days of CFPN-PI or AMPC treatment.

Protein F1 and Streptococcus pyogenes resistance to phagocytosis

Streptococcus pyogenes is a major cause of pharyngitis in humans and encodes several fibronectin-binding proteins. M protein and protein F1 (PrtF1/SfbI) are differentially regulated by CO(2) and O(2), respectively, and both mediate the invasion of epithelial cells. This study examined whether PrtF1/SfbI shares other properties with M protein. Expression of the PrtF1/SfbI protein by an M-negative mutant conferred resistance to phagocytosis and partial inhibition of C3 deposition on the S. pyogenes surface.

Paxillin phosphorylation: bifurcation point downstream of integrin-linked kinase (ILK) in streptococcal invasion

Efficient group A streptococcus (GAS) invasion of mammalian cells requires fibronectin (Fn) binding proteins, such as M1 and PrtF1/SfbI, that bridge bacteria to integrins and activate cellular signalling for ingestion. Previous studies of GAS invasion, mediated by both proteins, suggest a common signalling pathway. However, distinct cellular morphological changes at the port of bacterial entry suggest that different signals are also induced. Here we report that paxillin is phosphorylated in response to Fn-bound GAS that express either M1 or PrtF1/SfbI protein, but is not phosphorylated in response to a mutant deficient in both proteins. Inhibition of paxillin phosphorylation by a tyrosine kinase inhibitor, PP2, or by expression of a dominant negative form of paxillin significantly reduced invasion by M1+ but did not affect ingestion of PrtF1/SfbI+ strains. In contrast, another tyrosine inhibitor, genistein, did not significantly prevent paxillin phosphorylation and had no effect on ingestion of the M1+ strain, but reduced PrtF1/SfbI-mediated entry. This suggests that paxillin phosphorylation is induced by both proteins but only required for M1-mediated invasion. A bifurcation point, downstream of integrin-linked kinase (ILK) and phosphoinositide 3-kinase, likely accounts for the distinct morphological changes. Furthermore, ILK activity is indispensable for M1-induced paxillin recruitment and phosphorylation.

Autophagy in innate immunity against intracellular bacteria

Many pathogenic bacteria can invade phagocytic and non-phagocytic cells and colonize them intracellularly, then become disseminated to other cells. The endocytic degradation pathway is thought to be the only prevention against such intracellular pathogens. Autophagy, a fundamental cellular homeostasis pathway that operates with the intracellular degradation/recycling system, causes the turnover of cellular components by delivering portions of the cytoplasm and organelles to lysosomes. Recently, we reported that autophagic degradation is a previously unrecognized effector of host innate immunity. Streptococcus pyogenes (Group A Streptococcus; GAS) successfully enters human epithelial cells via endocytosis. GAS immediately escapes from the endosomes to the cytoplasm and gains a replicative niche, after which GAS in the cytoplasm is trapped in autophagosome-like compartments and degraded upon fusion with lysosomes. This process indicates that autophagy plays a protective role in infectious diseases. We also found that autophagic degradation was induced against Staphylococcus aureus, while methicillin-resistant S. aureus were resistant to autophagic degradation. The present review focuses on the protective function of autophagy against bacterial invasion of cells.

Serum opacity factor promotes group A streptococcal epithelial cell invasion and virulence

Serum opacity factor (SOF) is a bifunctional cell surface protein expressed by 40-50% of group A streptococcal (GAS) strains comprised of a C-terminal domain that binds fibronectin and an N-terminal domain that mediates opacification of mammalian sera. The sof gene was recently discovered to be cotranscribed in a two-gene operon with a gene encoding another fibronectin-binding protein, sfbX. We compared the ability of a SOF(+) wild-type serotype M49 GAS strain and isogenic mutants lacking SOF or SfbX to invade cultured HEp-2 human pharyngeal epithelial cells. Elimination of SOF led to a significant decrease in HEp-2 intracellular invasion while loss of SfbX had minimal effect. The hypoinvasive phenotype of the SOF(-) mutant could be restored upon complementation with the sof gene on a plasmid vector, and heterologous expression of sof49 in M1 GAS or Lactococcus lactis conferred marked increases in HEp-2 cell invasion. Studies using a mutant sof49 gene lacking the fibronectin-binding domain indicated that the N-terminal opacification domain of SOF contributes to HEp-2 invasion independent of the C-terminal fibronectin binding domain, findings corroborated by observations that a purified SOF N-terminal peptide could promote latex bead adherence to HEp-2 cells and inhibit GAS invasion of HEp-2 cells in a dose-dependent manner. Finally, the first in vivo studies to employ a single gene allelic replacement mutant of SOF demonstrate that this protein contributes to GAS virulence in a murine model of necrotizing skin infection.

Adherence and internalization of Streptococcus gordonii by epithelial cells involves beta1 integrin recognition by SspA and SspB (antigen I/II family) polypeptides

Streptococcus gordonii is a commensal bacterium that colonizes the hard and soft tissues present in the human mouth and nasopharynx. The cell wall-anchored polypeptides SspA and SspB expressed by S. gordonii mediate a wide range of interactions with host proteins and other bacteria. In this article we have determined the role of SspA and SspB proteins, which are members of the streptococcal antigen I/II (AgI/II) adhesin family, in S. gordonii adherence and internalization by epithelial cells. Wild-type S. gordonii DL1 expressing AgI/II polypeptides attached to and was internalized by HEp-2 cells, whereas an isogenic AgI/II- mutant was reduced in adherence and was not internalized. Association of S. gordonii DL1 with HEp-2 cells triggered protein tyrosine phosphorylation but no significant actin rearrangement. By contrast, Streptococcus pyogenes A40 showed 50-fold higher levels of internalization and this was associated with actin polymerization and interleukin-8 upregulation. Adherence and internalization of S. gordonii by HEp-2 cells involved beta1 integrin recognition but was not fibronectin-dependent. Recombinant SspA and SspB polypeptides bound to purified human alpha5beta1 integrin through sequences present within the NAV (N-terminal) region of AgI/II polypeptide. AgI/II polypeptides blocked interactions of S. gordonii and S. pyogenes with HEp-2 cells, and S. gordonii DL1 cells expressing AgI/II proteins inhibited adherence and internalization of S. pyogenes by HEp-2 cells. Conversely, S. gordonii AgI/II- mutant cells did not inhibit internalization of S. pyogenes. The results suggest that AgI/II proteins not only promote integrin-mediated internalization of oral commensal streptococci by host cells, but also potentially influence susceptibility of host tissues to more pathogenic bacteria.

P159 is a proteolytically processed, surface adhesin of Mycoplasma hyopneumoniae: defined domains of P159 bind heparin and promote adherence to eukaryote cells

Mycoplasma hyopneumoniae, the causative agent of porcine enzootic pneumonia, colonizes the respiratory cilia of affected swine causing significant economic losses to swine production worldwide. Heparin is known to inhibit adherence of M. hyopneumoniae to porcine respiratory epithelial cilia. M. hyopneumoniae cells bind heparin but the identity of the heparin-binding proteins is limited. Proteomic analysis of M. hyopneumoniae lysates identified 27 kDa (P27), 110 kDa (P110) and 52 kDa (P52) proteins representing different regions of a 159 kDa (P159) protein derived from mhp494. These cleavage fragments were surface located and present at all growth stages. Following purification of four recombinant proteins spanning P159 (F1P159, F2P159, F3P159 and F4P159), only F3P159 and F4P159 bound heparin in a dose-dependent manner (K(d) values 142.37 +/- 22.01 nM; 75.37 +/- 7.34 nM respectively). Scanning electron microscopic studies showed M. hyopneumoniae bound intimately to porcine kidney epithelial-like cells (PK15 cells) but these processes were inhibited by excess heparin and F4P159. Similarly, latex beads coated with F2P159 and F4P159 adhered to and entered PK15 cells, but heparin, F2P159 and F4P159 was inhibitory. These findings indicate that P159 is a post-translationally cleaved, glycosaminoglycan-binding adhesin of M. hyopneumoniae.

Streptococcal modulation of cellular invasion via TGF-beta1 signaling

Group A Streptococcus (GAS) and other bacterial pathogens are known to interact with integrins as an initial step in a complex pathway of bacterial ingestion by host cells. Efficient GAS invasion depends on the interaction of bound fibronectin (Fn) with integrins and activation of integrin signaling. TGF-beta1 regulates expression of integrins, Fn, and other extracellular matrix proteins, and positively controls the integrin signaling pathway. Therefore, we postulated that TGF-beta1 levels could influence streptococcal invasion of mammalian cells. Pretreatment of HEp-2 cells with TGF-beta1 increased their capacity to ingest GAS when the bacteria expressed fibronectin-binding proteins (M1 or PrtF1). Western blots revealed significant induction of alpha5 integrin and Fn expression by HEp-2 cells in response to TGF-beta1. Increased ingestion of streptococci by these cells was blocked by a specific inhibitor of the TGF-beta1 receptor I and antibodies directed against alpha5 integrin and Fn, indicating that increased invasion depends on TGF-beta1 up-regulation of both the alpha5 integrin and Fn. The capacity of TGF-beta1 to up-regulate integrin expression and intracellular invasion by GAS was reproduced in primary human tonsil fibroblasts, which could be a source of TGF-beta1 in chronically infected tonsils. The relationship between TGF-beta1 and GAS invasion was strengthened by the observation that TGF-beta1 production was stimulated in GAS-infected primary human tonsil fibroblasts. These findings suggest a mechanism by which GAS induce a cascade of changes in mammalian tissue leading to elevated expression of the alpha5beta1 receptor, enhanced invasion, and increased opportunity for survival and persistence in their human host.

Integrin-linked kinase is an essential link between integrins and uptake of bacterial pathogens by epithelial cells

Entry of Streptococcus pyogenes or group A streptococcus (GAS) into host cells is mediated by fibronectin bound to surface proteins, M1 or PrtF1, forming a bridge to alpha5beta1 integrins. This interaction leads to cytoskeletal rearrangement and uptake of streptococci. We postulated that integrin-linked kinase (ILK), which directly associates with integrins, is the universal link between integrins and several bacterial pathogens. We showed that inhibition of ILK expression by siRNA silencing, or ILK kinase activity by chemical inhibitors or expression of a dominant negative form of ILK reduced M1-mediated invasion of epithelial cells up to 80%. To evaluate the ILK requirement for PrtF1-mediated GAS invasion, a M1-PrtF1+ recombinant strain within the M1 background was constructed. Inhibition of ILK kinase activity also significantly reduced invasion of epithelial cells by this recombinant and wild-type strain JRS4 that expresses PrtF1. In addition, impaired ILK kinase activity results in significant reduction of integrin-dependent invasion mediated by invasins of two other important pathogens, Staphylococcus aureus and Yersinia spp. This study suggests that bacterial pathogens evolved different molecules and strategies to exploit the host integrin signalling pathway for their survival.

Active and passive intranasal immunizations with streptococcal surface protein C5a peptidase prevent infection of murine nasal mucosa-associated lymphoid tissue, a functional homologue of human tonsils

C5a peptidase, also called SCPA (surface-bound C5a peptidase), is a surface-bound protein on group A streptococci (GAS), etiologic agents for a variety of human diseases including pharyngitis, impetigo, toxic shock, and necrotizing fasciitis, as well as the postinfection sequelae rheumatic fever and rheumatic heart disease. This protein is highly conserved among different serotypes and is also expressed in human isolates of group B, C, and G streptococci. Human tonsils are the primary reservoirs for GAS, maintaining endemic disease across the globe. We recently reported that GAS preferentially target nasal mucosa-associated lymphoid tissue (NALT) in mice, a tissue functionally analogous to human tonsils. Experiments using a C5a peptidase loss-of-function mutant and an intranasal infection model showed that this protease is required for efficient colonization of NALT. An effective vaccine should prevent infection of this secondary lymphoid tissue; therefore, the potential of anti-SCPA antibodies to protect against streptococcal infection of NALT was investigated. Experiments showed that GAS colonization of NALT was significantly reduced following intranasal immunization of mice with recombinant SCPA protein administered alone or with cholera toxin, whereas a high degree of GAS colonization of NALT was observed in control mice immunized with phosphate-buffered saline only. Moreover, administration of anti-SCPA serum by the intranasal route protected mice against streptococcal infection. These results suggest that intranasal immunization with SCPA would prevent colonization and infection of human tonsils, thereby eliminating potential reservoirs that maintain endemic disease.