Expression of green fluorescent protein and its application in pathogenesis studies of serotype 2 Streptococcus suis

Transcriptome and metabolome profiling to elucidate the mechanism underlying the poor growth of Streptococcus suis serotype 2 after orphan response regulator CovR deletion

The deletion of orphan response regulator CovR reduces the growth rate of Streptococcus suis serotype 2 (S. suis 2). In this study, metabolome and transcriptome profiling were performed to study the mechanisms underlying the poor growth of S. suis 2 caused by the deletion of orphan response regulator CovR. By comparing S. suis 2 (ΔcovR) and S. suis 2 (SC19), 146 differentially accumulated metabolites (upregulated: 83 and downregulated: 63) and 141 differentially expressed genes (upregulated: 86 and downregulated: 55) were identified. Metabolome and functional annotation analysis revealed that the growth of ΔcovR was inhibited by the imbalance aminoacyl tRNA biosynthesis (the low contents of L-lysine, L-aspartic acid, L-glutamine, and L-glutamic acid, and the high content of L-methionine). These results provide a new insight into the underlying poor growth of S. suis 2 caused by the deletion of orphan response regulator CovR. Metabolites and candidate genes regulated by the orphan response regulator CovR and involved in the growth of S. suis 2 were reported in this study.

Neutrophils in Streptococcus suis Infection: From Host Defense to Pathology

Streptococcus suis is a swine pathogen and zoonotic agent responsible for economic losses to the porcine industry. Infected animals may develop meningitis, arthritis, endocarditis, sepsis and/or sudden death. The pathogenesis of the infection implies that bacteria breach mucosal host barriers and reach the bloodstream, where they escape immune-surveillance mechanisms and spread throughout the organism. The clinical manifestations are mainly the consequence of an exacerbated inflammation, defined by an exaggerated production of cytokines and recruitment of immune cells. Among them, neutrophils arrive first in contact with the pathogens to combat the infection. Neutrophils initiate and maintain inflammation, by producing cytokines and deploying their arsenal of antimicrobial mechanisms. Furthermore, neutrophilic leukocytosis characterizes S. suis infection, and lesions of infected subjects contain a large number of neutrophils. Therefore, this cell type may play a role in host defense and/or in the exacerbated inflammation. Nevertheless, a limited number of studies addressed the role or functions of neutrophils in the context of S. suis infection. In this review, we will explore the literature about S. suis and neutrophils, from their interaction at a cellular level, to the roles and behaviors of neutrophils in the infected host in vivo.

Colonization of the Intestinal Tract of the Polyphagous Pest Spodoptera littoralis with the GFP-Tagged Indigenous Gut Bacterium Enterococcus mundtii

The alkaline gut of Lepidopterans plays a crucial role in shaping communities of bacteria. Enterococcus mundtii has emerged as one of the predominant gut microorganisms in the gastrointestinal tract of the major agricultural pest, Spodoptera littoralis. Therefore, it was selected as a model bacterium to study its adaptation to harsh alkaline gut conditions in its host insect throughout different stages of development (larvae, pupae, adults, and eggs). To date, the mechanism of bacterial survival in insects' intestinal tract has been unknown. Therefore, we have engineered a GFP-tagged species of bacteria, E. mundtii, to track how it colonizes the intestine of S. littoralis. Three promoters of different strengths were used to control the expression of GFP in E. mundtii. The promoter ermB was the most effective, exhibiting the highest GFP fluorescence intensity, and hence was chosen as our main construct. Our data show that the engineered fluorescent bacteria survived and proliferated in the intestinal tract of the insect at all life stages for up to the second generation following ingestion.

Identification of genes preferentially expressed by highly virulent piscine Streptococcus agalactiae upon interaction with macrophages

Streptococcus agalactiae, long recognized as a mammalian pathogen, is an emerging concern with regard to fish. In this study, we used a mouse model and in vitro cell infection to evaluate the pathogenetic characteristics of S. agalactiae GD201008-001, isolated from tilapia in China. This bacterium was found to be highly virulent and capable of inducing brain damage by migrating into the brain by crossing the blood–brain barrier (BBB). The phagocytosis assays indicated that this bacterium could be internalized by murine macrophages and survive intracellularly for more than 24 h, inducing injury to macrophages. Further, selective capture of transcribed sequences (SCOTS) was used to investigate microbial gene expression associated with intracellular survival. This positive cDNA selection technique identified 60 distinct genes that could be characterized into 6 functional categories. More than 50% of the differentially expressed genes were involved in metabolic adaptation. Some genes have previously been described as associated with virulence in other bacteria, and four showed no significant similarities to any other previously described genes. This study constitutes the first step in further gene expression analyses that will lead to a better understanding of the molecular mechanisms used by S. agalactiae to survive in macrophages and to cross the BBB.

Construction and characterization of a Streptococcus suis serotype 2 recombinant expressing enhanced green fluorescent protein

Streptococcus suis serotype 2 (S. suis 2) is an important pathogen, responsible for diverse diseases in swine and humans. To obtain a S. suis 2 strain that can be tracked in vitro and in vivo, we constructed the Egfp-HA9801 recombinant S. suis 2 strain with egfp and spc^r genes inserted via homologous recombination. To assess the effects of the egfp and spc^r genes in HA9801, the biochemical characteristics, growth features and virulence in Balb/C mice were compared between the recombinant and the parent HA9801 strain. We detected the EGFP expression from Egfp-HA9801 by epifluorescence microscopy. The results showed that the biochemical characterization and growth features of the Egfp-HA9801 recombinant were highly similar to that of the parent HA9801. We did not find significant differences in lethality (50% lethal dose), morbidity and mortality between the two strains. Furthermore, the bacterial counts in each various tissues of Egfp-HA9801-infected mice displayed similar dynamic compared with the HA9801-infected mice. Our results also showed that the Egfp-HA9801 cells grown at 37°C for 36 h displayed greater green fluorescence signals than the cells grown at 28°C for 36 h and 37°C for 24 h. The fluorescence in the tissue cryosections of Egfp-HA9801-injected mice was also stronger than that of the HA9801 group. Together, these results indicate that the egfp and spc^r insertions into the Egfp-HA9801 recombinant did not significantly change the virulence when compared with HA980, and this EGFP labeled strain can be used for future S. suis 2 pathogenesis research.

High-level fluorescence labeling of gram-positive pathogens

Fluorescence labeling of bacterial pathogens has a broad range of interesting applications including the observation of living bacteria within host cells. We constructed a novel vector based on the E. coli streptococcal shuttle plasmid pAT28 that can propagate in numerous bacterial species from different genera. The plasmid harbors a promoterless copy of the green fluorescent variant gene egfp under the control of the CAMP-factor gene (cfb) promoter of Streptococcus agalactiae and was designated pBSU101. Upon transfer of the plasmid into streptococci, the bacteria show a distinct and easily detectable fluorescence using a standard fluorescence microscope and quantification by FACS-analysis demonstrated values that were 10-50 times increased over the respective controls. To assess the suitability of the construct for high efficiency fluorescence labeling in different gram-positive pathogens, numerous species were transformed. We successfully labeled Streptococcus pyogenes, Streptococcus agalactiae, Streptococcus dysgalactiae subsp. equisimilis, Enterococcus faecalis, Enterococcus faecium, Streptococcus mutans, Streptococcus anginosus and Staphylococcus aureus strains utilizing the EGFP reporter plasmid pBSU101. In all of these species the presence of the cfb promoter construct resulted in high-level EGFP expression that could be further increased by growing the streptococcal and enterococcal cultures under high oxygen conditions through continuous aeration.

Molecular cloning and analysis of the UDP-Glucose Pyrophosphorylase in Streptococcus equi subsp. zooepidemicus

UDP-Glucose Pyrophosphorylase (EC 2.7.7.9, UGPase) plays an important role in Streptococcus equi subsp. zooepidemicus (S. zooepidemicus) cell envelope Hyaluronic acid (HA) biosynthesis and it is also recognized as a virulence determinant in several bacterial species. HA is valuable biopolymer used in the pharmaceutical and cosmetic industry. In addition, encapsulation by HA is considered an important virulence factor in other streptococci. Research UGPase will contribute to the vaccine development of S. zooepidemicus and the production of HA. In this study, The UGPase gene fragment (789 bp) obtained from previous research was amplified using PCR, and located by Genome walking technology (Genebank No.GQ423507). The UGPase was expressed, purified and identified using UGPase antibody. The enzyme kinetic parameters were determined, the temperature and pH of the highest activity for the cloned UGPase were 37°C, pH 7.5. The Km and Kcat value against UTP and G-1-P was 8.5 μM, 69.05 s(-1) and 36.41 μM, 48.81 s(-1), respectively. The homology-modeling was operated. Overexpression of the UGPase in S. zooepidemicus, its virulence was slightly affected, and HA yield reduced. Real-time PCR was carried out to determine the UGPase expression levels of both SEZp and SEZugp in different grow period, the level is high in logarithmic phase and low in Decline phase.

GI-type T4SS-mediated horizontal transfer of the 89K pathogenicity island in epidemic Streptococcus suis serotype 2

Pathogenicity islands (PAIs), a distinct type of genomic island (GI), play important roles in the rapid adaptation and increased virulence of pathogens. 89K is a newly identified PAI in epidemic Streptococcus suis isolates that are related to the two recent large-scale outbreaks of human infection in China. However, its mechanism of evolution and contribution to the epidemic spread of S. suis 2 remain unknown. In this study, the potential for mobilization of 89K was evaluated, and its putative transfer mechanism was investigated. We report that 89K can spontaneously excise to form an extrachromosomal circular product. The precise excision is mediated by an 89K-borne integrase through site-specific recombination, with help from an excisionase. The 89K excision intermediate acts as a substrate for lateral transfer to non-89K S. suis 2 recipients, where it reintegrates site-specifically into the target site. The conjugal transfer of 89K occurred via a GI type IV secretion system (T4SS) encoded in 89K, at a frequency of 10(-6) transconjugants per donor. This is the first demonstration of horizontal transfer of a Gram-positive PAI mediated by a GI-type T4SS. We propose that these genetic events are important in the emergence, pathogenesis and persistence of epidemic S. suis 2 strains.

Single-cell techniques using chromosomally tagged fluorescent bacteria to study Listeria monocytogenes infection processes

Listeria monocytogenes is a Gram-positive facultative intracellular pathogen which invades different cell types, including nonphagocytic cells, where it is able to replicate and survive. The different steps of the cellular infectious process have been well described and consist of bacterial entry, lysis of the endocytic vacuole, intracellular replication, and spreading to neighboring cells. To study the listerial infectious process, gentamicin survival assays, plaque formation, and direct microscopy observations are typically used; however, there are some caveats with each of these techniques. In this study we describe new single-cell techniques based on use of an array of integrative fluorescent plasmids (green, cyan, and yellow fluorescent proteins) to easily, rapidly, and quantitatively detect L. monocytogenes in vitro and in vivo. We describe construction of 13 integrative and multicopy plasmids which can be used for detecting intracellular bacteria, for measuring invasion, cell-to-cell spreading, and intracellular replication, for monitoring in vivo infections, and for generating transcriptional or translational reporters. Furthermore, we tested these plasmids in a variety of epifluorescence- and flow cytometry-based assays. We showed that we could (i) determine the expression of a particular promoter during the cell cycle, (ii) establish in one rapid experiment at which step in the cell cycle a particular mutant is defective, and (iii) easily measure the number of infected cells in vitro and in mouse organs. The plasmids that are described and the methods to detect them are new powerful tools to study host-Listeria interactions in a fast, robust, and high-throughput manner.

The orphan response regulator CovR: a globally negative modulator of virulence in Streptococcus suis serotype 2

Streptococcus suis serotype 2 is an emerging zoonotic pathogen responsible for a wide range of life-threatening diseases in pigs and humans. However, the pathogenesis of S. suis serotype 2 infection is not well understood. In this study, we report that an orphan response regulator, CovR, globally regulates gene expression and negatively controls the virulence of S. suis 05ZYH33, a streptococcal toxic shock syndrome (STSS)-causing strain. A covR-defective (DeltacovR) mutant of 05ZYH33 displayed dramatic phenotypic changes, such as formation of longer chains, production of thicker capsules, and increased hemolytic activity. Adherence of the DeltacovR mutant to epithelial cells was greatly increased, and its resistance to phagocytosis and killing by neutrophils and monocytes was also significantly enhanced. More importantly, inactivation of covR increased the lethality of S. suis serotype 2 in experimental infection of piglets, and this phenotype was restored by covR complementation. Colonization experiments also showed that the DeltacovR mutant exhibited an increased ability to colonize susceptible tissues of piglets. The pleiotropic phenotype of the DeltacovR mutant is in full agreement with the large number of genes controlled by CovR as revealed by transcription profile analysis: 2 genes are positively regulated, and 193 are repressed, including many that encode known or putative virulence factors. These findings suggested that CovR is a global repressor in virulence regulation of STSS-causing S. suis serotype 2.

Characterization of strong promoters from an environmental Flavobacterium hibernum strain by using a green fluorescent protein-based reporter system

We developed techniques for the genetic manipulation of Flavobacterium species and used it to characterize several promoters found in these bacteria. Our studies utilized Flavobacterium hibernum strain W22, an environmental strain we isolated from tree hole habitats of mosquito larvae. Plasmids from F. hibernum strain W22 were more efficiently (approximately 1,250-fold) transferred by electroporation into F. hibernum strain W22 than those isolated from Escherichia coli, thus indicating that an efficient restriction barrier exists between these species. The strong promoter, tac, functional in proteobacteria, did not function in Flavobacterium strains. Therefore, a promoter-trap plasmid, pSCH03, containing a promoterless gfpmut3 gene was constructed. A library of 9,000 clones containing chromosomal fragments of F. hibernum strain W22 in pSCH03 was screened for their ability to drive expression of the promoterless gfpmut3 gene. Twenty strong promoters were used for further study. The transcription start points were determined from seven promoter clones by the 5' rapid amplification of cDNA ends technique. Promoter consensus sequences from Flavobacterium were identified as TAnnTTTG and TTG, where n is any nucleotide, centered approximately 7 and 33 bp upstream of the transcription start site, respectively. A putative novel ribosome binding site consensus sequence is proposed as TAAAA by aligning the 20-bp regions upstream of the translational start site in 25 genes. Our primary results demonstrate that at least some promoter and ribosome binding site motifs of Flavobacterium strains are unusual within the bacterial domain and suggest an early evolutionary divergence of this bacterial group. The techniques presented here allow for more detailed genetics-based studies and analyses of Flavobacterium species in the environment.

Construction of a multiple fluorescence labelling system for use in co-invasion studies of Listeria monocytogenes

BACKGROUND

Existing virulence models are often difficult to apply for quantitative comparison of invasion potentials of Listeria monocytogenes. Well-to-well variation between cell-line based in vitro assays is practically unavoidable, and variation between individual animals is the cause of large deviations in the observed capacity for infection when animal models are used. One way to circumvent this problem is to carry out virulence studies as competition assays between 2 or more strains. This, however, requires invasion-neutral markers that enable easy discrimination between the different strains.

RESULTS

A fluorescent marker system, allowing visualization and identification of single L. monocytogenes cells as well as colonies in a non-destructive manner, was developed. Five different fluorescent labels are available, and allowed simultaneous visual discrimination between three differently labelled strains at the single cell level by use of fluorescence microscopy. More than 90% of the L. monocytogenes host cells maintained the fluorescence tags for 40 generations. The fluorescence tags did not alter the invasive capacity of the L. monocytogenes cells in a traditional Caco-2 cell invasion assay, and visual discrimination between invaded bacteria carrying different fluorescent labels inside the cells was possible.

CONCLUSION

The constructed fluorescent marker system is stable, easy to use, does not affect the virulence of L. monocytogenes in Caco-2 cell assays, and allows discrimination between differently labelled bacteria after internalization in these cells.

Phagocytosis and killing of Streptococcus suis by porcine neutrophils

Streptococcus suis serotype 2 is an important swine pathogen responsible for diverse infections, mainly meningitis. Virulence factors and the pathogenesis of infection are not well understood. Neutrophils may play an important role in the pathogenesis of infection given that infiltration by neutrophils and mononuclear cells are frequently observed in lesions caused by S. suis. The objective of this work was to study the interactions between S. suis serotype 2 and porcine neutrophils. Results showed that suilysin is toxic to neutrophils and this could help S. suis evade innate immunity. Moreover, suilysin appears to affect complement-dependent killing by decreasing the opsonization of S. suis and the bactericidal capacity of neutrophils. Our results confirm that capsule polysaccharide protects S. suis against killing and phagocytosis by neutrophils. We also showed that the presence of specific IgG against S. suis serotype 2 promoted killing by neutrophils, indicating that the induction of a strong humoral response is beneficial for clearance of this pathogen.

Helicobacter pylori SabA adhesin evokes a strong inflammatory response in human neutrophils which is down-regulated by the neutrophil-activating protein

The human pathogen Helicobacter pylori expresses two dominant adhesins; the Lewis b blood group antigen binding adhesin, BabA, and the sialic acid-binding adhesin, SabA. These adhesins recognize specific carbohydrate moieties of the gastric epithelium, i.e. the Lewis b antigen, Le(b), and the sialyl-Lewis x antigen, sLe(x), respectively, which promote infection and inflammatory processes in the gastroduodenal tract. To assess the contribution of each of BabA, SabA and the neutrophil activating protein (HP-NAP) in a local inflammation, we investigated the traits of H. pylori mutants in their capacity to interact with and stimulate human neutrophils. We thence found that the SabA adhesin was not only the key inducer of oxidative metabolism (Unemo et al. J Biol Chem 280:15390-15397, 2005), but also essential in phagocytosis induction, as evaluated by flow cytometry, fluorescence microscopy and luminol-enhanced chemiluminescence. The napA deletion resulted in enhanced generation of reactive oxygen species and impaired adherence to the host cells. In conclusion, the SabA adhesin stimulates human neutrophils through selectin-mimicry. Interestingly, HP-NAP modulates the oxidative burst, which could tune the impact of the H. pylori infection for establishment of balanced and chronic inflammation of the gastric mucosa.

Putative mannose-specific phosphotransferase system component IID represses expression of suilysin in serotype 2 Streptococcus suis

In this study, we generated a genomic mutant library from a North American strain of serotype 2 Streptococcus suis using the pGh9:ISS1 transposition vector. Suilysin is the hemolysin made by S. suis. A hyper-hemolytic mutant was identified by screening for hemolytic phenotype using media with human blood. The hyper-hemolytic phenotype was characterised by a quantitative hemolysis microplate method. The use of green fluorescent protein (GFP) as a reporter also showed that suilysin gene expression was greater in the mutant. DNA sequence analysis of 3.8 kb surrounding the ISS1 insertion site revealed four open reading frames (ORFs) with three consecutive ORFs that belong to a putative mannose-specific phosphotransferase system (PTS). The S. suis gene homologous to mannose permease IID, manN, was interrupted by the transposon. A complementation test showed that manN repressed the expression of suilysin and the absence of manN was responsible for the hyper-hemolytic phenotype. However, both wild type and isogenic hyper-hemolytic mutant S. suis fermented mannose, glucose and lactose. Thus, despite its potential roles in carbohydrate transport, phosphorylation and metabolism, the manN homologue in the putative mannose-specific PTS regulates gene expression in S. suis.