Utilization of the ComRS system for the rapid markerless deletion of chromosomal genes in Streptococcus suis

A small RNA from Streptococcus suis epidemic ST7 strain promotes bacterial survival in host blood and brain by enhancing oxidative stress resistance

Streptococcus suis is a Gram-positive pathogen causing septicaemia and meningitis in pigs and humans. However, how S. suis maintains a high bacterial load in the blood and brain is poorly understood. In this study, we found that a small RNA rss03 is predominantly present in S. suis, Streptococcus parasuis, and Streptococcus ruminantium, implying a conserved biological function. rss03 with a size of 303 nt mainly exists in S. suis sequence type (ST) 1 and epidemic ST7 strains that are responsible for human infections in China. Using MS2-affinity purification coupled with RNA sequencing (MAPS), proteomics analysis, and CopraRNA prediction, 14 direct targets of rss03 from an ST7 strain were identified. These direct targets mainly involve substance transport, transcriptional regulation, rRNA modification, and stress response. A more detailed analysis reveals that rss03 interacts with the coding region of glpF mRNA, and unexpectedly rss03 protects glpF mRNA from degradation by RNase J1. The GlpF protein is an aquaporin, contributes to S. suis oxidative stress resistance by H2O2 efflux, and facilitates bacterial survival in murine macrophages RAW264.7. Finally, we showed that rss03 and GlpF are required to maintain a high bacterial load in mouse blood and brain. Our study presents the first sRNA targetome in streptococci, enriches the knowledge of sRNA regulation in streptococci, and identifies pathways contributing to S. suis pathogenesis.

Glycerol metabolic repressor GlpR contributes to Streptococcus suis oxidative stress resistance and virulence

Bacterial DeoR family transcription regulators regulate multiple physiological processes. Little is known about the function of DeoR family regulators in streptococci. Here, we identified a novel DeoR family regulator, GlpR, from Streptococcus suis, a pathogen causing severe diseases in pigs and humans. GlpR was involved in glycerol utilization and exhibited specific signature residues at positions 30-31 (KV) which are crucial for DNA binding. Deletion of glpR (ΔglpR) showed a significant increase in relative growth rate in glycerol medium compared to the wild-type (WT) and complementary strains (CΔglpR). Employing RNA-seq analysis, β-galactosidase activity analysis, and electrophoretic mobility shift assay, we discovered that GlpR directly represses the expression of glycerol metabolism-related genes pflB2, pflA1, and fsaA, encoding pyruvate formate-lyase and its activating enzyme, and fructose-6-phosphate aldolase, respectively. Compared to WT and CΔglpR, ΔglpR showed a reduced survival rate under oxidative stress and in murine macrophages and attenuated virulence in mice. GlpR probably enhances oxidative stress resistance and virulence in S. suis by functioning as a glycerol metabolic repressor decreasing energy consumption. These findings contribute to a better understanding of S. suis pathogenesis and enrich our knowledge of the biological functions of DeoR family regulators in streptococci.

Streptococcus suis Deploys Multiple ATP-Dependent Proteases for Heat Stress Adaptation

Streptococcus suis is an important zoonotic pathogen, causing cytokine storms of Streptococcal toxic shock-like syndrome amongst humans after a wound infection into the bloodstream. To overcome the challenges of fever and leukocyte recruitment, invasive S. suis must deploy multiple stress responses forming a network and utilize proteases to degrade short-lived regulatory and misfolded proteins induced by adverse stresses, thereby adapting and evading host immune responses. In this study, we found that S. suis encodes multiple ATP-dependent proteases, including single-chain FtsH and double-subunit Clp protease complexes ClpAP, ClpBP, ClpCP, and ClpXP, which were activated as the fever of infected mice in vivo. The expression of genes ftsH, clpA/B/C, and clpP, but not clpX, were significantly upregulated in S. suis in response to heat stress, while were not changed notably under the treatments with several other stresses, including oxidative, acidic, and cold stimulation. FtsH and ClpP were required for S. suis survival within host blood under heat stress in vitro and in vivo. Deletion of ftsH or clpP attenuated the tolerance of S. suis to heat, oxidative and acidic stresses, and significantly impaired the bacterial survival within macrophages. Further analysis identified that repressor CtsR directly binds and controls the clpA/B/C and clpP operons and is relieved by heat stress. In summary, the deployments of multiple ATP-dependent proteases form a flexible heat stress response network that appears to allow S. suis to fine-tune the degradation or refolding of the misfolded proteins to maintain cellular homeostasis and optimal survival during infection.

Identifying Cell-Penetrating Peptides for Effectively Delivering Antimicrobial Molecules into Streptococcus suis

Cell-penetrating peptides (CPPs) are promising carriers to effectively transport antisense oligonucleotides (ASOs), including peptide nucleic acids (PNAs), into bacterial cells to combat multidrug-resistant bacterial infections, demonstrating significant therapeutic potential. Streptococcus suis, a Gram-positive bacterium, is a major bacterial pathogen in pigs and an emerging zoonotic pathogen. In this study, through the combination of super-resolution structured illumination microscopy (SR-SIM), flow cytometry analysis, and toxicity analysis assays, we investigated the suitability of four CPPs for delivering PNAs into S. suis cells: HIV-1 TAT efficiently penetrated S. suis cells with low toxicity against S. suis; (RXR)4XB had high penetration efficiency with inherent toxicity against S. suis; (KFF)3K showed lower penetration efficiency than HIV-1 TAT and (RXR)4XB; K8 failed to penetrate S. suis cells. HIV-1 TAT-conjugated PNA specific for the essential gyrase A subunit gene (TAT-anti-gyrA PNA) effectively inhibited the growth of S. suis. TAT-anti-gyrA PNA exhibited a significant bactericidal effect on serotypes 2, 4, 5, 7, and 9 strains of S. suis, which are known to cause human infections. Our study demonstrates the potential of CPP-ASO conjugates as new antimicrobial compounds for combating S. suis infections. Furthermore, our findings demonstrate that applying SR-SIM and flow cytometry analysis provides a convenient, intuitive, and cost-effective approach to identifying suitable CPPs for delivering cargo molecules into bacterial cells.

Comparative Genome Analysis of Two Streptococcus suis Serotype 8 Strains Identifies Two New Virulence-Associated Genes

Streptococcus suis is an important zoonotic pathogen that can cause meningitis and septicemia in swine and humans. Among numerous pathogenic serotypes, S. suis serotype 8 has distinctive characteristics such as a high detection rate and causing multi-host infection. There is no complete genome of serotype 8 strains so far. In this study, the complete genome of two S. suis serotype 8 strains, virulent strain 2018WUSS151 and non-virulent strain WUSS030, were sequenced. Comparative genomic analysis showed that the homology of the two genomes reaches 99.68%, and the main difference is the distinctive prophages. There are 83 genes unique to virulent strain 2018WUSS151, including three putative virulence-associated genes (PVGs). Two PVGs, padR and marR, are passenger genes in ISSsu2 family transposons that are able to form circular DNA intermediates during transposition, indicating the possibility of horizontal transmission among S. suis strains. The deletion mutant of PVGs marR or atpase attenuated the virulence of serotype 2 virulent SC070731 in a mouse infection model, confirming their role in S. suis virulence. These findings contribute to clarifying the genomic characterization of S. suis serotype 8 and S. suis pathogenesis.

Techniques for Genetic Manipulation

In the last few decades, molecular techniques and genetic modification have been used in genotype and phenotype studies of S. suis. Genomic modification of S. suis requires DNA acquisition and its stable insertion into the chromosome by allelic exchange. In this chapter, we described two techniques for the preparation of genomic constructs (cloning and overlapping extension PCR) and for DNA uptake (electroporation and transformation). The protocols are accompanied with examples. All described protocols were successful on our hands with the reference S. suis strain P1/7.

A novel aquaporin Aagp contributes to Streptococcus suis H2O2 efflux and virulence

Streptococcus suis is a bacterium that can cause infections in pigs and humans. Although oxidative stress is common occurrence during bacterial growth and infection, the regulation networks of S. suis under oxidative stress remain poorly understood. To address this, we utilized RNA-Seq to reveal the transcriptional landscape of S. suis in response to H2O2 stress. We identified novel genes responsible for S. suis resistance to oxidative stress, including those involved in DNA repair or protection, and essential for the biosynthesis of amino acids and nucleic acids. In addition, we found that a novel aquaporin, Aagp, belonging to atypical aquaglyceroporins and widely distributed in diverse S. suis serotypes, plays a crucial role during H2O2 stress. By performing oxidative stress assays and measuring the intracellular H2O2 concentrations of the wild-type strain and Aagp mutants during H2O2 stress, we found that Aagp facilitated H2O2 efflux. Additionally, we found that Aagp might be involved in glycerol transport, as shown by the growth inhibition and H2O2 production in the presence of glycerol. Mice infection experiments indicated that Aagp contributed to S. suis virulence. This study contributes to understanding the mechanism of S. suis oxidative stress response, S. suis pathogenesis, and the function of aquaporins in prokaryotes.

Precision Genome Engineering in Streptococcus suis Based on a Broad-Host-Range Vector and CRISPR-Cas9 Technology

Streptococcussuis is an important zoonotic pathogen that causes severe invasive disease in pigs and humans. Current methods for genome engineering of S. suis rely on the insertion of antibiotic resistance markers, which is time-consuming and labor-intensive and does not allow the precise introduction of small genomic mutations. Here we developed a system for CRISPR-based genome editing in S. suis, utilizing linear DNA fragments for homologous recombination (HR) and a plasmid-based negative selection system for bacteria not edited by HR. To enable the use of this system in other bacteria, we engineered a broad-host-range replicon in the CRISPR plasmid. We demonstrated the utility of this system to rapidly introduce multiple gene deletions in successive rounds of genome editing and to make precise nucleotide changes in essential genes. Furthermore, we characterized a mechanism by which S. suis can escape killing by a targeted Cas9-sgRNA complex in the absence of HR. A characteristic of this new mechanism is the presence of very slow-growing colonies in a persister-like state that may allow for DNA repair or the introduction of mutations, alleviating Cas9 pressure. This does not impact the utility of CRISPR-based genome editing because the escape colonies are easily distinguished from genetically edited clones due to their small colony size. Our CRISPR-based editing system is a valuable addition to the genetic toolbox for engineering of S. suis, as it accelerates the process of mutant construction and simplifies the removal of antibiotic markers between successive rounds of genome editing.

Conjugative transfer of streptococcal prophages harboring antibiotic resistance and virulence genes

Prophages play important roles in the transduction of various functional traits, including virulence factors, but remain debatable in harboring and transmitting antimicrobial resistance genes (ARGs). Herein we characterize a prevalent family of prophages in Streptococcus, designated SMphages, which harbor twenty-five ARGs that collectively confer resistance to ten antimicrobial classes, including vanG-type vancomycin resistance locus and oxazolidinone resistance gene optrA. SMphages integrate into four chromosome attachment sites by utilizing three types of integration modules and undergo excision in response to phage induction. Moreover, we characterize four subtypes of Alp-related surface proteins within SMphages, the lethal effects of which are extensively validated in cell and animal models. SMphages transfer via high-frequency conjugation that is facilitated by integrative and conjugative elements from either donors or recipients. Our findings explain the widespread of SMphages and the rapid dissemination of ARGs observed in members of the Streptococcus genus.

Identification of the RNA-binding domain-containing protein RbpA that acts as a global regulator of the pathogenicity of Streptococcus suis serotype 2

Streptococcus suis serotype 2 (SS2), an emerging zoonotic pathogen, causes swine diseases and human cases of streptococcal toxic shock syndrome. RNA-binding proteins (RBPs) can modulate gene expression through post-transcriptional regulation. In this study, we identified an RBP harbouring an S1 domain, named RbpA, which facilitated SS2 adhesion to host epithelial cells and contributed to bacterial pathogenicity. Comparative proteomic analysis identified 145 proteins that were expressed differentially between ΔrbpA strain and wild-type strain, including several virulence-associated factors, such as the extracellular protein factor (EF), SrtF pilus, IgA1 protease, SBP2 pilus, and peptidoglycan-binding LysM’ proteins. The mechanisms underlying the regulatory effects of RbpA on their encoding genes were explored, and it was found that RbpA regulates gene expression through diverse mechanisms, including post-transcriptional regulation, and thus acts as a global regulator. These results partly reveal the pathogenic mechanism mediated by RbpA, improving our understanding of the regulatory systems of S. suis and providing new insights into bacterial pathogenicity.

Pathogenic investigations of Streptococcus pasteurianus, an underreported zoonotic pathogen, isolated from a diseased piglet with meningitis

Streptococcus pasteurianus, an underreported opportunistic pathogen, is considered an increasingly recognized cause of meningitis and bacteremia in many animals and humans worldwide. However, except for some epidemiological studies, there is no report about the gene-deletion mutagenesis, virulence factors, reservoir niches or animal infection models for this pathogen. In this study, we first isolated an S. pasteurianus strain from a newly weaned piglet's brain with meningitis. The genomic sequence of this swine isolate WUSP067 shared high homology with that of two human strains. The comparative genome analysis showed that strain WUSP067 contained a fucose utilization cluster absent in human strains, and it shared 91% identity with that of an integrative and conjugative element (ICE) ICEssuZJ20091101-2 from Streptococcus suis, another important swine bacterial pathogen. Strain WUSP067 was resistant to erythromycin, tulathromycin, lincomycin, clindamycin, doxycycline and gentamycin, and ICEs are vehicles for harbouring antimicrobial resistance genes. The infection model was established using the 3-week-old newly weaned ICR mice. The 50% lethal dose value of strain WUSP067 was 4.0 × 107 colony-forming units per mouse. The infected mice showed severe signs of meningitis and pathological changes in brains. Furthermore, the capsule-deficient mutant was generated using natural transformation, and we showed that capsule was an essential virulence factor for S. pasteurianus. In addition, we found that tonsils and hilar lymph nodes of healthy pigs may be reservoir niches for this bacterium. Thus, our study provided valuable information about the pathogenetic characteristics and antimicrobial resistance of S. pasteurianus and paved the way for studying its pathogenesis.

Highly Effective Markerless Genetic Manipulation of Streptococcus suis Using a Mutated PheS-Based Counterselectable Marker

Streptococcus suis is an important zoonotic pathogen, however, an efficient markerless genetic manipulation system is still lacking for further physiological and pathological studies on this bacterium. Several techniques have been developed for markerless genetic manipulation of S. suis utilizing either a temperature-sensitive vector or a counterselectable markers (CSMs), however, at present, the efficiency of these techniques is not very satisfactory. In this study, we developed a strategy for markerless genetic manipulation of S. suis employing a CSM based on a conditionally lethal mutant allele of pheS, which encodes the α-subunit of phenylalanyl-tRNA synthetase (PheS). This mutant pheS, mPheS, was constructed by introducing site-directed mutations for a T261S/A315G double-substitution and a number of silent mutations to decrease its similarity with the endogenous wild type pheS gene (wtPheS). Additionally, five potentially strong promoters from S. suis were screened for their ability to drive high-level expression of mPheS, thus endowing the carrier strain with sufficient sensitivity to the phenylalanine analog p-chloro-phenylalanine (p-Cl-phe). Insertion of these P-mPheS cassettes into a vector or into the chromosomal locus via a linked erythromycin resistance gene revealed that mPheS allele driven by promoters P₀₅₃₀ and P₁₅₀₃ renders S. suis sensitive to as low as 0.01% (or 0.5 mM) of p-Cl-phe. This offers two potential CSMs for S. suis with p-Cl-phe as a counterselective agent. P₁₅₀₃-mPheS was revealed to be 100% efficient for counter-selection in S. suis by application in a precise gene deletion. Using P₁₅₀₃-mPheS as a CSM, a two-step insertion and excision strategy for markerless genetic manipulation of S. suis were developed, supplying a powerful tool for markerless genetic manipulation of S. suis.

The antimicrobial systems of Streptococcus suis promote niche competition in pig tonsils

Streptococcus suis can cause severe infections in pigs and humans. The tonsils of pigs are major niches for S. suis, and different serotypes of S. suis can be found in the same tonsil. Pig tonsil colonization by S. suis is believed to be an important source of infection for humans and pigs. However, how S. suis competes for a stable tonsil niche is unknown. Here, we found that S. suis strain WUSS351, isolated from a healthy pig tonsil, is virulent and multidrug-resistant. The ABC transporter system SstFEG, conferring resistance to bacitracin, was reported to confer a competitive survival advantage in vivo. In addition, strain WUSS351 has several antimicrobial systems, including a novel type VII secretion system (T7SS), lantibiotic bacteriocin, and lactococcin972-like bacteriocin Lcn351. Bacterial competition experiments demonstrated T7SS-mediated cell contact-dependent antagonism of S. suis. Antibacterial activity analysis and 16S rRNA gene sequencing of the culture-independent and culture-dependent pig tonsillar microbiome revealed that Lcn351 mainly targets S. suis, one of the core microbiomes in pig tonsils. Taken together, our results revealed the mechanism of the stable persistence of S. suis in the tonsil niche, which might have important implications for S. suis epidemiology, potentially influencing strain prevalence and disease progression.

Contribution of quorum sensing to virulence and antibiotic resistance in zoonotic bacteria

Quorum sensing (QS), which is a key part of cell/cell communication, is widely distributed in microorganisms, especially in bacteria. Bacteria can produce and detect the presence of QS signal molecule, perceive the composition and density of microorganisms in their complex habitat, and then dynamically regulate their own gene expression to adapt to their environment. Among the many traits controlled by QS in pathogenic bacteria is the expression of virulence factors and antibiotic resistance. Many pathogenic bacteria rely on QS to govern the production of virulence factors and express drug-resistance, especially in zoonotic bacteria. The threat of antibiotic resistant zoonotic bacteria has called for alternative antimicrobial strategies that would mitigate the increase of classical resistance mechanism. Targeting QS has proven to be a promising alternative to conventional antibiotic for controlling infections. Here we review the QS systems in common zoonotic pathogenic bacteria and outline how QS may control the virulence and antibiotic resistance of zoonotic bacteria.

XRE family transcriptional regulator XtrSs modulates Streptococcus suis fitness under hydrogen peroxide stress

Streptococcus suis is an important emerging zoonosis that causes economic losses in the pig industry and severe threats to public health. Transcriptional regulators play essential roles in bacterial adaptation to host environments. In this study, we identified a novel XRE family transcriptional regulator in S. suis CZ130302, XtrSs, involved in the bacterial fitness to hydrogen peroxide stress. Based on electrophoretic mobility shift and β-galactosidase activity assays, we found that XtrSs auto-regulated its own transcription and repressed the expression of its downstream gene psePs, a surface protein with unknown function in S. suis, by binding to a palindromic sequence from the promoter region. Furthermore, we proved that the deletion of the psePs gene attenuated bacterial antioxidant response. Phylogenetic analysis revealed that XtrSs and PsePs naturally co-existed as a combination in most S. suis genomes. Collectively, we demonstrated the binding characteristics of XtrSs in S. suis and provided a new insight that XtrSs played a critical role in modulating psePs to the hydrogen peroxide resistance of S. suis.

erm(T)-Mediated Macrolide-Lincosamide Resistance in Streptococcus suis

To investigate the presence and location of erm(T) in clinical Streptococcus suis isolates and explore the transmission ability and fitness cost of erm(T)-carrying mobile genetic elements among S. suis isolates, MICs were determined by broth microdilution. The presence of erm(T) in S. suis was detected by PCR. The genetic environment of erm(T) in S. suis was explored by whole-genome sequencing (WGS) analysis. Intraspecies and interspecies transmission were examined by electrotransformation. The fitness cost associated with the carriage of an erm(T)-harboring plasmid or an integrative and conjugative element (ICE) was examined by competition experiments. Of 237 nonduplicate strains, erm(T) was detected in 2 S. suis strains (SC262-ST954 and SC117-ST1314), with its location on a 5,125-bp plasmid in S. suis SC262 and on a 64,013-bp ICESsuSC117 in S. suis SC117, respectively. Both the erm(T)-carrying plasmid pSC262 and the ICESsuSC117 were transmissible by transformation. Plasmid pSC262 can replicate and express macrolide-lincosamide resistance in heterologous hosts, including S. aureus and S. pneumoniae. Both the erm(T)-carrying plasmid and the ICE posed a fitness cost to the host S. suis isolate. To the best of our knowledge, this is the first report of the macrolide-lincosamide-streptogramin B resistance gene erm(T) in S. suis. Its location on a plasmid or an ICE will aid in its transmission. The low detection rate of erm(T) gene among the S. suis population might be due to the fitness cost of the erm(T)-carrying plasmid and ICE.

IMPORTANCE Macrolide and lincosamide resistance due to the presence of erm(T) have posed a challenge for the treatment of Gram-positive pathogens. Although the low detection rate of erm(T) gene among the S. suis population due to the fitness cost of the erm(T)-carrying plasmid and ICE, the presence of erm(T) in S. suis and its potential transmission to other Gram-positive pathogens will be of important significance.

YSIRK-G/S-directed translocation is required for Streptococcus suis to deliver diverse cell wall anchoring effectors contributing to bacterial pathogenicity

The Streptococcus suis serotype 2 (SS2) is a significant zoonotic pathogen that is responsible for various swine diseases, even causing cytokine storms of Streptococcal toxic shock-like syndromes amongst human. Cell wall anchoring proteins with a C-terminal LPxTG are considered to play vital roles during SS2 infection; however, their exporting mechanism across cytoplasmic membranes has remained vague. This study found that YSIRK-G/S was involved in the exportation of LPxTG-anchoring virulence factors MRP and SspA in virulent SS2 strain ZY05719. The whole-genome analysis indicated that diverse LPxTG proteins fused with an N-terminal YSIRK-G/S motif are encoded in strain ZY05719. Two novel LPxTG proteins SspB and YzpA were verified to be exported via a putative transport system that was dependent on the YSIRK-G/S directed translocation, and portrayed vital functions during the infection of SS2 strain ZY05719. Instead of exhibiting an inactivation of C5a peptidase in SspB, another LPxTG protein with an N-terminal YSIRK-G/S motif from Streptococcus agalactiae was depicted to cleave the C5a component of the host complement. The consequent domain-architecture retrieval determined more than 10,000 SspB/YzpA like proteins that are extensively distributed in the Gram-positive bacteria, and most of them harbor diverse glycosyl hydrolase or peptidase domains within their middle regions, thus presenting their capability to interact with host cells. The said findings provide compelling evidence that LPxTG proteins with an N-terminal YSIRK-G/S motif are polymorphic effectors secreted by Gram-positive bacteria, which can be further proposed to define as cell wall anchoring effectors in a new subset.

Emergence of a tet(M) Variant Conferring Resistance to Tigecycline in Streptococcus suis

The aim of this study was to gain insight into the resistance determinants conferring resistance to tigecycline in Streptococcus (S.) suis and to investigate the genetic elements involved in their horizontal transfer. A total of 31 tetracycline-resistant S. suis isolates were screened for tigecycline resistance by broth microdilution. S. suis isolate SC128 was subjected to whole genome sequencing with particular reference to resistance determinants involved in tigecycline resistance. Transferability of genomic island (GI) GISsuSC128 was investigated by transformation. The roles of tet(L) or tet(M) in contributing to tigecycline resistance in S. suis were confirmed by transformation using different tet(L)- or tet(M)-carrying constructs. Only S. suis SC128 showed a tigecycline resistance phenotype. A tet(L)-tet(M) and catA8 co-carrying GISsuSC128 was identified in this isolate. After transfer of the novel GI into a susceptible recipient, this recipient showed the same tigecycline resistance phenotype. Further transfer experiments with specific tet(L)- or tet(M)-carrying constructs confirmed that only tet(M), but not tet(L), contributes to resistance to tigecycline. Protein sequence analysis identified a Tet(M) variant, which is responsible for tigecycline resistance in S. suis SC128. It displayed 94.8% amino acid identity with the reference Tet(M) of Enterococcus faecium DO plasmid 1. To the best of our knowledge, this is the first time that a tet(M) variant conferring resistance to tigecycline was identified in S. suis. Its location on a GI will accelerate its transmission among the S. suis population.

A SacB-based system for diverse and multiple genome editing in Gluconobacter oxydans

Gluconobacter oxydans is an important industrial bacterial strain widely used to produce a lot of useful products. However, very few gene editing tools are available for G. oxydans. This study aimed to develop an efficient genome editing method for G. oxydans using SacB as a counter-selectable marker. A plasmid that could express the kanamycin resistance gene in both E. coli and G. oxydans was constructed using the screened shuttle promoter P₁₁₆. After optimizing the genome editing conditions, the derivative plasmids could be effectively utilized for diverse genome editing, such as gene deletion, insertion, replacement, and in situ modification in G. oxydans WSH-003. In addition, the SacB-based system also achieved multiple gene editing in G. oxydans. Moreover, the genome of the industrial strain G. oxydans WSH-003 was modified and the growth rate and substrate conversion rate of the strain successfully increased using this system. The system could also have potential to be applied in different G. oxydans strains. The process established in this study also provides a reference for constructing genetic tools for many other genetically recalcitrant bacteria.

Comparative genetic analyses provide clues about capsule switching in Streptococcus suis 2 strains with different virulence levels and genetic backgrounds

Streptococcus suis (S. suis) is a major bacterial pathogen in the swine industry and an emerging zoonotic agent. S. suis produces an important extracellular component, capsular polysaccharide (CPS), based on which dozens of serotypes have been identified. Through virulence genotyping, we revealed the relatedness between subpopulations of S. suis serotype 2 (SS2), S. suis serotype 3 (SS3) and S. suis serotype 7 (SS7) strains despite their serotype differences. Multilocus sequence typing (MLST) was used to characterize the whole S. suis population and revealed capsule switching between S. suis strains. Importantly, capsule switching occurred in the SS2, SS3 and SS7 strains belonging to CC28 and CC29, which are phylogenetically distinct from the main CC1 SS2 lineage. To further explore capsule switching in S. suis, comparative genomic analyses were performed using available complete S. suis genomes. Phylogenetic analyses suggested that the SS2 strains could be divided into two clades (1 and 2), and those classified into clade 2 colocalized with SS3 and SS7 strains, in accordance with the above virulence genotyping and MLST analyses. Clade 2 SS2 strains presented high genetic similarity to SS3 and SS7 and shared common competence and defensive elements with them but were significantly different from Clade 1 SS2 strains. Notably, although the cps loci shared by Clade 1 and 2 SS2 strains were almost identical, a specific region of the cps locus of strain NSUI002 (Clade 2 SS2) could be found in the SS3 cps locus but not in the Clade 1 SS2 strain. These data indicated that the SS2 strains in CC28 and CC29 might have acquired the cps locus through capsule switching, which could explain the distinct genetic lineages within the SS2 population.

A Markerless Gene Deletion System in Streptococcus suis by Using the Copper-Inducible Vibrio parahaemolyticus YoeB Toxin as a Counterselectable Marker

Streptococcus suis is an important zoonotic pathogen causing severe infections in swine and humans. Induction of the Vibrio parahaemolyticus YoeB toxin in Escherichia coli resulted in cell death, leading to the speculation that YoeBVp can be a counterselectable marker. Herein, the counterselection potential of YoeBVp was assessed in S. suis. The yoeBVp gene was placed under the copper-induced promoter PcopA. The PcopA-yoeBVp construct was cloned into the S. suis-E. coli shuttle vector pSET2 and introduced into S. suis to assess the effect of YoeBVp expression on S. suis growth. Reverse transcription quantitative PCR showed that copper induced yoeBVp expression. Growth curve analyses and spot dilution assays showed that YoeBVp expression inhibited S. suis growth both in liquid media and on agar plates, revealing that YoeBVp has the potential to be a counterselectable marker for S. suis. A SCIY cassette comprising the spectinomycin-resistance gene and copper-induced yoeBVp was constructed. Using the SCIY cassette and peptide-induced competence, a novel two-step markerless gene deletion method was established for S. suis. Moreover, using the ΔperR mutant generated by this method, we demonstrated that PmtA, a ferrous iron and cobalt efflux pump in S. suis, was negatively regulated by the PerR regulator.

CrfP, a fratricide protein, contributes to natural transformation in Streptococcus suis

Streptococcus suis (S. suis) is an important zoonotic pathogen that causes septicaemia, meningitis and streptococcal toxic shock-like syndrome in its host, and recent studies have shown that S. suis could be competent for natural genetic transformation. Transformation is an important mechanism for the horizontal transfer of DNA, but some elements that affect the transformation process need to be further explored. Upon entering the competent state, Streptococcus species stimulate the transcription of competence-related genes that are responsible for exogenous DNA binding, uptake and processing. In this study, we performed conserved promoter motif and qRT-PCR analyses and identified CrfP as a novel murein hydrolase that is widespread in S. suis and stimulated with a peptide pheromone in the competent state through a process controlled by ComX. A bioinformatics analysis revealed that CrfP consists of a CHAP hydrolase domain and two bacterial Src homology 3-binding (SH3b) domains. Further characterization showed that CrfP could be exported to extracellular bacterial cells and lytic S. suis strains of different serotypes, and this finding was verified by TEM and a turbidity assay. To investigate the potential effect of CrfP in vivo, a gene-deletion mutant (ΔcrfP) was constructed. Instead of stopping the natural transformation process, the inactivation of CrfP clearly reduced the effective transformation rate. Overall, these findings provide evidence showing that CrfP is important for S. suis serovar 2 competence.

LytR plays a role in normal septum formation and contributes to full virulence in Streptococcus suis

Streptococcus suis (S. suis) is a major zoonotic pathogen and is also responsible for variety of diseases in swine. LytR-CpsA-Psr (LCP) family proteins affect the biofilm formation and virulence of some Gram-positive bacteria, but we know nothing about their roles in S. suis. In this study, we constructed the LytR mutant and its revertant strains by natural transformation and verified them by PCR and western blot. We explored the effects of LytR on the cell morphology of S. suis. Transmission electron microscopic analysis showed that the mutant strain displayed aberrant septum placement with no obvious differences in capsular thickness. Crystal violet staining and laser-scanning confocal microscopy both revealed that LytR contributes to the biofilm formation of S. suis. The LytR mutant strain had reduced survival in whole human blood and was more sensitive to killing by polymorphonuclear leukocytes (PMNs). Furthermore, in a mouse infection model, the LytR mutant strain also exhibited significantly attenuated virulence and was more easily cleared in the blood. These results indicate that the LytR protein is involved in septum placement, biofilm formation and required for full virulence of S. suis during infection.

Streptococcus suis Uptakes Carbohydrate Source from Host Glycoproteins by N-glycans Degradation System for Optimal Survival and Full Virulence during Infection

Infection with the epidemic virulent strain of Streptococcus suis serotype 2 (SS2) can cause septicemia in swine and humans, leading to pneumonia, meningitis and even cytokine storm of Streptococcal toxic shock-like syndrome. Despite some progress concerning the contribution of bacterial adhesion, biofilm, toxicity and stress response to the SS2 systemic infection, the precise mechanism underlying bacterial survival and growth within the host bloodstream remains elusive. Here, we reported the SS2 virulent strains with a more than 20 kb endoSS-related insertion region that showed significantly higher proliferative ability in swine serum than low-virulent strains. Further study identified a complete N-glycans degradation system encoded within this insertion region, and found that both GH92 and EndoSS contribute to bacterial virulence, but that only DndoSS was required for optimal growth of SS2 in host serum. The supplement of hydrolyzed high-mannose-containing glycoprotein by GH92 and EndoSS could completely restore the growth deficiency of endoSS deletion mutant in swine serum. EndoSS only hydrolyzed a part of the model glycoprotein RNase B with high-mannose N-linked glycoforms into a low molecular weight form, and the solo activity of GH92 could not show any changes comparing with the blank control in SDS-PAGE gel. However, complete hydrolyzation was observed under the co-incubation of EndoSS and GH92, suggesting GH92 may degrade the high-mannose arms of N-glycans to generate a substrate for EndoSS. In summary, these findings provide compelling evidences that EndoSS-related N-glycans degradation system may enable SS2 to adapt to host serum-specific availability of carbon sources from glycoforms, and be required for optimal colonization and full virulence during systemic infection.

Bacitracin resistance and enhanced virulence of Streptococcus suis via a novel efflux pump

Background

Streptococcus suis is a prominent pathogen causing septicemia and meningitis in swine and humans. Bacitracin is used widely as a growth promoter in animal feed and to control the spread of necrotic enteritis in most developing countries. This study aimed to characterize a novel membrane transporter module Sst comprising SstE, SstF, and SstG for bacitracin resistance.

Results

Comparative genomics and protein homology analysis found a potential efflux pump SstFEG encoded upstream of well-known bacitracin-resistance genes bceAB and bceRS. A four-fold decrease in bacitracin susceptibility was observed in sstFEG deletion mutant comparing with S. suis wildtype strain CZ130302. Further studies indicated that the bacitracin tolerance mediated by SstFEG is not only independent of the BceAB transporter, but also regulated by the two-component system BceSR. Given that SstFEG are harbored by almost all virulent strains, but not in the avirulent strains, we managed to explore its potential role in bacterial pathogencity. Indeed, our results showed that SstFEG is involved in S. suis colonization and virulence in animal infection model by its potential competitive survival advantage against host bactericidal effect.

Conclusion

To our knowledge, this is the first study to functionally characterize the bacitracin efflux pump in S. suis to provide evidence regarding the important roles of the novel ABC transporter system SstFEG with respect to drug resistance and virulence.

A novel autolysin AtlASS mediates bacterial cell separation during cell division and contributes to full virulence in Streptococcus suis

Streptococcus suis (SS) is a major pathogen in the swine industry, and also an important zoonotic agent for humans. The novel SS cell surface protein, AtlA_SS, comprising the special GW module and N-acetylmuramidases domain, was designated as a putative autolysin. Indeed, the atlA_SS deletion mutant almost completely lost its activity in Triton X-100 induced bacterial autolysis, while the wild-type and CΔatlA_SS strains showed significant decrease, to less than 20% of the initial OD₆₀₀ values. Unexpectedly, both immunofluorescence and immunogold electron microscopy confirmed that AtlA_SS is mainly located in the cell division septum, suggesting autolytic activity in peptidoglycan hydrolysis may be required for cell separation, thus modulating and truncating bacterial chain length. The biofilm capacity of the AtlA_SS mutation was reduced ˜ 40%, as compared to the wild-type strain. The ΔatlA_SS strain also attenuated bacterial adherence in human brain microvessel endothelial cells (HBMECs). Furthermore, we confirmed that AtlA_SS has fibrinogen/fibronectin binding capacities. In mouse infection model, the AtlA_SS inactivation also significantly attenuated bacterial virulence and proliferation in vivo. In conclusion, these results indicate that AtlA_SS autolysin modulates bacterial chain length, and contributes to the full virulence of SS during infection.