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

The group A Streptococcus pathogenicity island RD2: virulence role and barriers to conjugative transfer

Serotype M28 isolates of the bacterial pathogen the group A Streptococcus (GAS; Streptococcus pyogenes), but not isolates of other serotypes, have a nonrandom association with cases of puerperal sepsis, a life-threatening infection that can occur in women following childbirth. In prior studies, we established that RD2, a pathogenicity island present in all M28 GAS isolates but mostly absent from other serotypes, is a factor in the M28-puerperal sepsis association. Here, we identified a significant reduction in the RD2 conjugation frequency in inter-serotype conjugation assays relative to intra-serotype assays. As isolates of most GAS serotypes produce an antiphagocytic hyaluronic acid capsule, while M28 isolates do not, we tested whether the capsule served as a barrier to RD2 acquisition or maintenance. The data showed that capsule production had no impact on the RD2 conjugation frequency or on the ability of RD2 to enhance vaginal colonization by GAS, but did inhibit the ability of RD2 to enhance GAS adherence to vaginal epithelial cell lines. Further molecular explanations for the inter-serotype barrier to RD2 conjugative transfer were investigated, and a conserved, chromosomally encoded Type I restriction-modification system was identified as being key. We also identified that RD2 modifies the GAS transcriptome, including mRNAs encoding virulence factors with adherence and dissemination roles, following exposure to human plasma. Our data provide insights into factors that contribute to the restriction of the RD2 pathogenicity island to discrete subsets of the GAS population.

The burden of group A Streptococcus (GAS) infections: The challenge continues in the twenty-first century

Streptococcus pyogenes is a Gram-positive bacterium, also known as Group A Streptococcus (GAS), that has become a significant threat to the healthcare system, infecting more than 18 million people and resulting in more than 500,000 deaths annually worldwide. GAS infection rates decreased gradually during the 20th century in Western countries, largely due to improved living conditions and access to antibiotics. However, post-COVID-19, the situation has led to a steep increase in GAS infection rates in Europe, the United States, Australia, and New Zealand, which triggers a global concern. GAS infections are normally moderate, with symptoms of fever, pharyngitis, and pyoderma; nevertheless, if left untreated or with continued exposure to GAS or with recurring infections it can result in fatal outcomes. GAS produces a variety of virulence factors and exotoxins that can lead to deadly infections such as necrotizing fasciitis, impetigo, cellulitis, pneumonia, empyema, streptococcal toxic shock syndrome, bacteremia, and puerperal sepsis. In addition, post-immune mediated disorders such as post-streptococcal glomerulonephritis, acute rheumatic fever, and rheumatic heart disease contribute to extremely high death rates in developing nations. Despite substantial research on GAS infections, it is still unclear what molecular pathways are responsible for their emergence and how to best manage them. This review thus provides insights into the most recent research on the pathogenesis, virulence, resistance, and host interaction mechanisms of GAS, as well as novel management options to assist scientific communities in combating GAS infections.

Efficient and rapid one-step method to generate gene deletions in Streptococcus pyogenes

Streptococcus pyogenes or Group A Streptococcus (GAS) remains a significant infectious problem around the world, particularly in low- and middle-income settings. Moreover, a recent invasive GAS infection (iGAS) upsurge has been observed in high-income settings. However, to date, no vaccine is available. Finding a good vaccine antigen and understanding the role of virulence factors in GAS infections have been hampered, in part, by technical difficulties to transform the many different strains and generate knockout mutants. Using colE1-type plasmid as a suicide vector, we have set up a method allowing the generation of non-polar mutants of GAS in 3 days.

IMPORTANCE

Group A Streptococcus (GAS) is a major human pathogen, causing diseases ranging from mild and superficial infections of the skin and pharyngeal epithelium to severe systemic and invasive diseases. Since June 2022, several European countries, the US, and Australia are facing an upsurge of invasive life-threatening GAS infections. Finding a good vaccine antigen and understanding the role of virulence factors in GAS infections have been hampered, in part, by technical difficulties to transform the many different GAS strains and generate knockout mutants. Moreover, these tools must be adapted to a large range of different strains, since GAS are divided into more than 260 emm-types (M-type). We have set up a method allowing the generation of non-polar mutants of GAS in 3 days and in diverse backgrounds, which contrasts with previously published protocols.

C. Rocha E. Intragenomic conflicts with plasmids and chromosomal mobile genetic elements drive the evolution of natural transformation within species

Natural transformation is the only mechanism of genetic exchange controlled by the recipient bacteria. We quantified its rates in 786 clinical strains of the human pathogens Legionella pneumophila (Lp) and 496 clinical and environmental strains of Acinetobacter baumannii (Ab). The analysis of transformation rates in the light of phylogeny revealed they evolve by a mixture of frequent small changes and a few large quick jumps across 6 orders of magnitude. In standard conditions close to half of the strains of Lp and a more than a third in Ab are below the detection limit and thus presumably non-transformable. Ab environmental strains tend to have higher transformation rates than the clinical ones. Transitions to non-transformability were frequent and usually recent, suggesting that they are deleterious and subsequently purged by natural selection. Accordingly, we find that transformation decreases genetic linkage in both species, which might accelerate adaptation. Intragenomic conflicts with chromosomal mobile genetic elements (MGEs) and plasmids could explain these transitions and a GWAS confirmed systematic negative associations between transformation and MGEs: plasmids and other conjugative elements in Lp, prophages in Ab, and transposable elements in both. In accordance with the hypothesis of modulation of transformation rates by genetic conflicts, transformable strains have fewer MGEs in both species and some MGEs inactivate genes implicated in the transformation with heterologous DNA (in Ab). Innate defense systems against MGEs are associated with lower transformation rates, especially restriction-modification systems. In contrast, CRISPR-Cas systems are associated with higher transformation rates suggesting that adaptive defense systems may facilitate cell protection from MGEs while preserving genetic exchanges by natural transformation. Ab and Lp have different lifestyles, gene repertoires, and population structure. Nevertheless, they exhibit similar trends in terms of variation of transformation rates and its determinants, suggesting that genetic conflicts could drive the evolution of natural transformation in many bacteria.

An efficient in vivo-inducible CRISPR interference system for group A Streptococcus genetic analysis and pathogenesis studies

While genome-wide transposon mutagenesis screens have identified numerous essential genes in the significant human pathogen Streptococcus pyogenes (group A Streptococcus or GAS), many of their functions remain elusive. This knowledge gap is attributed in part to the limited molecular toolbox for controlling GAS gene expression and the bacterium's poor genetic transformability. CRISPR interference (CRISPRi), using catalytically inactive GAS Cas9 (dCas9), is a powerful approach to specifically repress gene expression in both bacteria and eukaryotes, but ironically, it has never been harnessed for controlled gene expression in GAS. In this study, we present a highly transformable and fully virulent serotype M1T1 GAS strain and introduce a doxycycline-inducible CRISPRi system for efficient repression of bacterial gene expression. We demonstrate highly efficient, oligo-based single guide RNA cloning directly to GAS, enabling the construction of a gene knockdown strain in just 2 days, in contrast to the several weeks typically required. The system is shown to be titratable and functional both in vitro and in vivo using a murine model of GAS infection. Furthermore, we provide direct in vivo evidence that the expression of the conserved cell division gene ftsZ is essential for GAS virulence, highlighting its promise as a target for emerging FtsZ inhibitors. Finally, we introduce SpyBrowse (https://veeninglab.com/SpyBrowse), a comprehensive and user-friendly online resource for visually inspecting and exploring GAS genetic features. The tools and methodologies described in this work are poised to facilitate fundamental research in GAS, contribute to vaccine development, and aid in the discovery of antibiotic targets.

IMPORTANCE

While group A Streptococcus (GAS) remains a predominant cause of bacterial infections worldwide, there are limited genetic tools available to study its basic cell biology. Here, we bridge this gap by creating a highly transformable, fully virulent M1T1 GAS strain. In addition, we established a tight and titratable doxycycline-inducible system and developed CRISPR interference (CRISPRi) for controlled gene expression in GAS. We show that CRISPRi is functional in vivo in a mouse infection model. Additionally, we present SpyBrowse, an intuitive and accessible genome browser (https://veeninglab.com/SpyBrowse). Overall, this work overcomes significant technical challenges of working with GAS and, together with SpyBrowse, represents a valuable resource for researchers in the GAS field.

Genetic modification of Streptococcus dysgalactiae by natural transformation

Streptococcus dysgalactiae is an emerging human and animal pathogen. Functional studies of genes involved in virulence of S. dysgalactiae and other pyogenic group streptococci are often hampered by limited genetic tractability. It is known that pyogenic streptococci carry genes required for competence for natural transformation; however, in contrast to other streptococcal subgroups, there is limited evidence for gene transfer by natural transformation in these bacteria. In this study, we systematically assessed the genomes of 179 S. dysgalactiae strains of both human and animal origins (subsp. equisimilis and dysgalactiae, respectively) for the presence of genes required for natural transformation. While a considerable fraction of the strains contained inactive genes, the majority (64.2%) of the strains had an intact gene set. In selected strains, we examined the dynamics of competence activation after addition of competence-inducing pheromones using transcriptional reporter assays and exploratory RNA-seq. Based on these findings, we were able to establish a protocol allowing us to utilize natural transformation to construct deletion mutants by allelic exchange in several S. dysgalactiae strains of both subspecies. As part of the work, we deleted putative lactose utilization genes to study their role in growth on lactose. The data presented here provide new knowledge on the potential of horizonal gene transfer by natural transformation in S. dysgalactiae and, importantly, demonstrates the possibility to exploit natural transformation for genetic engineering in these bacteria.

IMPORTANCE

Numerous Streptococcus spp. exchange genes horizontally through natural transformation, which also facilitates efficient genetic engineering in these organisms. However, for the pyogenic group of streptococci, including the emerging pathogen Streptococcus dysgalactiae, there is limited experimental evidence for natural transformation. In this study, we demonstrate that natural transformation in vitro indeed is possible in S. dysgalactiae strains under optimal conditions. We utilized this method to perform gene deletion through allelic exchange in several strains, thereby paving the way for more efficient gene engineering methods in pyogenic streptococci.

The elimination of two restriction enzyme genes allows for electroporation-based transformation and CRISPR-Cas9-based base editing in the non-competent Gram-negative bacterium Acinetobacter sp. Tol 5

Environmental isolates are promising candidates for new chassis of synthetic biology because of their inherent capabilities, which include efficiently converting a wide range of substrates into valuable products and resilience to environmental stresses; however, many remain genetically intractable and unamenable to established genetic tools tailored for model bacteria. Acinetobacter sp. Tol 5, an environmentally isolated Gram-negative bacterium, possesses intriguing properties for use in synthetic biology applications. Despite the previous development of genetic tools for the engineering of strain Tol 5, its genetic manipulation has been hindered by low transformation efficiency via electroporation, rendering the process laborious and time-consuming. This study demonstrated the genetic refinement of the Tol 5 strain, achieving efficient transformation via electroporation. We deleted two genes encoding type I and type III restriction enzymes. The resulting mutant strain not only exhibited marked efficiency of electrotransformation but also proved receptive to both in vitro and in vivo DNA assembly technologies, thereby facilitating the construction of recombinant DNA without reliance on intermediate Escherichia coli constructs. In addition, we successfully adapted a CRISPR-Cas9-based base-editing platform developed for other Acinetobacter species. Our findings provide genetic modification strategies that allow for the domestication of environmentally isolated bacteria, streamlining their utilization in synthetic biology applications.IMPORTANCERecent synthetic biology has sought diverse bacterial chassis from environmental sources to circumvent the limitations of laboratory Escherichia coli strains for industrial and environmental applications. One of the critical barriers in cell engineering of bacterial chassis is their inherent resistance to recombinant DNA, propagated either in vitro or within E. coli cells. Environmental bacteria have evolved defense mechanisms against foreign DNA as a response to the constant threat of phage infection. The ubiquity of phages in natural settings accounts for the genetic intractability of environmental isolates. The significance of our research is in demonstrating genetic modification strategies for the cell engineering of such genetically intractable bacteria. This research marks a pivotal step in the domestication of environmentally isolated bacteria, promising candidates for emerging synthetic biology chassis. Our work thus significantly contributes to advancing their applications across industrial, environmental, and biomedical fields.

Methylome-dependent transformation of emm1 group A streptococci

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

Streptolysin O Deficiency in Streptococcus pyogenes M1T1 covR/S Mutant Strain Attenuates Virulence in In Vitro and In Vivo Infection Models

Mutation within the Streptococcus pyogenes (Streptococcus group A; Strep A) covR/S regulatory system has been associated with a hypervirulent phenotype resulting from the upregulation of several virulence factors, including the pore-forming toxin, streptolysin O (SLO). In this study, we utilized a range of covR/S mutants, including M1T1 clonal strains (5448 and a covS mutant generated through mouse passage designated 5448AP), to investigate the contribution of SLO to the pathogenesis of covR/S mutant Strep A disease. Up-regulation of slo in 5448AP resulted in increased SLO-mediated hemolysis, decreased dendritic cell (DC) viability post coculture with Strep A, and increased production of tumor necrosis factor (TNF) and monocyte chemoattractant protein 1 (MCP-1) by DCs. Mouse passage of an isogenic 5448 slo-deletion mutant resulted in recovery of several covR/S mutants within the 5448Δslo background. Passage also introduced mutations in non-covR/S genes, but these were considered to have no impact on virulence. Although slo-deficient mutants exhibited the characteristic covR/S-controlled virulence factor upregulation, these mutants caused increased DC viability with reduced inflammatory cytokine production by infected DCs. In vivo, slo expression correlated with decreased DC numbers in infected murine skin and significant bacteremia by 3 days postinfection, with severe pathology at the infection site. Conversely, the absence of slo in the infecting strain (covR/S mutant or wild-type) resulted in detection of DCs in the skin and attenuated virulence in a murine model of pyoderma. slo-sufficient and -deficient covR/S mutants were susceptible to immune clearance mediated by a combination vaccine consisting of a conserved M protein peptide and a peptide from the CXC chemokine protease SpyCEP. IMPORTANCE Streptococcus pyogenes is responsible for significant numbers of invasive and noninvasive infections which cause significant morbidity and mortality globally. Strep A isolates with mutations in the covR/S system display greater propensity to cause severe invasive diseases, which are responsible for more than 163,000 deaths each year. This is due to the upregulation of virulence factors, including the pore-forming toxin streptolysin O. Utilizing covR/S and slo-knockout mutants, we investigated the role of SLO in virulence. We found that SLO alters interactions with host cell populations and increases Strep A viability at sterile sites of the host, such as the blood, and that its absence results in significantly less virulence. This work underscores the importance of SLO in Strep A virulence while highlighting the complex nature of Strep A pathogenesis. This improved insight into host-pathogen interactions will enable a better understanding of host immune evasion mechanisms and inform streptococcal vaccine development programs.

Highly efficient polyhydroxyalkanoate production from lignin using genetically engineered Halomonas sp. Y3

Lignin degradation represents a significant challenge in biological valorization, but it is suffering from insufficiency, putting barriers to efficient lignin conversion. Herein, the study first develops a highly efficient laccase secretion apparatus, enabling high enzyme activity of 184 U/mL, complementing the biochemical limits on lignin depolymerization well in Halomonas sp. Y3. Further engineering of PHA biosynthesis produces a significantly high PHA titer of 286, 742, and 868 mg/L from alkaline lignin, catechol, and protocatechuate, respectively. The integration of laccase-secretion and PHA production modules enables a record titer of 693 and 1209 mg/L in converting lignin and lignin-containing stream to PHA, respectively. The titer is improved furtherly to 740 and 1314 mg/L by developing a non-sterilized fermentation. This study advances a cheaper and greener production of valuable chemicals from lignin by constructing a biosynthetic platform for PHA production and provides novel insight into the lignin conversion by extremophilic microbes.

Characterization of the Staphylococcus xylosus methylome reveals a new variant of type I restriction modification system in staphylococci

Restriction modification (RM) systems are known to provide a strong barrier to the exchange of DNA between and within bacterial species. Likewise, DNA methylation is known to have an important function in bacterial epigenetics regulating essential pathways such as DNA replication and the phase variable expression of prokaryotic phenotypes. To date, research on staphylococcal DNA methylation focused mainly on the two species Staphylococcus aureus and S. epidermidis. Less is known about other members of the genus such as S. xylosus, a coagulase-negative commensal of mammalian skin. The species is commonly used as starter organism in food fermentations but is also increasingly considered to have an as yet elusive function in bovine mastitis infections. We analyzed the methylomes of 14 S. xylosus strains using single-molecular, real-time (SMRT) sequencing. Subsequent in silico sequence analysis allowed identification of the RM systems and assignment of the respective enzymes to the discovered modification patterns. Hereby the presence of type I, II, III and IV RM systems in varying numbers and combinations among the different strains was revealed, clearly distinguishing the species from what is known for other members of the genus so far. In addition, the study characterizes a newly discovered type I RM system, encoded by S. xylosus but also by a variety of other staphylococcal species, with a hitherto unknown gene arrangement that involves two specificity units instead of one (hsdRSMS). Expression of different versions of the operon in E. coli showed proper base modification only when genes encoding both hsdS subunits were present. This study provides new insights into the general understanding of the versatility and function of RM systems as well as the distribution and variations in the genus Staphylococcus.

Characterization of the Type I Restriction Modification System Broadly Conserved among Group A Streptococci

Although prokaryotic DNA methylation investigations have long focused on immunity against exogenous DNA, it has been recently recognized that DNA methylation impacts gene expression and phase variation in Streptococcus pneumoniae and Streptococcus suis. A comprehensive analysis of DNA methylation is lacking for beta-hemolytic streptococci, and thus we sought to examine DNA methylation in the major human pathogen group A Streptococcus (GAS). Using a database of 224 GAS genomes encompassing 80 emm types, we found that nearly all GAS strains encode a type I restriction modification (RM) system that lacks the hsdS′ alleles responsible for impacting gene expression in S. pneumoniae and S. suis. The GAS type I system is located on the core chromosome, while sporadically present type II orphan methyltransferases were identified on prophages. By combining single-molecule real-time (SMRT) analyses of 10 distinct emm types along with phylogenomics of 224 strains, we were able to assign 13 methylation patterns to the GAS population. Inactivation of the type I RM system, occurring either naturally through phage insertion or through laboratory-induced gene deletion, abrogated DNA methylation detectable via either SMRT or MinION sequencing. Contrary to a previous report, inactivation of the type I system did not impact transcript levels of the gene (mga) encoding the key multigene activator protein (Mga) or Mga-regulated genes. Inactivation of the type I system significantly increased plasmid transformation rates. These data delineate the breadth of the core chromosomal type I RM system in the GAS population and clarify its role in immunity rather than impacting Mga regulon expression.

IMPORTANCE The advent of whole-genome approaches capable of detecting DNA methylation has markedly expanded appreciation of the diverse roles of epigenetic modification in prokaryotic physiology. For example, recent studies have suggested that DNA methylation impacts gene expression in some streptococci. The data described herein are from the first systematic analysis of DNA methylation in a beta-hemolytic streptococcus and one of the few analyses to comprehensively characterize DNA methylation across hundreds of strains of the same bacterial species. We clarify that DNA methylation in group A Streptococcus (GAS) is primarily due to a type I restriction modification (RM) system present in the core genome and does not impact mga-regulated virulence gene expression, but does impact immunity against exogenous DNA. The identification of the DNA motifs recognized by each type I RM system may assist with optimizing methods for GAS genetic manipulation and help us understand how bacterial pathogens acquire exogenous DNA elements.

DNA Methyltransferase HsdM Induce Drug Resistance on Mycobacterium tuberculosis via Multiple Effects

Besides the genomic variants, epigenetic mechanisms such as DNA methylation also have an effect on drug resistance. This study aimed to investigate the methylomes of totally/extensively drug-resistant M. tuberculosis clinical isolates using the PacBio single-molecule real-time technology. The results showed they were almost the same as the pan-susceptible ones. Genetics and bioinformatics analysis confirmed three DNA methyltransferases-MamA, MamB, and HsdM. Moreover, anti-tuberculosis drug treatment did not change the methylomes. In addition, the knockout of the DNA methyltransferase hsdM gene in the extensively drug-resistant clinical isolate 11826 revealed that the motifs of GTAYN4ATC modified by HsdM were completely demethylated. Furthermore, the results of the methylated DNA target analysis found that HsdM was mainly involved in redox-related pathways, especially the prodrug isoniazid active protein KatG. HsdM also targeted three drug-targeted genes, eis, embB, and gyrA, and three drug transporters (Rv0194, Rv1410, and Rv1877), which mildly affected the drug susceptibility. The overexpression of HsdM in M. smegmatis increased the basal mutation rate. Our results suggested that DNA methyltransferase HsdM affected the drug resistance of M. tuberculosis by modulating the gene expression of redox, drug targets and transporters, and gene mutation.

Identification of Group A Streptococcus Genes Directly Regulated by CsrRS and Novel Intermediate Regulators

Adaptation of group A Streptococcus (GAS) to its human host is mediated by two-component systems that transduce external stimuli to regulate bacterial physiology. Among such systems, CsrRS (also known as CovRS) is the most extensively characterized for its role in regulating ∼10% of the GAS genome, including several virulence genes. Here, we show that extracellular magnesium and the human antimicrobial peptide LL-37 have opposing effects on the phosphorylation of the response regulator CsrR by the receptor kinase CsrS. Genetic inactivation of CsrS phosphatase or kinase activity, respectively, had similar but more pronounced effects on CsrR phosphorylation compared to growth in magnesium or LL-37. These changes in CsrR phosphorylation were correlated with the repression or activation of CsrR-regulated genes as assessed by NanoString analysis. Chromatin immunoprecipitation and DNA sequencing (ChIP-seq) revealed CsrR occupancy at CsrRS-regulated promoters and lower-affinity associations at many other locations on the GAS chromosome. Because ChIP-seq did not detect CsrR occupancy at promoters associated with some CsrR-regulated genes, we investigated whether these genes might be controlled indirectly by intermediate regulators whose expression is modulated by CsrR. Transcriptional profiling of mutant strains deficient in the expression of either of two previously uncharacterized transcription regulators in the CsrR regulon indicated that one or both proteins participated in the regulation of 22 of the 42 CsrR-regulated promoters for which no CsrR association was detected by ChIP-seq. Taken together, these results illuminate CsrRS-mediated regulation of GAS gene expression through modulation of CsrR phosphorylation, CsrR association with regulated promoters, and the control of intermediate transcription regulators. IMPORTANCE Group A Streptococcus (GAS) is an important public health threat as a cause of sore throat, skin infections, life-threatening invasive infections, and the postinfectious complications of acute rheumatic fever, a leading cause of acquired heart disease. This work characterizes CsrRS, a GAS system for the detection of environmental signals that enables adaptation of the bacteria for survival in the human throat by regulating the production of products that allow the bacteria to resist clearance by the human immune system. CsrRS consists of two proteins: CsrS, which is on the bacterial surface to detect specific stimuli, and CsrR, which receives signals from CsrS and, in response, represses or activates the expression of genes coding for proteins that enhance bacterial survival. Some of the genes regulated by CsrR encode proteins that are themselves regulators of gene expression, thereby creating a regulatory cascade.