Extracellular deoxyribonuclease made by group A Streptococcus assists pathogenesis by enhancing evasion of the innate immune response

How do microbes evade neutrophil killing?

Many microbial pathogens evolved to circumvent the attack of neutrophils, which are essential effector cells of the innate immune system. Here we review six major strategies that pathogenic bacteria and fungi use to evade neutrophil defences: (i) turning on survival and stress responses, (ii) avoiding contact, (iii) preventing phagocytosis, (iv) surviving intracellularly, (v) inducing cell death and (vi) evading killing by neutrophil extracellular traps. For each category we give examples and further focus on one particular pathogenic microbe in more detail. Pathogens include Candida albicans, Cryptococcus neoformans, Yersinia ssp., Helicobacter pylori, Staphylococcus aureus, Streptococcus pyogenes and Streptococcus pneumoniae.

Emergence of a bacterial clone with enhanced virulence by acquisition of a phage encoding a secreted phospholipase A2

The molecular basis of pathogen clone emergence is relatively poorly understood. Acquisition of a bacteriophage encoding a previously unknown secreted phospholipase A(2) (designated SlaA) has been implicated in the rapid emergence in the mid-1980s of a new hypervirulent clone of serotype M3 group A Streptococcus. Although several lines of circumstantial evidence suggest that SlaA is a virulence factor, this issue has not been addressed experimentally. We found that an isogenic DeltaslaA mutant strain was significantly impaired in ability to adhere to and kill human epithelial cells compared with the wild-type parental strain. The mutant strain was less virulent for mice than the wild-type strain, and immunization with purified SlaA significantly protected mice from invasive disease. Importantly, the mutant strain was significantly attenuated for colonization in a monkey model of pharyngitis. We conclude that transductional acquisition of the ability of a GAS strain to produce SlaA enhanced the spread and virulence of the serotype M3 precursor strain. Hence, these studies identified a crucial molecular event underlying the evolution, rapid emergence, and widespread dissemination of unusually severe human infections caused by a distinct bacterial clone.

Analysis of the transcriptome of group A Streptococcus in mouse soft tissue infection

Molecular mechanisms mediating group A Streptococcus (GAS)-host interactions remain poorly understood but are crucial for diagnostic, therapeutic, and vaccine development. An optimized high-density microarray was used to analyze the transcriptome of GAS during experimental mouse soft tissue infection. The transcriptome of a wild-type serotype M1 GAS strain and an isogenic transcriptional regulator knockout mutant (covR) also were compared. Array datasets were verified by quantitative real-time reverse transcriptase-polymerase chain reaction and in situ immunohistochemistry. The results unambiguously demonstrate that coordinated expression of proven and putative GAS virulence factors is directed toward overwhelming innate host defenses leading to severe cellular damage. We also identified adaptive metabolic responses triggered by nutrient signals and hypoxic/acidic conditions in the host, likely facilitating pathogen persistence and proliferation in soft tissues. Key discoveries included that oxidative stress genes, virulence genes, genes related to amino acid and maltodextrin utilization, and several two-component transcriptional regulators were highly expressed in vivo. This study is the first global analysis of the GAS transcriptome during invasive infection. Coupled with parallel analysis of the covR mutant strain, novel insights have been made into the regulation of GAS virulence in vivo, resulting in new avenues for targeted therapeutic and vaccine research.

Maltodextrin utilization plays a key role in the ability of group A Streptococcus to colonize the oropharynx

Analysis of multiple group A Streptococcus (GAS) genomes shows that genes encoding proteins involved in carbohydrate utilization comprise some 15% of the core GAS genome. Yet there is a limited understanding of how carbohydrate utilization contributes to GAS pathogenesis. Previous genome-wide GAS studies led us to a focused investigation of MalE, a putative maltodextrin-binding protein. Analysis of 28 strains of 22 distinct M protein serotypes showed that MalE is highly conserved among diverse GAS strains. malE transcript levels were significantly increased during growth in human saliva compared to growth in a chemically defined glucose-containing medium or a nutrient-rich medium. MalE was accessible to antibody binding, indicating that it is expressed on the GAS cell surface. Moreover, growth in human saliva appeared to increase MalE surface expression compared to growth in a nutrient-rich medium. Analysis of a delta malE isogenic mutant strain revealed decreased growth in human saliva compared to wild-type GAS. Radiolabeled carbohydrate binding assays showed that MalE was required for the binding of maltose but not glucose. The delta malE isogenic mutant strain colonized a lower percentage of GAS-challenged mice compared to wild-type and genetically complemented strains. Furthermore, decreased numbers of CFU were recovered from mice infected with the delta malE strain compared to those infected with wild-type GAS. These data demonstrate that maltodextrin acquisition is likely to be a key factor in the ability of GAS to successfully infect the oropharynx. Further investigation into carbohydrate transport and metabolism pathways may yield novel insights into GAS pathogenesis.

Neutrophil extracellular trap formation by bovine neutrophils is not inhibited by milk

Neutrophils are the first line of defense in a mammary gland infection. However, the process of neutrophil transmigration across a membrane and ingestion of fat and/or casein when incubated in milk have been shown to inhibit bacterial phagocytosis and oxidative burst functions. Recently, a killing mechanism has been described whereby stimulated neutrophils release nuclear and granule material in fibrous webs that physically trap and kill bacteria. We demonstrate that these neutrophil extracellular traps are also produced by bovine blood neutrophils stimulated with PMA/ionomycin. Importantly, neutrophil extracellular traps can be formed when neutrophils have been incubated for up to 6h in milk prior to stimulation. This contrasts milk's rapid inhibition of bacterial phagocytosis and oxidative burst functions in the neutrophil. Furthermore, stimulation of neutrophils with bacteria common to mammary gland infections leads to neutrophil extracellular traps being formed in milk. Some bacteria tested stimulated enhanced formation of neutrophil extracellular traps in milk compared to culture media. Therefore, being unaffected by incubation in milk may indicate an important role for neutrophil extracellular traps in defense against mastitis.

An endonuclease allows Streptococcus pneumoniae to escape from neutrophil extracellular traps

Streptococcus pneumoniae (pneumococcus) is the most common cause of community-acquired pneumonia, with high morbidity and mortality worldwide. A major feature of pneumococcal pneumonia is an abundant neutrophil infiltration . It was recently shown that activated neutrophils release neutrophil extracellular traps (NETs), which contain antimicrobial proteins bound to a DNA scaffold. NETs provide a high local concentration of antimicrobial components and bind, disarm, and kill microbes extracellularly. Here, we show that pneumococci are trapped but, unlike many other pathogens, not killed by NETs. NET trapping in the lungs, however, may allow the host to confine the infection, reducing the likelihood for the pathogen to spread into the bloodstream. DNases are expressed by many Gram-positive bacterial pathogens, but their role in virulence is not clear. Expression of a surface endonuclease encoded by endA is a common feature of many pneumococcal strains. We show that EndA allows pneumococci to degrade the DNA scaffold of NETs and escape. Furthermore, we demonstrate that escaping NETs promotes spreading of pneumococci from the upper airways to the lungs and from the lungs into the bloodstream during pneumonia.

DNase expression allows the pathogen group A Streptococcus to escape killing in neutrophil extracellular traps

The innate immune response plays a crucial role in satisfactory host resolution of bacterial infection. In response to chemotactic signals, neutrophils are early responding cells that migrate in large numbers to sites of infection. The recent discovery of secreted neutrophil extracellular traps (NETs) composed of DNA and histones opened a novel dimension in our understanding of the microbial killing capacity of these specialized leukocytes. M1 serotype strains of the pathogen Group A Streptococcus (GAS) are associated with invasive infections including necrotizing fasciitis (NF) and express a potent DNase (Sda1). Here we apply a molecular genetic approach of allelic replacement mutagenesis, single gene complementation, and heterologous expression to demonstrate that DNase Sda1 is both necessary and sufficient to promote GAS neutrophil resistance and virulence in a murine model of NF. Live fluorescent microscopic cell imaging and histopathological analysis are used to establish for the first time a direct linkage between NET degradation and bacterial pathogenicity. Inhibition of GAS DNase activity with G-actin enhanced neutrophil clearance of the pathogen in vitro and reduced virulence in vivo. The results demonstrate a significant role for NETs in neutrophil-mediated innate immunity, and at the same time identify a novel therapeutic target against invasive GAS infection.

Group A streptococcal infections of the skin: molecular advances but limited therapeutic progress

PURPOSE OF REVIEW

With the sequencing of several Streptococcus pyogenes (group A Streptococcus) genomes have come major advances in understanding the pathogenesis of group A Streptococcus-associated diseases. This review focuses on group A Streptococcus skin infections and summarizes data published in the English language medical literature in 2004 and 2005.

RECENT FINDINGS

Group A Streptococcus shows enormous and evolving molecular diversity driven by horizontal transmission between group A Streptococcus strains and between group A Streptococcus and other streptococci. Acquisition of prophages accounts for much of the diversity, conferring both virulence through phage-associated virulence factors and increased bacterial survival against host defences. Studies of group A Streptococcus isolates outside the US also question the generalizability of classic group A Streptococcus M serotype associations with specific disease entities such as acute rheumatic fever and necrotizing fasciitis. The distinction between throat and skin group A Streptococcus has become blurred. Although there have been few advances in treatment of group A Streptococcus skin infections, developments towards group A Streptococcus vaccines are promising.

SUMMARY

The diversity of group A Streptococcus remains a challenge for vaccine development. As acute rheumatic fever and streptococcal pyoderma occur predominantly in disadvantaged populations, international funding support will be necessary for any group A Streptococcus vaccine to have a sustained impact on the global burden of disease.

Analysis of the role of CovR and CovS in the dissemination of Streptococcus pyogenes in invasive skin disease

The global regulatory two-component system CovR/S controls expression of about 15% of the Streptococcus pyogenes (group A streptococcus; GAS) genome. Recently, we found that CovS plays a pivotal role in general stress response of this strictly human pathogen. Therefore, we expected that both CovS and CovR might affect virulence. In this work, mice were inoculated subcutaneously with isogenic nonpolar covR and covS deletion-substitution mutants and the isogenic wild-type strain. The covS mutant behaved like the wild-type parental strain in terms of resulting lesion appearance and invasive disease leading to death. This is in agreement with previous results suggesting that the absence of its cognate sensor kinase does not affect the ability of CovR to become phosphorylated and cause repression of its regulon. However, two different covR deletion-substitution mutants caused significantly less invasive disease and death in the mice than the wild-type parental strain, although the local lesions produced by the covR mutants were more severe and purulent than those resulting from the wild-type GAS strain. Thus, it appears that production of CovR increases the ability of S. pyogenes to cause severe invasive disease in this mouse model and therefore is an important virulence factor for this organism.

Expression microarray and mouse virulence analysis of four conserved two-component gene regulatory systems in group a streptococcus

Group A streptococcus (GAS) is a gram-positive human bacterial pathogen that causes diseases ranging from relatively mild epithelial cell surface infections to life-threatening invasive episodes. Much is known about the extracellular molecules that contribute to host-pathogen interactions, but in contrast, far less information is available about regulatory genes that control the expression of individual or multiple GAS virulence factors. The eight GAS genomes that have been sequenced have 12 conserved two-component gene regulatory systems (TCSs), but only 3 of these 12 have been studied in detail. Using an allelic replacement strategy with a nonpolar cassette, we inactivated the response regulator of four TCSs that have only weak homology with TCS genes of known or inferred function in other bacteria. The mutant strains were analyzed by expression microarray analysis at four time points and tested in two mouse infection models. Each TCS influenced expression (directly or indirectly) of 12 to 41% of all chromosomal genes, as assessed by growth in Todd-Hewitt broth and a custom Affymetrix GeneChip. None of the isogenic mutant strains was significantly altered for mouse virulence based on intraperitoneal inoculation. Similarly, compared to the wild-type strain, there was no significant difference in skin lesion size for three of the four mutants. In contrast, the DeltaM5005_Spy_0680 mutant strain produced significantly larger abscesses after subcutaneous inoculation into mice, consistent with a hypervirulence phenotype. The mutant strain had significantly higher in vitro expression of several proven and putative virulence genes, including scpA, encoding a peptidase that inactivates complement protein C5a. Together, the data provide new information about previously uncharacterized GAS TCSs.

Genome-wide analysis of group a streptococci reveals a mutation that modulates global phenotype and disease specificity

Many human pathogens produce phenotypic variants as a means to circumvent the host immune system and enhance survival and, as a potential consequence, exhibit increased virulence. For example, it has been known for almost 90 y that clinical isolates of the human bacterial pathogen group A streptococci (GAS) have extensive phenotypic heterogeneity linked to variation in virulence. However, the complete underlying molecular mechanism(s) have not been defined. Expression microarray analysis of nine clinical isolates identified two fundamentally different transcriptomes, designated pharyngeal transcriptome profile (PTP) and invasive transcriptome profile (ITP). PTP and ITP GAS differed in approximately 10% of the transcriptome, including at least 23 proven or putative virulence factor genes. ITP organisms were recovered from skin lesions of mice infected subcutaneously with PTP GAS and were significantly more able to survive phagocytosis and killing by human polymorphonuclear leukocytes. Complete genome resequencing of a mouse-derived ITP GAS revealed that the organism differed from its precursor by only a 7-bp frameshift mutation in the gene (covS) encoding the sensor kinase component of a two-component signal transduction system implicated in virulence. Genetic complementation, and sequence analysis of covR/S in 42 GAS isolates confirmed the central role of covR/S in transcriptome, exoproteome, and virulence modulation. Genome-wide analysis provides a heretofore unattained understanding of phenotypic variation and disease specificity in microbial pathogens, resulting in new avenues for vaccine and therapeutics research.

Phenotypic heterogeneity within an infecting population is a strategy commonly used by bacterial pathogens to evade the host immune system and enhance survival. Such phenotypic variation has been observed for the human pathogen group A streptococci (GAS), which can cause a wide range of diseases with differing severity. However, the underlying mechanisms that control this variation, and the survival- and virulence-associated effects of this variation, have not been fully elucidated.

By assaying total gene expression the authors found that clinical GAS isolates from invasive and pharyngeal diseases had distinct gene expression patterns during growth in standard laboratory media. These two gene expression patterns conferred distinct virulence-associated attributes on the expressing GAS strain, as assessed using bacteremia and soft-tissue infection models of disease. Likewise, the ability to survive the bactericidal activity of human neutrophils was significantly different between GAS strains with the two distinct expression patterns. Transition from one gene expression pattern to the other required the mutation of the two-component signal transduction system CovRS (control of virulence R/S). The authors conclude that the ability of GAS to remodel its transcriptome plays a major contribution in its ability to colonize distinct niches of the human body and cause disease.

Central role of a bacterial two-component gene regulatory system of previously unknown function in pathogen persistence in human saliva

The molecular genetic mechanisms used by bacteria to persist in humans are poorly understood. Group A Streptococcus (GAS) causes the majority of bacterial pharyngitis cases in humans and is prone to persistently inhabit the upper respiratory tract. To gain information about how GAS survives in and infects the oropharynx, we analyzed the transcriptome of a serotype M1 strain grown in saliva. The dynamic pattern of changes in transcripts of genes [spy0874/0875, herein named sptR and sptS (sptR/S), for saliva persistence] encoding a two-component gene regulatory system of unknown function suggested that SptR/S contributed to persistence of GAS in saliva. Consistent with this idea, an isogenic nonpolar mutant strain (DeltasptR) was dramatically less able to survive in saliva compared with the parental strain. Iterative expression microarray analysis of bacteria grown in saliva revealed that transcripts of several known and putative GAS virulence factor genes were decreased significantly in the DeltasptR mutant strain. Compared with the parental strain, the isogenic mutant strain also had altered transcripts of multiple genes encoding proteins involved in complex carbohydrate acquisition and utilization pathways. Western immunoblot analysis and real-time PCR analysis of GAS in throat swabs taken from humans with pharyngitis confirmed the findings. We conclude that SptR/S optimizes persistence of GAS in human saliva, apparently by strategically influencing metabolic pathways and virulence factor production. The discovery of a genetic program that significantly increased persistence of a major human pathogen in saliva enhances understanding of how bacteria survive in the host and suggests new therapeutic strategies.

Longitudinal analysis of the group A Streptococcus transcriptome in experimental pharyngitis in cynomolgus macaques

Identification of the genetic events that contribute to host-pathogen interactions is important for understanding the natural history of infectious diseases and developing therapeutics. Transcriptome studies conducted on pathogens have been central to this goal in recent years. However, most of these investigations have focused on specific end points or disease phases, rather than analysis of the entire time course of infection. To gain a more complete understanding of how bacterial gene expression changes over time in a primate host, the transcriptome of group A Streptococcus (GAS) was analyzed during an 86-day infection protocol in 20 cynomolgus macaques with experimental pharyngitis. The study used 260 custom Affymetrix (Santa Clara, CA) chips, and data were confirmed by TaqMan analysis. Colonization, acute, and asymptomatic phases of disease were identified. Successful colonization and severe inflammation were significantly correlated with an early onset of superantigen gene expression. The differential expression of two-component regulators covR and spy0680 (M1_spy0874) was significantly associated with GAS colony-forming units, inflammation, and phases of disease. Prophage virulence gene expression and prophage induction occurred predominantly during high pathogen cell densities and acute inflammation. We discovered that temporal changes in the GAS transcriptome were integrally linked to the phase of clinical disease and host-defense response. Knowledge of the gene expression patterns characterizing each phase of pathogen-host interaction provides avenues for targeted investigation of proven and putative virulence factors and genes of unknown function and will assist vaccine research.