Attribution of the Various Inhibitory Actions of the Streptococcal Inhibitor of Complement (SIC) to Regions within the Molecule

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.

Vaccine-induced, but not natural immunity, against the Streptococcal inhibitor of complement protects against invasive disease

Highly pathogenic emm1 Streptococcus pyogenes strains secrete the multidomain Streptococcal inhibitor of complement (SIC) that binds and inactivates components of the innate immune response. We aimed to determine if naturally occurring or vaccine-induced antibodies to SIC are protective against invasive S. pyogenes infection. Immunisation with full-length SIC protected mice against systemic bacterial dissemination following intranasal or intramuscular infection with emm1 S. pyogenes. Vaccine-induced rabbit anti-SIC antibodies, but not naturally occurring human anti-SIC antibodies, enhanced bacterial clearance in an ex vivo whole-blood assay. SIC vaccination of both mice and rabbits resulted in antibody recognition of all domains of SIC, whereas naturally occurring human anti-SIC antibodies recognised the proline-rich region of SIC only. We, therefore, propose a model whereby natural infection with S. pyogenes generates non-protective antibodies against the proline-rich region of SIC, while vaccination with full-length SIC permits the development of protective antibodies against all SIC domains.

Streptococcal inhibitor of complement (SIC) modulates fibrinolysis and enhances bacterial survival within fibrin clots

Some strains of the bacterial pathogen Streptococcus pyogenes secrete protein SIC (streptococcal inhibitor of complement), including strains of the clinically relevant M1 serotype. SIC neutralizes the effect of a number of antimicrobial proteins/peptides and interferes with the function of the host complement system. Previous studies have shown that some S. pyogenes proteins bind and modulate coagulation and fibrinolysis factors, raising the possibility that SIC also may interfere with the activity of these factors. Here we show that SIC interacts with both human thrombin and plasminogen, key components of coagulation and fibrinolysis. We found that during clot formation, SIC binds fibrin through its central region and that SIC inhibits fibrinolysis by interacting with plasminogen. Flow cytometry results indicated that SIC and plasminogen bind simultaneously to S. pyogenes bacteria, and fluorescence microscopy revealed co-localization of the two proteins at the bacterial surface. As a consequence, SIC-expressing bacteria entrapped in clots inhibit fibrinolysis, leading to delayed bacterial escape from the clots as compared with mutant bacteria lacking SIC. Moreover, within the clots SIC-expressing bacteria were protected against killing. In an animal model of subcutaneous infection, SIC-expressing bacteria exhibited a delayed systemic spread. These results demonstrate that the bacterial protein SIC interferes with coagulation and fibrinolysis and thereby enhances bacterial survival, a finding that has significant implications for S. pyogenes virulence.

Protein SIC Secreted from Streptococcus pyogenes Forms Complexes with Extracellular Histones That Boost Cytokine Production

Innate immunity relies on an effective recognition of the pathogenic microorganism as well as on endogenous danger signals. While bacteria in concert with their secreted virulence factors can cause a number of inflammatory reactions, danger signals released at the site of infection may in addition determine the amplitude of such responses and influence the outcome of the disease. Here, we report that protein SIC, Streptococcal Inhibitor of Complement, an abundant secreted protein from Streptococcus pyogenes, binds to extracellular histones, a group of danger signals released during necrotizing tissue damage. This interaction leads to the formation of large aggregates in vitro. Extracellular histones and SIC are abundantly expressed and seen colocalized in biopsies from patients with necrotizing soft-tissue infections caused by S. pyogenes. In addition, binding of SIC to histones neutralized their antimicrobial activity. Likewise, the ability of histones to induce hemolysis was inhibited in the presence of SIC. However, when added to whole blood, SIC was not able to block the pro-inflammatory effect of histones. Instead SIC boosted the histone-triggered release of a broad range of cytokines and chemokines, including IL-6, TNF-α, IL-8, IL-1β, IL-1ra, G-CSF, and IFN-γ. These results demonstrate that the interaction between SIC and histones has multiple effects on the host response to S. pyogenes infection.

Staphylokinase has distinct modes of interaction with antimicrobial peptides, modulating its plasminogen-activation properties

Staphylokinase (Sak) is a plasminogen activator protein that is secreted by many Staphylococcus aureus strains. Sak also offers protection by binding and inhibiting specific antimicrobial peptides (AMPs). Here, we evaluate Sak as a more general interaction partner for AMPs. Studies with melittin, mCRAMP, tritrpticin and bovine lactoferricin indicate that the truncation of the first ten residues of Sak (SakΔN10), which occurs in vivo and uncovers important residues in a bulge region, improves its affinity for AMPs. Melittin and mCRAMP have a lower affinity for SakΔN10, and in docking studies, they bind to the N-terminal segment and bulge region of SakΔN10. By comparison, lactoferricin and tritrpticin form moderately high affinity 1:1 complexes with SakΔN10 and their cationic residues form several electrostatic interactions with the protein's α-helix. Overall, our work identifies two distinct AMP binding surfaces on SakΔN10 whose occupation would lead to either inhibition or promotion of its plasminogen activating properties.

Seropositivity for Antibodies to DRS-G, a Virulence Factor from Streptococcus dysgalactiae subsp. equisimilis, Is an Independent Risk Factor for Poststreptococcus Glomerulonephritis and Chronic Kidney Disease in Mumbai, India

The disease spectrum caused by Streptococcus dysgalactiae subsp. equisimilis resembles that of S. pyogenes (group A streptococcus [GAS]). These two bacterial species are closely related and possess many common virulence characteristics. While some GAS strains express virulence factors called streptococcal inhibitor of complement (SIC) and distantly related to SIC (DRS), some S. dysgalactiae subsp. equisimilis isolates express an orthologue of DRS, which is referred to as DRS-G. We reported previously that seropositivity for either anti-SIC or anti-DRS antibodies (Abs) is associated with poststreptococcal glomerulonephritis (PSGN). However, only seropositivity for anti-SIC Abs is associated with chronic kidney disease (CKD). We now extend the study to test whether seropositivity for anti-DRS-G Abs is also associated with these renal diseases. Stored serum samples collected for our previous study were tested by an enzyme-linked immunosorbent assay (ELISA) for Abs to DRS-G. The samples represented sera from 100 CKD adult patients, 70 adult end-stage renal disease (ESRD) patients, 25 PSGN pediatric patients, and corresponding age-matched control subjects. The proportion of PSGN, CKD, and ESRD patients who showed seroreaction to anti-DRS-G Abs was significantly higher than that of the corresponding age-matched controls, who in general exhibited seropositivity rates commensurate with the isolation rate of drsG-positive S. dysgalactiae subsp. equisimilis in the community during this study period. Since higher rates of seropositivity for anti-DRS-G Abs in the renal disease categories are resultant of previous infections with DRS-G-positive S. dysgalactiae subsp. equisimilis strains, we conclude the seropositivity is an additional risk factor for these renal diseases. In this regard, anti-DRS-G Abs have attributes similar to those of the anti-SIC Abs.

DrsG from Streptococcus dysgalactiae subsp. equisimilis inhibits the antimicrobial peptide LL-37

SIC and DRS are related proteins present in only 4 of the >200 Streptococcus pyogenes emm types. These proteins inhibit complement-mediated lysis and/or the activity of certain antimicrobial peptides (AMPs). A gene encoding a homologue of these proteins, herein called DrsG, has been identified in the related bacterium Streptococcus dysgalactiae subsp. equisimilis. Here we show that geographically dispersed isolates representing 14 of 50 emm types examined possess variants of drsG. However, not all isolates within the drsG-positive emm types possess the gene. Sequence comparisons also revealed a high degree of conservation in different S. dysgalactiae subsp. equisimilis emm types. To examine the biological activity of DrsG, recombinant versions of two major DrsG variants, DrsGS and DrsGL, were expressed and purified. Western blot analysis using antisera raised to these proteins demonstrated both variants to be expressed and secreted into culture supernatants. Unlike SIC, but similar to DRS, DrsG does not inhibit complement-mediated lysis. However, like both SIC and DRS, DrsG is a ligand of the cathelicidin LL-37 and is inhibitory to its bactericidal activity in in vitro assays. Conservation of prolines in the C-terminal region also suggests that these residues are important in the biology of this family of proteins. This is the first report demonstrating the activity of an AMP-inhibitory protein in S. dysgalactiae subsp. equisimilis and suggests that inhibition of AMP activity is the primary function of this family of proteins. The acquisition of the complement-inhibitory activity of SIC may reflect its continuing evolution.

Seroprevalence of Streptococcal Inhibitor of Complement (SIC) suggests association of streptococcal infection with chronic kidney disease

BACKGROUND

Group A streptococcus (GAS) is an etiological agent for the immune mediated sequela post streptococcal glomerulonephritis (PSGN). In some populations PSGN is recognized as a risk factor for chronic kidney disease (CKD) and end-stage renal disease (ESRD). It was found that a significantly greater proportion of subjects with past history of PSGN than without the history exhibited seroreactions to streptococcal antigens called streptococcal inhibitor of complement (SIC) and to distantly related SIC (DRS). These antigens are expressed by major PSGN-associated GAS types. We therefore predicted that in populations such as India, which is endemic for streptococcal diseases and which has high prevalence of CKD and ESRD, greater proportions of CKD and ESRD patients exhibit seroreaction to SIC and DRS than healthy controls.

METHODS

To test this we conducted a SIC and DRS seroprevalence study in subjects from Mumbai area. We recruited 100 CKD, 70 ESRD and 70 healthy individuals.

RESULTS

Nineteen and 35.7% of CKD and ESRD subjects respectively were SIC antibody-positive, whereas only 7% of healthy cohort was seropositive to SIC. Furthermore, significantly greater proportion of the ESRD patients than the CKD patients is seropositive to SIC (p=0.02; odds ratio 2.37). No association was found between the renal diseases and DRS-antibody-positivity.

CONCLUSIONS

Past infection with SIC-positive GAS is a risk factor for CKD and ESRD in Mumbai population. Furthermore, SIC seropositivity is predictive of poor prognosis of CKD patients.

Molecular analysis of an outbreak of lethal postpartum sepsis caused by Streptococcus pyogenes

Sepsis is now the leading direct cause of maternal death in the United Kingdom, and Streptococcus pyogenes is the leading pathogen. We combined conventional and genomic analyses to define the duration and scale of a lethal outbreak. Two postpartum deaths caused by S. pyogenes occurred within 24 h; one was characterized by bacteremia and shock and the other by hemorrhagic pneumonia. The women gave birth within minutes of each other in the same maternity unit 2 days earlier. Seven additional infections in health care and household contacts were subsequently detected and treated. All cluster-associated S. pyogenes isolates were genotype emm1 and were initially indistinguishable from other United Kingdom emm1 isolates. Sequencing of the virulence gene sic revealed that all outbreak isolates had the same unique sic type. Genome sequencing confirmed that the cluster was caused by a unique S. pyogenes clone. Transmission between patients occurred on a single day and was associated with casual contact only. A single isolate from one patient demonstrated a sequence change in sic consistent with longer infection duration. Transmission to health care workers was traced to single clinical contacts with index cases. The last case was detected 18 days after the first case. Following enhanced surveillance, the outbreak isolate was not detected again. Mutations in bacterial regulatory genes played no detectable role in this outbreak, illustrating the intrinsic ability of emm1 S. pyogenes to spread while retaining virulence. This fast-moving outbreak highlights the potential of S. pyogenes to cause a range of diseases in the puerperium with rapid transmission, underlining the importance of immediate recognition and response by clinical infection and occupational health teams.

The lysozyme-induced peptidoglycan N-acetylglucosamine deacetylase PgdA (EF1843) is required for Enterococcus faecalis virulence

Lysozyme is a key component of the innate immune response in humans that provides a first line of defense against microbes. The bactericidal effect of lysozyme relies both on the cell wall lytic activity of this enzyme and on a cationic antimicrobial peptide activity that leads to membrane permeabilization. Among Gram-positive bacteria, the opportunistic pathogen Enterococcus faecalis has been shown to be extremely resistant to lysozyme. This unusual resistance is explained partly by peptidoglycan O-acetylation, which inhibits the enzymatic activity of lysozyme, and partly by d-alanylation of teichoic acids, which is likely to inhibit binding of lysozyme to the bacterial cell wall. Surprisingly, combined mutations abolishing both peptidoglycan O-acetylation and teichoic acid alanylation are not sufficient to confer lysozyme susceptibility. In this work, we identify another mechanism involved in E. faecalis lysozyme resistance. We show that exposure to lysozyme triggers the expression of EF1843, a protein that is not detected under normal growth conditions. Analysis of peptidoglycan structure from strains with EF1843 loss- and gain-of-function mutations, together with in vitro assays using recombinant protein, showed that EF1843 is a peptidoglycan N-acetylglucosamine deacetylase. EF1843-mediated peptidoglycan deacetylation was shown to contribute to lysozyme resistance by inhibiting both lysozyme enzymatic activity and, to a lesser extent, lysozyme cationic antimicrobial activity. Finally, EF1843 mutation was shown to reduce the ability of E. faecalis to cause lethality in the Galleria mellonella infection model. Taken together, our results reveal that peptidoglycan deacetylation is a component of the arsenal that enables E. faecalis to thrive inside mammalian hosts, as both a commensal and a pathogen.

Streptococcal inhibitor of complement-mediated lysis (SIC): an anti-inflammatory virulence determinant

Since the late 1980s, a worldwide increase of severe Streptococcus pyogenes infections has been associated with strains of the M1 serotype, strains which all secrete the streptococcal inhibitor of complement-mediated lysis (SIC). Previous work has shown that SIC blocks complement-mediated haemolysis, inhibits the activity of antibacterial peptides and has affinity for the human plasma proteins clusterin and histidine-rich glycoprotein; the latter is a member of the cystatin protein family. The present work demonstrates that SIC binds to cystatin C, high-molecular-mass kininogen (HK) and low-molecular-mass kininogen, which are additional members of this protein family. The binding sites in HK are located in the cystatin-like domain D3 and the endothelial cell-binding domain D5. Immobilization of HK to cellular structures plays a central role in activation of the human contact system. SIC was found to inhibit the binding of HK to endothelial cells, and to reduce contact activation as measured by prolonged blood clotting time and impaired release of bradykinin. These results suggest that SIC modifies host defence systems, which may contribute to the virulence of S. pyogenes strains of the M1 serotype.

The extracytoplasmic function sigma factor SigV plays a key role in the original model of lysozyme resistance and virulence of Enterococcus faecalis

Background

Enterococcus faecalis is one of the leading agents of nosocomial infections. To cause diseases, pathogens or opportunistic bacteria have to adapt and survive to the defense systems encountered in the host. One of the most important compounds of the host innate defense response against invading microorganisms is lysozyme. It is found in a wide variety of body fluids, as well as in cells of the innate immune system. Lysozyme could act either as a muramidase and/or as a cationic antimicrobial peptide. Like Staphylococcus aureus, E. faecalis is one of the few bacteria that are completely lysozyme resistant.

Results

This study revealed that oatA (O-acetyl transferase) and dlt (D-Alanylation of lipoteicoic acids) genes contribute only partly to the lysozyme resistance of E. faecalis and that a specific transcriptional regulator, the extracytoplasmic function SigV sigma factor plays a key role in this event. Indeed, the sigV single mutant is as sensitive as the oatA/dltA double mutant, and the sigV/oatA/dltA triple mutant displays the highest level of lysozyme sensitivity suggesting synergistic effects of these genes. In S. aureus, mutation of both oatA and dlt genes abolishes completely the lysozyme resistance, whereas this is not the case in E. faecalis. Interestingly SigV does not control neither oatA nor dlt genes. Moreover, the sigV mutants clearly showed a reduced capacity to colonize host tissues, as they are significantly less recovered than the parental JH2-2 strain from organs of mice subjected to intravenous or urinary tract infections.

Conclusions

This work led to the discovery of an original model of lysozyme resistance mechanism which is obviously more complex than those described for other Gram positive pathogens. Moreover, our data provide evidences for a direct link between lysozyme resistance and virulence of E. faecalis.

The 4.5S RNA component of the signal recognition particle is required for group A Streptococcus virulence

The signal recognition particle (SRP) is a ribonucleoprotein complex that targets proteins for secretion in a co-translational manner. While originally thought to be essential in all bacteria, recent data show that the SRP is dispensable in at least some streptococcal species. The SRP from the human pathogen group A Streptococcus (GAS, Streptococcus pyogenes) is predicted to be composed of protein Ffh and 4.5S RNA. Deletion of ffh alters the secretion of several GAS proteins, and leads to a severe reduction in virulence. Here, we report that mutation of the gene encoding 4.5S RNA results in phenotypes both similar to and distinct from that observed following ffh mutation. Similarities include a reduction in secretion of the haemolysin streptolysin O, and attenuation of virulence as assessed by a murine soft tissue infection model. Differences include a reduction in transcript levels for the genes encoding streptolysin O and NAD-glycohydrolase, and the reduced secretion of the SpeB protease. Several differences in transcript abundance between the parental and mutant strain were shown to be dependent on the sensor-kinase-encoding gene covS. Using growth in human saliva as an ex vivo model of upper respiratory tract infection we identified that 4.5S RNA mutation leads to a 10-fold reduction in colony-forming units over time, consistent with the 4.5S RNA contributing to GAS growth and persistence during upper respiratory tract infections. Finally, we determined that the 4.5S RNA was essential for GAS to cause lethal infections in a murine bacteraemia model of infection. The data presented extend our knowledge of the contribution of the SRP to the virulence of an important Gram-positive pathogen.

Seroreactivity against streptococcal DRS (distantly related to SIC) protein is a predictor for end-stage renal failure

We hypothesized that immunoreactivity against antigens from nephritic strains of Streptococcus pyogenes may be elevated in patients with end-stage renal failure (ESRF). Additionally, we investigated whether a difference in seroreactivity exists between nonindigenous and indigenous (Aboriginal/Torres Strait Islander) patients. To examine these possibilities, antibodies against potentially nephritogenic proteins, streptokinase (Ska1) (from M1), streptococcal pyrogenic exotoxin type B (SpeB) (from M1), the streptococcal inhibitor of complement-mediated cell lysis (SIC) (from M1) and its two variants, closely related to SIC (CRS) (from M57) and distantly related to SIC (DRS) (from M12) were determined in 66 patients and 31 healthy controls by enzyme-linked immunosorbent assays. A significantly higher proportion of patients compared to controls were seropositive to Ska1 (P = 0.004), DRS (P = 0.0003), CRS (P = 0.001), and SIC (P = 0.018). Regression analysis showed that seroreactivity to DRS (r(2) = 0.85, P = 0.001) predicted the development of ESRF and that being diabetic was positively associated with being an ESRF patient (r(2) = 0.37, P < 0.0001) and being indigenous (r(2) = 0.47, P < 0.0001). These results suggest that these ESRF patients were exposed to strains of S. pyogenes that secrete Ska1, DRS, CRS, and SIC and may have pathological significance. No significant difference was observed between the indigenous patients and nonindigenous patients.

Enterococcus faecalis constitutes an unusual bacterial model in lysozyme resistance

Lysozyme is an important and widespread compound of the host constitutive defense system, and it is assumed that Enterococcus faecalis is one of the few bacteria that are almost completely lysozyme resistant. On the basis of the sequence analysis of the whole genome of E. faecalis V583 strain, we identified two genes that are potentially involved in lysozyme resistance, EF_0783 and EF_1843. Protein products of these two genes share significant homology with Staphylococcus aureus peptidoglycan O-acetyltransferase (OatA) and Streptococcus pneumoniae N-acetylglucosamine deacetylase (PgdA), respectively. In order to determine whether EF_0783 and EF_1843 are involved in lysozyme resistance, we constructed their corresponding mutants and a double mutant. The DeltaEF_0783 mutant and DeltaEF_0783 DeltaEF_1843 double mutant were shown to be more sensitive to lysozyme than the parental E. faecalis JH2-2 strain and DeltaEF_1843 mutant were. However, compared to other bacteria, such as Listeria monocytogenes or S. pneumoniae, the tolerance of DeltaEF_0783 and DeltaEF_0783 DeltaEF_1843 mutants towards lysozyme remains very high. Peptidoglycan structure analysis showed that EF_0783 modifies the peptidoglycan by O acetylation of N-acetyl muramic acid, while the EF_1843 deletion has no obvious effect on peptidoglycan structure under the same conditions. Moreover, the EF_0783 and EF_1843 deletions seem to significantly affect the ability of E. faecalis to survive within murine macrophages. In all, while EF_0783 is currently involved in the lysozyme resistance of E. faecalis, peptidoglycan O acetylation and de-N-acetylation are not the main mechanisms conferring high levels of lysozyme resistance to E. faecalis.

Defensins in the immunology of bacterial infections

Defensins are a component of the host response against bacterial infections. Multiple studies suggest a linked upregulation of beta-defensins and pro-inflammatory cytokines expression in various tissues, as well as the possibility of mutual induction. Recent data demonstrate the importance of nucleotide-binding oligomerization proteins for the expression of defensins, and associate low levels of alpha-defensins expression by intestinal Paneth cells with susceptibility to Crohn's disease of the ileum. A novel anti-toxin activity has been identified for several alpha- and theta-defensins, expanding the repertoire of the antimicrobial functions of defensins. It has been shown that bacterial proteins can inactivate the action of defensins, and that pathogen type III secretion systems (T3SS) manipulate defensins expression via T3SS-mediated inhibition of the NF-kappaB pathway.

Streptococcal DRS (distantly related to SIC) and SIC inhibit antimicrobial peptides, components of mucosal innate immunity: a comparison of their activities

"Streptococcal inhibitor of complement" (SIC) and "distantly related to SIC" (DRS) are related virulence factors secreted by M1 and M12 strains of GAS, respectively. The human mucosal innate immune system, important components of which are beta-defensins, secretory leukocyte proteinase inhibitor (SLPI) and lysozyme, provides the first line of defence against microorganisms. We report the interaction between DRS and these proteins; further investigations into the interaction of SIC with the beta-defensins; and compare the sensitivity of M12 and M1 GAS to SLPI. We show that SLPI, which kills M1 GAS and is inhibited by SIC, cannot kill M12 GAS. DRS cannot inhibit SLPI killing of M1 GAS, although ELISA shows binding of DRS to SLPI. We suggest that the target for SLPI on M1 GAS resembles SIC, and soluble SIC inhibits by acting as a decoy for SLPI. M12 GAS may not have this target and cannot interact with SLPI. DRS inhibits the antibacterial action of hBD-2 and hBD-3. Binding of both SIC and DRS to hBD-2, and DRS to hBD-3, shows small positive enthalpy, suggesting that binding is largely hydrophobic. The data for SIC and hBD-3 indicate that this is not a homogeneous bimolecular interaction. We conclude that DRS shares several of the properties of SIC, and therefore can be considered an important virulence factor of M12 GAS and an aid to colonization of the host mucosae.

DRS is far less divergent than streptococcal inhibitor of complement of group A streptococcus

When 100 group A streptococcus isolates were screened, drs, a variant of sic, was identified in emm12 and emm55 isolates. Molecular characterization showed that the drs gene sequence is highly conserved, unlike the sic gene sequence. However, the variation in gene size observed was due to the presence of extra internal repeat sequences.

Genetic characterization and virulence role of the RALP3/LSA locus upstream of the streptolysin s operon in invasive M1T1 Group A Streptococcus

Group A Streptococcus (GAS) is a leading human pathogen associated with a wide spectrum of mucosal and invasive infections. GAS expresses a large number of virulence determinants whose expression is under the control of several transcriptional regulatory networks. Here we performed the first mutational analysis of a genetic locus immediately upstream of the streptolysin S biosynthetic operon in several GAS genome sequences, including that of the M1T1 serotype, the leading isolates associated with serious invasive disease. The locus consists of a predicted RofA-like stand-alone transcriptional regulator (RALP3) and the largest open reading frame in the GAS genome, encoding a predicted LPXSG motif cell wall-anchored protein we have named LSA (for "large surface-anchored" protein). Comparative reverse transcription-PCR analysis of wild-type M1T1 GAS and an isogenic RALP3-deficient mutant identifies RALP3 as a global transcriptional regulator affecting expression of numerous virulence factor genes, including those for strong repression of the hyaluronic acid capsule and cysteine protease production. RALP3 contributed to GAS epithelial cell invasion and bloodstream survival. LSA was found to be under negative regulation by RALP3 and to influence GAS-epithelial cell interactions and GAS antimicrobial peptide sensitivity. Isogenic M1T1 GAS mutants lacking either RALP3 or LSA were attenuated in a murine model of systemic infection, indicating that this locus plays a role in the virulence potential of the organism.

Cell wall substrate specificity of six different lysozymes and lysozyme inhibitory activity of bacterial extracts

We have investigated the specificity of six different lysozymes for peptidoglycan substrates obtained by extraction of a number of gram-negative bacteria and Micrococcus lysodeikticus with chloroform/Tris-HCl buffer (chloroform/buffer). The lysozymes included two that are commercially available (hen egg white lysozyme or HEWL, and mutanolysin from Streptomyces globisporus or M1L), and four that were chromatographically purified (bacteriophage lambda lysozyme or LaL, bacteriophage T4 lysozyme or T4L, goose egg white lysozyme or GEWL, and cauliflower lysozyme or CFL). HEWL was much more effective on M. lysodeikticus than on any of the gram-negative cell walls, while the opposite was found for LaL. Also the gram-negative cell walls showed remarkable differences in susceptibility to the different lysozymes, even for closely related species like Escherichia coli and Salmonella Typhimurium. These differences could not be due to the presence of lysozyme inhibitors such as Ivy from E. coli in the cell wall substrates because we showed that chloroform extraction effectively removed this inhibitor. Interestingly, we found strong inhibitory activity to HEWL in the chloroform/buffer extracts of Salmonella Typhimurium, and to LaL in the extracts of Pseudomonas aeruginosa, suggesting that other lysozyme inhibitors than Ivy exist and are probably widespread in gram-negative bacteria.

The human beta-defensin-3, an antibacterial peptide with multiple biological functions

A group of interesting molecules called defensins exhibit multiple functions but have been primarily recognized to possess a broad spectrum of antimicrobial activities. Studies have reported two different types of defensins (alpha and beta) from human and animals, a cyclic theta defensin from rhesus, and several defensin-like peptides from plants. There is no amino acid sequence homology between these peptides, but they all contain three Cys-Cys disulfide linkages while the connectivities are different. Human beta-defensin-3 (HbetaD-3) is the most recently discovered member of the host-defense peptide family that has attracted much attention. This molecule is expressed either constitutively or induced upon a challenge, and a growing evidence indicates the involvement of such molecules in adaptive immunity as well. It has been shown to exhibit antibacterial activities towards Gram-negative and Gram-positive bacteria as well as an ability to act as a chemo-attractant. Analysis of NMR structural data suggested a symmetrical dimeric form of this peptide in solution, which consists of three beta strands and a short helix in the N-terminal region. While the disulfide linkages are known to provide the structural stability and stability against proteases, the biological relevance of this dimeric form was contradicted by another biological study. Since there is considerable current interest in developing HbetaD-3 for possible pharmaceutical applications, studies to further our understanding on the determinants of antibacterial activities and immunomodulatory function of HbetaD-3 are considered to be highly significant. The knowledge of its biosynthetic regulation will also help in understanding the role of HbetaD-3 in immunity. This article presents an overview of the expression and regulation of HbetaD-3 in humans, and the structure-function correlations among HbetaD-3 and its modified peptides are discussed emphasizing the functional importance. The future scope for studies on HbetaD-3 and design of short potent antimicrobial peptides, based on the native HbetaD-3 molecule, that do not interfere in the immunomodulatory function is also outlined.

Inhibition of antimicrobial peptides by group A streptococci: SIC and DRS

SIC (streptococcal inhibitor of complement) is a 31 kDa protein secreted by a few highly virulent strains of GAS (group A streptococci), predominantly by the M1 strain. Initially described as an inhibitor of the membrane attack complex of complement, it has turned out to be a polyfunctional inhibitor of the innate mucosal immune response. The SIC protein sequence contains three domains: an N-terminal SRR (short repeat region), followed by three longer tandem repeats [LRR (long repeat region)] and a C-terminal PRR (proline-rich region). SIC inhibits the antibacterial activity of a wide range of antimicrobial peptides and proteins: i.e. lysozyme, SLPI (secretory leucocyte proteinase inhibitor), LL-37, hNP-1 (human neutrophil peptide-1) and the human β-defensins 1, 2 and 3. Analysis of the functional properties of recombinant domains of SIC shows that binding and inhibition of lysozyme and human β-defensin-3 require the SRR+LRR, as does binding to SLPI. Complement inhibition is confined to the SRR. M12 GAS secrete a protein ‘distantly related to SIC’ (DRS). DRS contains a C-terminal PRR which is significantly similar to that of SIC, but it has no central LRR and the N-terminal SRR is very different. DRS inhibits human β-defensin-3, but has no effect on lysozyme, SLPI or complement.