Method and parameters for genetic transformation of Streptococcus sanguis Challis

Spatiotemporal monitoring of a periodontal multispecies biofilm model: demonstration of prebiotic treatment responses

Biofilms are complex polymicrobial communities which are often associated with human infections such as the oral disease periodontitis. Studying these complex communities under controlled conditions requires in vitro biofilm model systems that mimic the natural environment as close as possible. This study established a multispecies periodontal model in the drip flow biofilm reactor in order to mimic the continuous flow of nutrients at the air-liquid interface in the oral cavity. The design is engineered to enable real-time characterization. A community of five bacteria, Streptococcus gordonii-GFPmut3*, Streptococcus oralis-GFPmut3*, Streptococcus sanguinis-pVMCherry, Fusobacterium nucleatum, and Porphyromonas gingivalis-SNAP26 is visualized using two distinct fluorescent proteins and the SNAP-tag. The biofilm in the reactor develops into a heterogeneous, spatially uniform, dense, and metabolically active biofilm with relative cell abundances similar to those in a healthy individual. Metabolic activity, structural features, and bacterial composition of the biofilm remain stable from 3 to 6 days. As a proof of concept for our periodontal model, the 3 days developed biofilm is exposed to a prebiotic treatment with L-arginine. Multifaceted effects of L-arginine on the oral biofilm were validated by this model setup. L-arginine showed to inhibit growth and incorporation of the pathogenic species and to reduce biofilm thickness and volume. Additionally, L-arginine is metabolized by Streptococcus gordonii-GFPmut3* and Streptococcus sanguinis-pVMCherry, producing high levels of ornithine and ammonium in the biofilm. In conclusion, our drip flow reactor setup is promising in studying spatiotemporal behavior of a multispecies periodontal community.ImportancePeriodontitis is a multifactorial chronic inflammatory disease in the oral cavity associated with the accumulation of microorganisms in a biofilm. Not the presence of the biofilm as such, but changes in the microbiota (i.e., dysbiosis) drive the development of periodontitis, resulting in the destruction of tooth-supporting tissues. In this respect, novel treatment approaches focus on maintaining the health-associated homeostasis of the resident oral microbiota. To get insight in dynamic biofilm responses, our research presents the establishment of a periodontal biofilm model including Streptococcus gordonii, Streptococcus oralis, Streptococcus sanguinis, Fusobacterium nucleatum, and Porphyromonas gingivalis. The added value of the model setup is the combination of simulating continuously changing natural mouth conditions with spatiotemporal biofilm profiling using non-destructive characterization tools. These applications are limited for periodontal biofilm research and would contribute in understanding treatment mechanisms, short- or long-term exposure effects, the adaptation potential of the biofilm and thus treatment strategies.

Development of a Streptococcus gordonii vaccine strain expressing Schistosoma japonicum Sj-F1 and evaluation of using this strain for intranasal immunization in mice

Schistosomiasis is a worldwide parasitic disease. Currently, chemotherapy is the main effective method to treat schistosomiasis; however, it does not prevent reinfection. No effective vaccine is currently available to prevent schistosomiasis. Sj-F1 (GenBank accession number AY261995) is a novel gene that was discovered through screening adult Schistosoma japonicum worm cDNA library with female S. japonicum antigen-immunized sera. Streptococcus gordonii, a normal inhabitant of the human oral cavity, has been a prime candidate in recent investigations toward developing a live oral vaccine vector. One of the approaches for the surface expression of heterologous antigens in S. gordonii is to surface-localize them with the M6 protein from Streptococcus pyogenes. Here, we develop a recombinant S. gordonii strain that expresses the M6-Sj-F1 fusion protein on the bacterial surface. Intranasal immunization in mice with such M6-Sj-F1-expressing S. gordonii bacteria induced strong serum IgG, serum IgA, and saliva IgA against Sj-F1. The results of protective immunity against a challenge with cercariae of S. japonicum showed statistically significant protection following this treatment, with a worm reduction rate of 21.45% and an egg reduction rate of 34.77%. Our data indicate that the described M6-Sj-F1-expressing S. gordonii is highly immunogenic and can partially protect mice from challenge infection with S. japonicum. Intranasal immunization with recombinant S. gordonii may be an alternative to developing a novel S. japonicum vaccine in a safe, effective, and feasible way.

Regulation of neuraminidase expression in Streptococcus pneumoniae

Background

Sialic acid (N-acetylneuraminic acid; NeuNAc) is one of the most important carbohydrates for Streptococcus pneumoniae due of its role as a carbon and energy source, receptor for adhesion and invasion and molecular signal for promotion of biofilm formation, nasopharyngeal carriage and invasion of the lung.

Results

In this work, NeuNAc and its metabolic derivative N-acetyl mannosamine (ManNAc) were used to analyze regulatory mechanisms of the neuraminidase locus expression. Genomic and metabolic comparison to Streptococcus mitis, Streptococcus oralis, Streptococcus gordonii and Streptococcus sanguinis elucidates the metabolic association of the two amino sugars to different parts of the locus coding for the two main pneumococcal neuraminidases and confirms the substrate specificity of the respective ABC transporters. Quantitative gene expression analysis shows repression of the locus by glucose and induction of all predicted transcriptional units by ManNAc and NeuNAc, each inducing with higher efficiency the operon encoding for the transporter with higher specificity for the respective amino sugar. Cytofluorimetric analysis demonstrated enhanced surface exposure of NanA on pneumococci grown in NeuNAc and ManNAc and an activity assay allowed to quantify approximately twelve times as much neuraminidase activity on induced cells as opposed to glucose grown cells.

Conclusions

The present data increase the understanding of metabolic regulation of the nanAB locus and indicate that experiments aimed at the elucidation of the relevance of neuraminidases in pneumococcal virulence should possibly not be carried out on bacteria grown in glucose containing media.

A single mutation in enzyme I of the sugar phosphotransferase system confers penicillin tolerance to Streptococcus gordonii

Tolerance is a poorly understood phenomenon that allows bacteria exposed to a bactericidal antibiotic to stop their growth and withstand drug-induced killing. This survival ability has been implicated in antibiotic treatment failures. Here, we describe a single nucleotide mutation (tol1) in a tolerant Streptococcus gordonii strain (Tol1) that is sufficient to provide tolerance in vitro and in vivo. It induces a proline-to-arginine substitution (P483R) in the homodimerization interface of enzyme I of the sugar phosphotransferase system, resulting in diminished sugar uptake. In vitro, the susceptible wild-type (WT) and Tol1 cultures lost 4.5 and 0.6 log(10) CFU/ml, respectively, after 24 h of penicillin exposure. The introduction of tol1 into the WT (WT P483R) conferred tolerance (a loss of 0.7 log(10) CFU/ml/24 h), whereas restitution of the parent sequence in Tol1 (Tol1 R483P) restored antibiotic susceptibility. Moreover, penicillin treatment of rats in an experimental model of endocarditis showed a complete inversion in the outcome, with a failure of therapy in rats infected with WT P483R and the complete disappearance of bacteria in animals infected with Tol1 R483P.

Primary activation of antigen-specific naive CD4+ and CD8+ T cells following intranasal vaccination with recombinant bacteria

The primary activation of T-helper and T-cytotoxic cells following mucosal immunization with recombinant Streptococcus gordonii was studied in vivo by adoptive transfer of ovalbumin (OVA)-specific transgenic CD8(+) (OT-I) and CD4(+) (OT-II) T cells. A recombinant strain, expressing on the surface the vaccine antigen Ag85B-ESAT-6 from Mycobacterium tuberculosis fused to OVA T-helper and T-cytotoxic epitopes (peptides 323 to 339 and 257 to 264), was constructed and used to immunize C57BL/6 mice by the intranasal route. Recombinant, but not wild-type, bacteria induced OVA-specific CD4(+) and CD8(+) T-cell clonal expansion in cervical lymph nodes, lung, and spleen. OVA-specific CD4(+) and CD8(+) T-cell proliferation appeared first in cervical lymph nodes and later in the spleen, suggesting a possible migration of activated cells from the inductive site to the systemic district. A significant correlation between the percentages of CD4(+) and CD8(+) proliferating T cells was observed for each animal. The expression of CD69, CD44, and CD45RB on proliferating T lymphocytes changed as a function of the cell division number, confirming T-cell activation following the antigen encounter. These data indicate that intranasal immunization with recombinant S. gordonii is capable of inducing primary activation of naive antigen-specific CD4(+) and CD8(+) T cells, both locally and systemically.

Fitness cost and impaired survival in penicillin-resistant Streptococcus gordonii isolates selected in the laboratory

Cycling of Streptococcus gordonii (115 times) with penicillin resulted in a MIC increase of more than 100-fold, from 0.008 to 2 microg/ml. The 2-microg/ml MIC maximum was already reached after 36 passages but resulted in impaired fitness. Although the MIC did not increase further, fitness was partially recovered during the 79 additional cycles.

Comparison of transformation protocols in Streptococcus gordonii and evaluation of native promoter strength using a multiple-copy plasmid

An active area of research in the development of Streptococcus gordonii for use as a bacterial commensal vector involves the identification and utilization of strong promoters for high-level expression of heterologous products. Escherichia coli plasmid vectors containing different streptococcal promoters often fail to become established in E. coli for unknown reasons. Therefore, it is desirable at times to transform S. gordonii, which is naturally competent, with small quantities of nascently ligated DNA without using E. coli first to amplify or screen the product. By comparing the efficiency of two methods used to induce competence in S. gordonii, it was shown that the use of a synthetic competence stimulating peptide substantially enhanced plasmid uptake by S. gordonii. We amplified the amylase-binding protein (abpA) promoter from the S. gordonii genome and, using a synthetic peptide to induce competence, directly introduced plasmid DNA containing this promoter into S. gordonii as an unamplified product of ligation. This plasmid facilitated abundant secretion of a heterologous product by S. gordonii. By assessing the levels of heterologous product secreted by two plasmid constructs, it was possible to evaluate the relative strength of two native promoters.

Promoter and transcription analysis of penicillin-binding protein genes in Streptococcus gordonii

An optimally cross-linked peptidoglycan requires both transglycosylation and transpeptidation, provided by class A and class B penicillin-binding proteins (PBPs). Streptococcus gordonii possesses three class A PBPs (PBPs 1A, 1B, and 2A) and two class B PBPs (PBPs 2B and 2X) that are important for penicillin resistance. High-level resistance (MIC, > or =2 microg/ml) requires mutations in class B PBPs. However, although unmutated, class A PBPs are critical to facilitate resistance development (M. Haenni and P. Moreillon, Antimicrob. Agents Chemother. 50:4053-4061, 2006). Thus, their overexpression might be important to sustain the drug. Here, we determined the promoter regions of the S. gordonii PBPs and compared them to those of other streptococci. The extended -10 box was highly conserved and complied with a sigma(A)-type promoter consensus sequence. In contrast, the -35 box was poorly conserved, leaving the possibility of differential PBP regulation. Gene expression in a penicillin-susceptible parent (MIC, 0.008 microg/ml) and a high-level-resistant mutant (MIC, 2 microg/ml) was monitored using luciferase fusions. In the absence of penicillin, all PBPs were constitutively expressed, but their expression was globally increased (1.5 to 2 times) in the resistant mutant. In the presence of penicillin, class A PBPs were specifically overexpressed both in the parent (PBP 2A) and in the resistant mutant (PBPs 1A and 2A). By increasing transglycosylation, class A PBPs could promote peptidoglycan stability when transpeptidase is inhibited by penicillin. Since penicillin-related induction of class A PBPs occurred in both susceptible and resistant cells, such a mutation-independent facilitating mechanism could be operative at each step of resistance development.

Effects of alpha-phosphoglucomutase deficiency on cell wall properties and fitness in Streptococcus gordonii

Streptococcus gordonii alpha-phosphoglucomutase, which converts glucose 6-phosphate to glucose 1-phosphate, is encoded by pgm. The pgm transcript is monocistronic and is initiated from a sigma(A)-like promoter. Mutants with a gene disruption in pgm exhibited an altered cell wall muropeptide pattern and a lower teichoic acid content, and had reduced fitness both in vitro and in vivo. In vitro, the reduced fitness included reduced growth, reduced viability in the stationary phase and increased autolytic activity. In vivo, the pgm-deficient strain had a lower virulence in a rat model of experimental endocarditis.

Loss of penicillin tolerance by inactivating the carbon catabolite repression determinant CcpA in Streptococcus gordonii

OBJECTIVES

Antibiotic tolerance is a phenomenon allowing bacteria to withstand drug-induced killing. Here, we studied a penicillin-tolerant mutant of Streptococcus gordonii (Tol1), which was shown to be deregulated in the expression of the arginine deiminase operon (arc). arc was not directly responsible for tolerance, but is controlled by the global regulator CcpA. Therefore, we sought whether CcpA might be implicated in tolerance.

METHODS

The ccpA gene was characterized and subsequently inactivated by PCR ligation mutagenesis in both the susceptible wild-type (WT) and Tol1. The minimal inhibitory concentration and time-kill curves for the strains were determined and the outcome of penicillin treatment in experimental endocarditis assessed.

RESULTS

ccpA sequence and expression were similar between the WT and Tol1 strains. In killing assays, the WT lost 3.5 +/- 0.6 and 5.3 +/- 0.6 log(10) cfu/mL and Tol1 lost 0.4 +/- 0.2 and 1.4 +/- 0.9 log(10) cfu/mL after 24 and 48 h of penicillin exposure, respectively. Deletion of ccpA almost totally restored Tol1 kill susceptibility (loss of 2.5 +/- 0.7 and 4.9 +/- 0.7 log(10) cfu/mL at the same endpoints). In experimental endocarditis, penicillin treatment induced a significant reduction in vegetation bacterial densities between Tol1 (4.1 log(10) cfu/g) and Tol1DeltaccpA (2.4 log(10) cfu/g). Restitution of ccpA re-established the tolerant phenotype both in vitro and in vivo.

CONCLUSIONS

CcpA, a global regulator of the carbon catabolite repression system, is implicated in penicillin tolerance both in vitro and in vivo. This links antibiotic survival to bacterial sugar metabolism. However, since ccpA sequence and expression were similar between the WT and Tol1 strains, other factors are probably involved in tolerance.

Mutations in penicillin-binding protein (PBP) genes and in non-PBP genes during selection of penicillin-resistant Streptococcus gordonii

Penicillin resistance in Streptococcus spp. involves multiple mutations in both penicillin-binding proteins (PBPs) and non-PBP genes. Here, we studied the development of penicillin resistance in the oral commensal Streptococcus gordonii. Cyclic exposure of bacteria to twofold-increasing penicillin concentrations selected for a progressive 250- to 500-fold MIC increase (from 0.008 to between 2 and 4 microg/ml). The major MIC increase (> or = 35-fold) was related to non-PBP mutations, whereas PBP mutations accounted only for a 4- to 8-fold additional increase. PBP mutations occurred in class B PBPs 2X and 2B, which carry a transpeptidase domain, but not in class A PBP 1A, 1B, or 2A, which carry an additional transglycosylase domain. Therefore, we tested whether inactivation of class A PBPs affected resistance development in spite of the absence of mutations. Deletion of PBP 1A or 2A profoundly slowed down resistance development but only moderately affected resistance in already highly resistant mutants (MIC = 2 to 4 microg/ml). Thus, class A PBPs might facilitate early development of resistance by stabilizing penicillin-altered peptidoglycan via transglycosylation, whereas they might be less indispensable in highly resistant mutants which have reestablished a penicillin-insensitive cell wall-building machinery. The contribution of PBP and non-PBP mutations alone could be individualized in DNA transformation. Both PBP and non-PBP mutations conferred some level of intrinsic resistance, but combining the mutations synergized them to ensure high-level resistance (> or = 2 microg/ml). The results underline the complexity of penicillin resistance development and suggest that inhibition of transglycosylase might be an as yet underestimated way to interfere with early resistance development.

Mutational analysis of class A and class B penicillin-binding proteins in Streptococcus gordonii

High-molecular-weight (HMW) penicillin-binding proteins (PBPs) are divided into class A and class B PBPs, which are bifunctional transpeptidases/transglycosylases and monofunctional transpeptidases, respectively. We determined the sequences for the HMW PBP genes of Streptococcus gordonii, a gingivo-dental commensal related to Streptococcus pneumoniae. Five HMW PBPs were identified, including three class A (PBPs 1A, 1B, and 2A) and two class B (PBPs 2B and 2X) PBPs, by homology with those of S. pneumoniae and by radiolabeling with [3H]penicillin. Single and double deletions of each of them were achieved by allelic replacement. All could be deleted, except for PBP 2X, which was essential. Morphological alterations occurred after deletion of PBP 1A (lozenge shape), PBP 2A (separation defect and chaining), and PBP 2B (aberrant septation and premature lysis) but not PBP 1B. The muropeptide cross-link patterns remained similar in all strains, indicating that cross-linkage for one missing PBP could be replaced by others. However, PBP 1A mutants presented shorter glycan chains (by 30%) and a relative decrease (25%) in one monomer stem peptide. Growth rate and viability under aeration, hyperosmolarity, and penicillin exposure were affected primarily in PBP 2B-deleted mutants. In contrast, chain-forming PBP 2A-deleted mutants withstood better aeration, probably because they formed clusters that impaired oxygen diffusion. Double deletion could be generated with any PBP combination and resulted in more-altered mutants. Thus, single deletion of four of the five HMW genes had a detectable effect on the bacterial morphology and/or physiology, and only PBP 1B seemed redundant a priori.

Binding ofStreptococcus gordoniito extracellular matrix proteins

Knock-out mutants of Streptococcus gordonii Challis were constructed and assayed for binding to extracellular matrix proteins (EMPs) by enzyme-linked immunosorbent assay (ELISA). It was shown that (i) the mutant lacking the cell wall polysaccharide receptor could no longer bind type I and type II collagen, (ii) the mutant lacking the fibronectin-binding proteins CshA and FbpA was also strongly impaired in collagen binding and (iii) the mutant lacking the methionine sulfoxide reductase MsrA was significantly impaired in fibronectin binding. Our results indicate that binding to EMPs by S. gordonii is a multifactorial process controlled by genes located at three different chromosomal sites.

Lack of in vitro biofilm formation does not attenuate the virulence of Streptococcus gordonii in experimental endocarditis

The ability to induce experimental endocarditis of biofilm-deficient mutants of Streptococcus gordonii was studied in an isogenic background. Strains were inactivated in either comD, fruK or pbp2b genes, which are involved in biofilm formation. These strains were clearly impaired (>75% reduction) in biofilm production in vitro. However, this did not result in a decreased severity of infection in vivo.

Oral immunization of BALB/c mice by intragastric delivery of Streptococcus gordonii-expressing Giardia cyst wall protein 2 decreases cyst shedding in challenged mice

Giardia lamblia (Giardia duodenalis or Giardia intestinalis) is a protozoan parasite of vertebrates with broad host specificity. Specific antibodies directed against cyst antigens can interfere with the cyst wall-building process. In this study, we engineered Streptococcus gordonii to express a 26 kDa fragment of cyst wall protein 2 (CWP2), containing a relevant B cell epitope, on the cell surface. This is the first report of S. gordonii expressing a protein of parasite origin. As S. gordonii was intended for intestinal delivery of CWP2, it was determined that this oral commensal bacterium is able to persist in the murine intestine for 30 days. Immunization with recombinant streptococci expressing the 26 kDa fragment resulted in higher antibody levels. Specific anti-CWP2 IgA antibodies were detected in fecal samples and anti-CWP2 IgG antibodies were detected in serum demonstrating the efficacy of S. gordonii for intragastric antigen delivery. In a pilot challenge experiment, immunized mice demonstrated a significant 70% reduction in cyst output.

Granular layer in the periplasmic space of gram-positive bacteria and fine structures of Enterococcus gallinarum and Streptococcus gordonii septa revealed by cryo-electron microscopy of vitreous sections

High-resolution structural information on optimally preserved bacterial cells can be obtained with cryo-electron microscopy of vitreous sections. With the help of this technique, the existence of a periplasmic space between the plasma membrane and the thick peptidoglycan layer of the gram-positive bacteria Bacillus subtilis and Staphylococcus aureus was recently shown. This raises questions about the mode of polymerization of peptidoglycan. In the present study, we report the structure of the cell envelope of three gram-positive bacteria (B. subtilis, Streptococcus gordonii, and Enterococcus gallinarum). In the three cases, a previously undescribed granular layer adjacent to the plasma membrane is found in the periplasmic space. In order to better understand how nascent peptidoglycan is incorporated into the mature peptidoglycan, we investigated cellular regions known to represent the sites of cell wall production. Each of these sites possesses a specific structure. We propose a hypothetic model of peptidoglycan polymerization that accommodates these differences: peptidoglycan precursors could be exported from the cytoplasm to the periplasmic space, where they could diffuse until they would interact with the interface between the granular layer and the thick peptidoglycan layer. They could then polymerize with mature peptidoglycan. We report cytoplasmic structures at the E. gallinarum septum that could be interpreted as cytoskeletal elements driving cell division (FtsZ ring). Although immunoelectron microscopy and fluorescence microscopy studies have demonstrated the septal and cytoplasmic localization of FtsZ, direct visualization of in situ FtsZ filaments has not been obtained in any electron microscopy study of fixed and dehydrated bacteria.

Systemic and mucosal immunity to respiratory syncytial virus induced by recombinantStreptococcus gordoniisurface-displaying a domain of viral glycoprotein G

A conserved fragment comprising amino acid residues 130-230 of the G glycoprotein of human respiratory syncytial virus subtype A was expressed in the commensal bacterium Streptococcus gordonii. Recombinant streptococci displaying the G domain at the cell surface were used to immunize mice via both parenteral and mucosal routes. Subcutaneous immunization induced respiratory syncytial virus-specific serum immunoglobin G (IgG) capable of partially controlling virus replication in the lungs. Intranasal immunization with live bacteria stimulated the production of IgA against both the whole virus and the G domain in serum and bronchoalveolar fluid. Upon challenge, immunized animals had significantly lower virus titres in the lungs than the controls. Our results show for the first time that the G domain-expressing S. gordonii strain elicits both systemic and mucosal immunity that reduced respiratory syncytial virus replication in the lungs of mice.

Multilevel control of competence development and stress tolerance in Streptococcus mutans UA159

Genetic competence appears to be important in establishment of biofilms and tolerance of environmental insults. We report here that the development of competence is controlled at multiple levels in a complex network that includes two signal-transducing two-component systems (TCS). Using Streptococcus mutans strain UA159, we demonstrate that the histidine kinase CiaH, but not the response regulator CiaR, causes a dramatic decrease in biofilm formation and in transformation efficiency. Inactivation of comE or comD had no effect on stress tolerance, but transformability of the mutants was poor and was not restored by addition of competence-stimulating peptide (CSP). Horse serum (HS) or bovine serum albumin (BSA) had no impact on transformability of any strains. Interestingly, though, the presence of HS or BSA in combination with CSP was required for efficient induction of comD, comX, and comYA, and induction was dependent on ComDE and CiaH, but not CiaR. Inactivation of comC, encoding CSP, had no impact on transformation, and CiaH was shown to be required for optimal comC expression. This study reveals that S. mutans integrates multiple environmental signals through CiaHR and ComDE to coordinate induction of com genes and that CiaH can exert its influence through CiaR and as-yet-unidentified regulators. The results highlight critical differences in the role and regulation of CiaRH and com genes in different S. mutans isolates and between S. mutans and Streptococcus pneumoniae, indicating that substantial divergence in the role and regulation of TCS and competence genes has occurred in streptococci.

In vivo activation of naive CD4+ T cells in nasal mucosa-associated lymphoid tissue following intranasal immunization with recombinant Streptococcus gordonii

The antigen-specific primary activation of CD4+ T cells was studied in vivo by adoptive transfer of ovalbumin-specific transgenic T cells (KJ1-26+ CD4+) following intranasal immunization with recombinant Streptococcus gordonii. A strain of S. gordonii expressing on its surface a model vaccine antigen fused to the ovalbumin (OVA) peptide from position 323 to 339 was constructed and used to study the OVA-specific T-cell activation in nasal mucosa-associated lymphoid tissue (NALT), lymph nodes, and spleens of mice immunized by the intranasal route. The recombinant strain, but not the wild type, activated the OVA-specific CD4+ T-cell population in the NALT (89% of KJ1-26+ CD4+ T cells) just 3 days following immunization. In the cervical lymph nodes and in the spleen, the percentage of proliferating cells was initially low, but it reached the peak of activation at day 5 (90%). This antigen-specific clonal expansion of KJ1-26+ CD4+ T cells after intranasal immunization was obtained with live and inactivated recombinant bacteria, and it indicates that the NALT is the site of antigen-specific T-cell priming.

Development of PLEX, a plasmid-based expression system for production of heterologous gene products by the gram-positive bacteria Streptococcus gordonii

While Escherichia coli expression systems have been widely utilized for the production of heterologous proteins, these systems have limitations with regard to the production of particular protein products, including poor expression, expression of insoluble proteins into inclusion bodies, and/or expression of a truncated product. Using the surface protein expression (SPEX) system, chromosomally integrated heterologous genes are expressed and secreted into media by the naturally competent gram-positive organism Streptococcus gordonii. After E. coli turned out to be an inappropriate expression system to produce sufficient quantities of intact product, we successfully utilized SPEX to produce the heterologous antigen BH4XCRR that is designed from sequences homologous to the S. pyogenes M-protein C-repeat region. To further enhance production of this product by S. gordonii, we sought to develop a novel system for the production and secretion of heterologous proteins. We observed that under various growth conditions, S. gordonii secreted high levels of a 172 kDa protein, which was identified by N-terminal sequence analysis as the glucosyltransferase GTF. Here we report on the development of a plasmid-based expression system, designated as PLEX, which we used to enhance production of BH4XCRR by S. gordonii. A region from the S. gordonii chromosome that contains the positive regulatory gene rgg, putative gtfG promoter, and gtfG secretion-signal sequence was cloned into the E. coli/Streptococcus shuttle plasmid pVA838. Additionally, the bh4xcrr structural gene was cloned into the same plasmid downstream and in-frame with rgg and gtfG. This plasmid construct was transformed into S. gordonii and BH4XCRR was detected in culture supernatants from transformants at greater concentrations than in supernatants from a SPEX strain expressing the same product. BH4XCRR was easily purified from culture supernatant using a scalable two-step purification process involving hydrophobic-interaction and gel-filtration chromatography.

Marker rescue studies of the transfer of recombinant DNA to Streptococcus gordonii in vitro, in foods and gnotobiotic rats

A plasmid marker rescue system based on restoration of the nptII gene was established in Streptococcus gordonii to study the transfer of bacterial and transgenic plant DNA by transformation. In vitro studies revealed that the marker rescue efficiency depends on the type of donor DNA. Plasmid and chromosomal DNA of bacteria as well as DNA of transgenic potatoes were transferred with efficiencies ranging from 8.1 x 10(-6) to 5.8 x 10(-7) transformants per nptII gene. Using a 792-bp amplification product of nptII the efficiency was strongly decreased (9.8 x 10(-9)). In blood sausage, marker rescue using plasmid DNA was detectable (7.9 x 10(-10)), whereas in milk heat-inactivated horse serum (HHS) had to be added to obtain an efficiency of 2.7 x 10(-11). No marker rescue was detected in extracts of transgenic potatoes despite addition of HHS. In vivo transformation of S. gordonii LTH 5597 was studied in monoassociated rats by using plasmid DNA. No marker rescue could be detected in vivo, although transformation was detected in the presence of saliva and fecal samples supplemented with HHS. It was also shown that plasmid DNA persists in rat saliva permitting transformation for up to 6 h of incubation. It is suggested that the lack of marker rescue is due to the absence of competence-stimulating factors such as serum proteins in rat saliva.

Antibiotic-dependent correlation between drug-induced killing and loss of luminescence in Streptococcus gordonii expressing luciferase

Measuring antibiotic-induced killing relies on time-consuming biological tests. The firefly luciferase gene (luc) was successfully used as a reporter gene to assess antibiotic efficacy rapidly in slow-growing Mycobacterium tuberculosis. We tested whether luc expression could also provide a rapid evaluation of bactericidal drugs in Streptococcus gordonii. The suicide vectors pFW5luc and a modified version of pJDC9 carrying a promoterless luc gene were used to construct transcriptional-fusion mutants. One mutant susceptible to penicillin-induced killing (LMI2) and three penicillin-tolerant derivatives (LMI103, LMI104, and LMI105) producing luciferase under independent streptococcal promoters were tested. The correlation between antibiotic-induced killing and luminescence was determined with mechanistically unrelated drugs. Chloramphenicol (20 times the MIC) inhibited bacterial growth. In parallel, luciferase stopped increasing and remained stable, as determined by luminescence and Western blots. Ciprofloxacin (200 times the MIC) rapidly killed 1.5 log10 CFU/ml in 2-4 hr. Luminescence decreased simultaneously by 10-fold. In contrast, penicillin (200 times the MIC) gave discordant results. Although killing was slow (< or = 0.5 log10 CFU/ml in 2 hr), luminescence dropped abruptly by 50-100-times in the same time. Inactivating penicillin with penicillinase restored luminescence, irrespective of viable counts. This was not due to altered luciferase expression or stability, suggesting some kind of post-translational modification. Luciferase shares homology with aminoacyl-tRNA synthetase and acyl-CoA ligase, which might be regulated by macromolecule synthesis and hence affected in penicillin-inhibited cells. Because of resemblance, luciferase might be down-regulated simultaneously. Luminescence cannot be universally used to predict antibiotic-induced killing. Thus, introducing reporter enzymes sharing mechanistic similarities with normal metabolic reactions might reveal other effects than those expected.

The novel conjugative transposon tn1207.3 carries the macrolide efflux gene mef(A) in Streptococcus pyogenes

The macrolide efflux gene mef(A) of the Streptococcus pyogenes clinical strain 2812A was found to be carried by a 52-kb chromosomal genetic element that could be transferred by conjugation to the chromosome of other streptococcal species. The characteristics of this genetic element are typical of conjugative transposons and was named Tn1207.3. The size of Tn1207.3 was established by pulsed-field gel electrophoresis (PFGE), and DNA sequencing analysis showed that the 7,244 bp at the left end of Tn1207.3 were identical to those of the pneumococcal Tn1207.1 element. Tn1207.3-like genetic elements were found to be inserted at a single specific chromosomal site in 12 different clinical isolates S. pyogenes exhibiting the M phenotype of resistance to macrolides and carrying the mef(A) gene. Tn1207.3 was transferred from S. pyogenes 2812A to Streptococcus pneumoniae, and sequence analysis carried out on six independent transconjugants showed that insertion of Tn1207.3 in the pneumococcal genome always occurred at a single specific site as in Tn1207.1. Using MF2, a representative S. pneumoniae transconjugant, as a donor, Tn1207.3 was transferred again by conjugation to S. pyogenes and Streptococcus gordonii. The previously described nonconjugative element Tn1207.1 of S. pneumoniae appears to be a defective element, part of a longer conjugative transposon that carries mef(A) and is found in clinical isolates of S. pyogenes.

In vivo mucosal delivery of bioactive human interleukin 1 receptor antagonist produced by Streptococcus gordonii

BACKGROUND

Interleukin-1 (IL-1) is a cytokine involved in the initiation and amplification of the defence response in infectious and inflammatory diseases. IL-1 receptor antagonist (IL-1ra) is an inactive member of the IL-1 family and represents one of the most potent mechanisms for controlling IL-1-dependent inflammation. IL-1ra has proven effective in the therapy of acute and chronic inflammatory diseases in experimental animal models and also in preliminary clinical trials. However, optimisation of therapeutic schedules is still needed. For instance, the use of drug delivery systems targeting specific mucosal sites may be useful to improve topical bioavailability and avoid side effects associated with systemic administration.

RESULTS

In order to develop systems for the delivery of IL-1ra to mucosal target sites, a Streptococcus gordonii strain secreting human IL-1ra was constructed. The recombinant IL-1ra produced by S. gordonii was composed of the four amino acid residues RVFP of the fusion partner at the N-terminus, followed by the mature human IL-1ra protein. RFVP/IL-1ra displayed full biological activity in vitro in assays of inhibition of IL-1beta-induced lymphocyte proliferation and was released by recombinant S. gordonii in vivo both at the vaginal and the gastrointestinal mucosa of mice. RFVP/IL-1ra appeared beneficial in the model of ulcerative colitis represented by IL-2-/- mice (knock-out for the interleukin-2 gene), as shown by the body weight increase of IL-2-/- mice locally treated with S. gordonii producing RFVP/IL-1ra.

CONCLUSIONS

These results indicate that recombinant S. gordonii can be successfully used as a delivery system for the selective targeting of mucosal surfaces with therapeutic proteins.

Biological consequences of antigen and cytokine co-expression by recombinant Streptococcus gordonii vaccine vectors

To test the effect of co-expression of immunomodulatory molecules, together with target antigen, two recombinant Streptococcus gordonii strains were constructed which secreted either murine interleukin-2 (IL-2) or interferon-gamma (IFN-gamma) in addition to a surface anchored test antigen (the conserved C-repeat region (CRR) of the M6 protein of Streptococcus pyogenes). The secretion of functional cytokines by S. gordonii was achieved by in-frame fusion of sequences encoding mature IL-2 or IFN-gamma to the sequences encoding the leader signal of the M6 protein. Expression of the M protein CRR region from a separate chromosomal site produced double recombinants expressing a secreted cytokine and the M protein CRR region anchored to the surface. Protein expression was verified by streak blot, immunoblot, and ELISA on both the single and double recombinants. A cytokine bioassay using HT-2 cells verified biological activity of recombinant IL-2 secreted from S. gordonii. When mice were immunized subcutaneously with the different S. gordonii expression strains, cytokine co-expression apparently modulated the systemic immune response. These results show that streptococci can deliver biologically active molecules such as cytokines along with antigens to the immune system. These results demonstrate that a cytokine-secreting, noninvasive, bacterial vaccine vector can be used to modulate immune responses to a co-expressed antigen.

Analysis of factors affecting surface expression and immunogenicity of recombinant proteins expressed by gram-positive commensal vectors

Several key protein structural attributes were altered in an effort to optimize expression and immunogenicity of a foreign protein (M protein from Streptococcus pyogenes) exposed on the surface of Streptococcus gordonii commensal bacterial vectors: (i) a shorter N-terminal region, (ii) the addition of a 94-amino-acid spacer, and (iii) the addition of extra C-repeat regions (CRR) from the M6 protein. A decrease in the amount of cell surface M6 was observed upon deletion of 10 or more amino acid residues at the N terminus. On the other hand, reactivity of monoclonal antibody to surface M6 increased with the addition of the spacer adjacent to the proline- and glycine-rich region, and an increase in epitope dosage was obtained by adding another CRR immediately downstream of the original CRR. The results obtained should facilitate the design of improved vaccine candidates using this antigen delivery technology.

Genetic engineering of Streptococcus gordonii for the simultaneous display of two heterologous proteins at the bacterial surface

The Gram-positive bacterium Streptococcus gordonii has been genetically engineered to allow the simultaneous expression of two heterologous proteins at the cell surface. A family of recombinant streptococci displaying two different antigens was constructed. All the strains were genetically stable and expressed both proteins at the surface of the same bacterial cell. S. gordonii co-expressing the immunomodulating molecule LTB (B monomer of Escherichia coli heat-labile toxin) and the V3 domain of HIV-1 gp120 were inoculated subcutaneously to BALB/c mice. Animals were capable of responding to both antigens, producing LTB- and V3-specific serum IgG. The V3-specific IgG titer was four-fold higher in mice immunised with the double protein-expressing bacteria, as compared to control animals inoculated either with S. gordonii expressing the V3 domain alone or with a mixture of the two strains expressing LTB and V3, separately. Therefore, LTB was able to potentiate the antibody response towards the V3 domain, and this effect was observed only when LTB was co-expressed on the same bacterial cell.

Cleavage and purification of intein fusion proteins using the Streptococcus gordonii spex system

A gram-positive bacterial expression vector using Streptococcus gordonii has been developed for expression and secretion, or surface anchoring of heterologous proteins. This system, termed Surface Protein Expression system or SPEX, has been used to express a variety of surface anchored and secreted proteins. In this study, the Mycobacterium xenopi (Mxe) GyrA intein and chitin binding domain from Bacillus circulans chitinase Al were used in conjunction with SPEX to express a fusion protein to facilitate secretion, cleavage, and purification. Streptococcus gordonii was transformed to express a secreted fusion protein consisting of a target protein with a C-terminal intein and chitin-binding domain. Two target proteins, the C-repeat region of the Streptococcus pyogenes M6 protein (M6) and the nuclease A (NucA) enzyme of Staphylococcus aureus, were expressed and tested for intein cleavage. The secreted fusion proteins were purified from culture medium by binding to chitin beads and subjected to reaction conditions to induce intein self-cleavage to release the target protein. The M6 and NucA fusion proteins were shown to bind chitin beads and elute under cleavage reaction conditions. In addition, NucA demonstrated enzyme activity both before and after intein cleavage.

Tts, a processive beta-glucosyltransferase of Streptococcus pneumoniae, directs the synthesis of the branched type 37 capsular polysaccharide in Pneumococcus and other gram-positive species

The type 37 capsule of Streptococcus pneumoniae is a homopolysaccharide built up from repeating units of [beta-d-Glcp-(1-->2)]-beta-d-Glcp-(1-->3). The elements governing the expression of the tts gene, coding for the glucosyltransferase involved in the synthesis of the type 37 pneumococcal capsular polysaccharide, have been studied. Primer extension analysis and functional tests demonstrated the presence of four new transcriptional start points upstream of the previously reported tts promoter (ttsp). Most interesting, three of these transcriptional start points are located in a RUP element thought to be involved in recombinational events (Oggioni, M. R., and Claverys, J. P. (1999) Microbiology 145, 2647-2653). Transformation experiments using either a recombinant plasmid containing the whole transcriptional unit of tts or chromosomal DNA from a type 37 pneumococcus showed that tts is the only gene required to drive the biosynthesis of a type 37 capsule in S. pneumoniae and other Gram-positive bacteria, namely Streptococcus oralis, Streptococcus gordonii, and Bacillus subtilis. The Tts synthase was overproduced in S. pneumoniae and purified as a membrane-associated enzyme. These membrane preparations used UDP-Glc as substrate to catalyze the synthesis of a high molecular weight polysaccharide immunologically identical to the type 37 capsule. In addition, UDP-Gal was also a substrate to produce type 37 polysaccharide since a strong UDP-Glc-4'-epimerase activity is associated to the membrane fraction of S. pneumoniae. These results indicated that Tts has a dual biochemical activity that leads to the synthesis of the branched type 37 polysaccharide.

Dendritic cells express tight junction proteins and penetrate gut epithelial monolayers to sample bacteria

Penetration of the gut mucosa by pathogens expressing invasion genes is believed to occur mainly through specialized epithelial cells, called M cells, that are located in Peyer's patches. However, Salmonella typhimurium that are deficient in invasion genes encoded by Salmonella pathogenicity island 1 (SPI1) are still able to reach the spleen after oral administration. This suggests the existence of an alternative route for bacterial invasion, one that is independent of M cells. We report here a new mechanism for bacterial uptake in the mucosa tissues that is mediated by dendritic cells (DCs). DCs open the tight junctions between epithelial cells, send dendrites outside the epithelium and directly sample bacteria. In addition, because DCs express tight-junction proteins such as occludin, claudin 1 and zonula occludens 1, the integrity of the epithelial barrier is preserved.

Immunoglobulin-binding domains of peptostreptococcal protein L enhance vaginal colonization of mice by Streptococcus gordonii

Protein L, an immunoglobulin-binding protein of some strains of the anaerobic bacterium Peptostreptococcus magnus, has been hypothesized to be a virulence determinant in bacterial vaginosis. In order to investigate the role of protein L in peptostreptococcal virulence, the Ig-binding domains of protein L were expressed at the surface of the human oral commensal Streptococcus gordonii. Recombinant streptococci were used in vaginal colonization experiments, and protein L-expressing S. gordonii demonstrated enhanced ability to colonize the vaginal mucosa. Compared to the control strain, they also persisted for a longer period in the murine vagina.

Immunization with recombinant Streptococcus gordonii expressing tetanus toxin fragment C confers protection from lethal challenge in mice

Tetanus toxin fragment C (TTFC) was expressed on the surface of the vaccine vector Streptococcus gordonii, a Gram-positive commensal bacterium of the human oral cavity. The immunogenicity of recombinant S. gordonii expressing TTFC was assayed in mice immunized by the parenteral and mucosal routes. High serum TTFC-specific IgG responses were induced in both BALB/c and C57BL/6 mice immunized subcutaneously. A total of 82% of vaccinated BALB/c mice were protected from the lethal challenge with 50 LD(50) of tetanus toxin (TT) and a direct correlation between the serum TTFC-specific IgG concentration and survival time of unprotected animals was observed. Intranasal immunization of BALB/c mice was also effective in inducing TTFC-specific serum IgG and local IgA in lung washes. Furthermore, 38% of animals immunized intranasally were protected from the lethal challenge with 10 LD(50) of TT while all control animals died within 24 h. Analysis of the serum IgG subclasses showed that the IgG1 subclass was predominant after parenteral immunization in BALB/c mice (IgG1/IgG2a ratio congruent with6) while following mucosal immunization a mixed IgG1 and IgG2a pattern (IgG1/IgG2a ratio congruent with1) was observed. These data show that TTFC expressed on the surface of S. gordonii is immunogenic by the subcutaneous and mucosal routes and the immune response induced is capable of conferring protection from the lethal challenge with TT.

Study of Staphylococcus aureus pathogenic genes by transfer and expression in the less virulent organism Streptococcus gordonii

Because Staphylococcus aureus strains contain multiple virulence factors, studying their pathogenic role by single-gene inactivation generated equivocal results. To circumvent this problem, we have expressed specific S. aureus genes in the less virulent organism Streptococcus gordonii and tested the recombinants for a gain of function both in vitro and in vivo. Clumping factor A (ClfA) and coagulase were investigated. Both gene products were expressed functionally and with similar kinetics during growth by streptococci and staphylococci. ClfA-positive S. gordonii was more adherent to platelet-fibrin clots mimicking cardiac vegetations in vitro and more infective in rats with experimental endocarditis (P < 0.05). Moreover, deleting clfA from clfA-positive streptococcal transformants restored both the low in vitro adherence and the low in vivo infectivity of the parent. Coagulase-positive transformants, on the other hand, were neither more adherent nor more infective than the parent. Furthermore, coagulase did not increase the pathogenicity of clfA-positive streptococci when both clfA and coa genes were simultaneously expressed in an artificial minioperon in streptococci. These results definitively attribute a role for ClfA, but not coagulase, in S. aureus endovascular infections. This gain-of-function strategy might help solve the role of individual factors in the complex the S. aureus-host relationship.

Inactivation of the srtA gene in Streptococcus gordonii inhibits cell wall anchoring of surface proteins and decreases in vitro and in vivo adhesion

The srtA gene product, SrtA, has been shown to be required for cell wall anchoring of protein A as well as virulence in the pathogenic bacterium Staphylococcus aureus. There are five major mechanisms for displaying proteins at the surface of gram-positive bacteria (P. Cossart and R. Jonquieres, Proc. Natl. Acad. Sci. USA 97:5013-5015, 2000). However, since many of the known surface proteins of gram-positive bacteria are believed to be exported and anchored via the sortase pathway, it was of interest to determine if srtA plays a similar role in other gram-positive bacteria. To that end, the srtA gene in the human oral commensal organism Streptococcus gordonii was insertionally inactivated. The srtA mutant S. gordonii exhibited a marked reduction in quantity of a specific anchored surface protein. Furthermore, the srtA mutant had reduced binding to immobilized human fibronectin and had a decreased ability to colonize the oral mucosa of mice. Taken together, these results suggest that the activity of SrtA plays an important role in the biology of nonpathogenic as well as pathogenic gram-positive cocci.

Co-operative binding of human fibronectin to Sfbl protein triggers streptococcal invasion into respiratory epithelial cells

Streptococcal fibronectin binding protein I (SfbI) mediates adherence to and invasion of Streptococcus pyogenes into human epithelial cells. In this study, we analysed the binding activity of distinct domains of SfbI protein towards its ligand, the extracellular matrix component fibronectin, as well as the biological implication of the binding events during the infection process. By using purified recombinant SfbI derivatives as well as in vivo expressed SfbI domains on the surface of heterologous organism Streptococcus gordonii, we were able to dissociate the two major streptococcal target domains on the human fibronectin molecule. The SfbI repeat region exclusively bound to the 30 kDa N-terminal fragment of fibronectin, whereas the SfbI spacer region exclusively bound to the 45 kDa collagen-binding fragment of fibronectin. In the case of native surface-expressed SfbI protein, an induced fit mode of bacteria-fibronectin interaction was identified. We demonstrate that binding of the 30 kDa fibronectin fragment to the repeat region of SfbI protein co-operatively activates the adjacent SfbI spacer domain to bind the 45 kDa fibronectin fragment. The biological consequence arising from this novel mode of fibronectin targeting was analysed in eukaryotic cell invasion assays. The repeat region of SfbI protein is mediating adherence and constitutes a prerequisite for subsequent invasion, whereas the SfbI spacer domain efficiently triggers the invasion process of streptococci into the eukaryotic cell. Thus, we were able to dissect bacterial adhesion from invasion by manipulating one protein. SfbI protein therefore represents a highly evolved prokaryotic molecule that exploits the host factor fibronectin not only for extracellular targeting but also for its subsequent activation that leads to efficient cellular invasion.

Expression and purification of histidine-tagged proteins from the gram-positive Streptococcus gordonii SPEX system

Streptococcus gordonii (S. gordonii) has been used as a gram-positive bacterial expression vector for secreted or surface-anchored recombinant proteins. Fusion of the gram-positive bacterial N-terminal signal sequence to the target protein is all that is required for efficient export. This system is termed SPEX for Surface Protein EXpression and has been used to express proteins for a variety of uses. In this study, the SPEX system has been further developed by the construction of vectors that express polyhistidine-tagged fusion proteins. SPEX vectors were constructed with an N-terminal or C-terminal histidine tag. The C-repeat region (CRR) from Streptococcus pyogenes M6 protein and the Staphylococcus aureus nuclease A (NucA) enzyme were tested for expression. The fusion proteins were purified using metal affinity chromatography (MAC). Results show that the fusion proteins were expressed and secreted from S. gordonii with the His tag at either the N- or C-terminal position and could be purified using MAC. The M6 fusions retained immunoreactivity after expression and purification as determined by immunoblots and ELISA analyses. In addition, NucA fusions retained functional activity after MAC purification. The M6-His and NucA-His fusions were purified approximately 15- and 10-fold respectively with approximately 30% recovery of protein using MAC. This study shows that the polyhistidine tag in either the N- or C-terminal position is a viable way to purify secreted heterologous proteins from the supernatant of recombinant S. gordonii cultures. This study further illustrates the value of the SPEX system for secreted expression and purification of proteins.

Deregulation of the arginine deiminase (arc) operon in penicillin-tolerant mutants of Streptococcus gordonii

Penicillin tolerance is an incompletely understood phenomenon that allows bacteria to resist drug-induced killing. Tolerance was studied with independent Streptococcus gordonii mutants generated by cyclic exposure to 500 times the MIC of penicillin. Parent cultures lost 4 to 5 log(10) CFU/ml of viable counts/24 h. In contrast, each of four independent mutant cultures lost < or =2 log(10) CFU/ml/24 h. The mutants had unchanged penicillin-binding proteins but contained increased amounts of two proteins with respective masses of ca. 50 and 45 kDa. One mutant (Tol1) was further characterized. The two proteins showing increased levels were homologous to the arginine deiminase and ornithine carbamoyl transferase of other gram-positive bacteria and were encoded by an operon that was >80% similar to the arginine-deiminase (arc) operon of these organisms. Partial nucleotide sequencing and insertion inactivation of the S. gordonii arc locus indicated that tolerance was not a direct consequence of arc alteration. On the other hand, genetic transformation of tolerance by Tol1 DNA always conferred arc deregulation. In nontolerant recipients, arc was repressed during exponential growth and up-regulated during postexponential growth. In tolerant transformants, arc was constitutively expressed. Tol1 DNA transformed tolerance at the same rate as transformation of a point mutation (10(-2) to 10(-3)). The tolerance mutation mapped on a specific chromosomal fragment but was physically distant from arc. Importantly, arc deregulation was observed in most (6 of 10) of additional independent penicillin-tolerant mutants. Thus, although not exclusive, the association between arc deregulation and tolerance was not fortuitous. Since penicillin selection mimicked the antibiotic pressure operating in the clinical environment, arc deregulation might be an important correlate of naturally occurring tolerance and help in understanding the mechanism(s) underlying this clinically problematic phenotype.

Characterization of a genetic element carrying the macrolide efflux gene mef(A) in Streptococcus pneumoniae

The mef(A) gene from a clinical isolate of Streptococcus pneumoniae exhibiting the M-type resistance to macrolides was found to be part of the 7,244-bp chromosomal element Tn1207.1, which contained 8 open reading frames. orf2 encodes a resolvase/invertase, and orf5 is a homolog of the macrolide-streptogramin B resistance gene msr(SA).

Immunogenicity of the B monomer of Escherichia coli heat-labile toxin expressed on the surface of Streptococcus gordonii

The B monomer of the Escherichia coli heat-labile toxin (LTB) was expressed on the surface of the human oral commensal bacterium Streptococcus gordonii. Recombinant bacteria expressing LTB were used to immunize BALB/c mice subcutaneously and intragastrically. The LTB monomer expressed on the streptococcal surface proved to be highly immunogenic, as LTB-specific immunoglobulin G (IgG) serum titers of 140,000 were induced after systemic immunization. Most significantly, these antibodies were capable of neutralizing the enterotoxin in a cell neutralization assay. Following mucosal delivery, antigen-specific IgA antibodies were found in feces and antigen-specific IgG antibodies were found in sera. Analysis of serum IgG subclasses showed a clear predominance of IgG1 when recombinant bacteria were inoculated subcutaneously, while a prevalence of IgG2a was observed upon intragastric delivery, suggesting, in this case, the recruitment of a Th1 type of immune response.

Broad-host-range shuttle vectors for screening of regulated promoter activity in viridans group streptococci: isolation of a pH-regulated promoter

Viridans group streptococci are major constituents of the normal human oral flora and are also identified as the predominant pathogenic bacteria in native valve infective endocarditis. Little information is available regarding the regulation of gene expression in viridans group streptococci, either in response to changes in the oral environment or during development of endocarditis. We therefore constructed a set of broad-host-range vectors for the isolation of promoters from viridans group streptococci that are activated by specific environmental stimuli in vitro or in vivo. A genomic library of Streptococcus gordonii strain CH1 was constructed in one of the new vectors, and this library was introduced into a homologous bacterium by using an optimized electroporation protocol for viridans group streptococci. Because viridans group streptococci entering the bloodstream from the oral cavity encounter an increase in pH, we selected promoters upregulated by this specific stimulus. One of the selected promoter sequences showed homology to the promoter region of the hydA gene from Clostridium acetobutylicum, the expression of which is known to be regulated by the environmental pH. The isolation of this pH-regulated promoter shows that S. gordonii can sense an increase in the environmental pH, which serves as a signal for bacterial gene activation. Furthermore, this demonstrates the usefulness of these new selection vectors in research on adaptive gene expression of viridans group streptococci and possibly also of other gram-positive bacteria.

Engineering the gram-positive cell surface for construction of bacterial vaccine vectors

A genetic system for surface display of heterologous proteins has been developed in Streptococcus gordonii, a gram-positive human oral commensal that is naturally competent for genetic transformation. Our approach is based on chromosomal integration downstream from a resident promoter and translational fusion to an M6 protein. Using this strategy a variety of proteins, of different origin and size, were displayed on the cell surface and were shown to be stably expressed both in vitro and in vivo. Animal models of mucosal colonization (oral and vaginal) and intragastric immunization with recombinant S. gordonii were developed and the local and systemic immune responses were studied. Here we report the techniques for the construction of recombinant bacteria, use of animal models, and analysis of the immune response.

Identification and characterization of a novel fibronectin-binding protein on the surface of group A streptococci

Understanding the role surface proteins play in the interaction of group A streptococci with epithelial cells is an important step toward the development of new strategies to fight infections. Fibronectin-binding proteins in streptococci and staphylococci have been described as important mediators for adherence to eukaryotic cells. In the present study we describe a new Streptococcus pyogenes fibronectin-binding protein (PFBP). The gene encoding the PFBP protein (pfbp) was identified from an M12 strain genomic library. It encodes a protein of 127.4 kDa which contains the LPXTGX motif characteristic of cell wall-associated proteins in gram-positive organisms and is among the largest surface molecules described for group A streptococci. The pfbp gene is transcribed during cell growth and was present in several class I and II streptococcal strains tested. The deduced amino acid sequence of PFBP exhibits a variable N-terminal region and a conserved C-terminal region when compared to most fibronectin-binding proteins identified from other gram-positive bacteria. The N-terminal region presents a stretch of 105 amino acids with no homology with N-terminal regions of previously described fibronectin-binding molecules, while the C-terminal region contains three repeat domains that share significant similarity with the repeat regions of fibronectin-binding proteins from S. pyogenes, S. dysgalactiae, and S. equisimilis. The PFBP repeated region, when expressed on the surface of S. gordonii, a commensal organism, binds to soluble and immobilized fibronectin. This study also shows that, in addition to pfbp, a second gene homologous with that of protein F1 (which also codes for a fibronectin-binding protein) is transcribed during cell growth in the same S. pyogenes strain.

SPEX, a system for the expression of recombinant proteins from gram-positive bacterial vectors

Using a conserved pathway for surface protein extrusion, a system has been developed for the expression and secretion of proteins from gram-positive bacteria. As proof-of-concept, the Streptococcus gordonii Challis strain has been engineered to express a series of recombinant proteins fused to the conserved region of the M6 protein of Streptococcus pyogenes. In the prototype surface protein expression system, the recombinant M6 protein is anchored to the surface of S. gordonii cells expressing it. In order to overexpress the protein and easily purify it away from the bacteria, the protein was modified to enable it to be secreted into the medium. To accomplish this, a stop codon was introduced into the gene just prior to the anchor region using site-directed mutagenesis. Using enzyme-linked immunosorbent assays, it was possible to quantitate the amount of protein expressed using this system. With little or no optimization, 3 mg of protein per liter of culture was expressed and secreted into the medium of a bacterial culture grown to an OD600 equal to 1.0. This system should be broadly applicable for the expression and secretion of a variety of proteins (antigens, hormones, and enzymes) directly into the medium.

Bacteria-induced neo-biosynthesis, stabilization, and surface expression of functional class I molecules in mouse dendritic cells

Here, we show that bacteria induce de novo synthesis of both major histocompatability complex (MHC) class I and II molecules in a mouse dendritic cell culture system. The neo-biosynthesis of MHC class I molecules is delayed as compared with that of MHC class II. Furthermore, bacteria stabilize MHC class I molecules by a 3-fold increase of their half-life. This has important consequences for the capacity of dendritic cells to present bacterial antigens in the draining lymph nodes. In addition, a model antigen, ovalbumin, expressed on the surface of recombinant Streptococcus gordonii is processed and presented on MHC class I molecules. This presentation is 10(6) times more efficient than that of soluble OVA protein. This exogenous pathway of MHC class I presentation is transporter associated with antigen processing (TAP)-dependent, indicating that there is a transport from phagolysosome to cytosol in dendritic cells. Thus, bacteria are shown to be a potentially useful mean for the correct delivery of exogenous antigens to be presented efficiently on MHC class I molecules.

Vaginal immunization with recombinant gram-positive bacteria

PROBLEM

Many viral and bacterial pathogens enter the body through the genital mucosa. Therefore, one of the major goals of a vaccine against sexually transmitted diseases (STDs) should be to induce an immune response in the genital mucosa capable of controlling the entry of the pathogen. Our approach for the development of vaccines against STDs is based on the use of nonpathogenic Gram-positive bacteria as live vaccine vectors.

METHOD OF STUDY

Recombinant Gram-positive bacteria expressing vaccine antigens were constructed using genetic systems developed in our laboratory. Balb/c mice and Cynomolgus monkeys were inoculated by the vaginal route and vaginal samples were collected using absorbent wicks. Colonization was evaluated by the presence of recombinant bacteria in the vaginal samples. Local and systemic immune responses were studied.

RESULTS

We have developed genetic systems for the expression of heterologous antigens on the surface of the human commensals Streptococcus gordonii and Lactobacillus spp. Both S. gordonii and L. casei stably colonized the murine vagina after a single inoculum. Vaginal colonization of mice with recombinant strains of S. gordonii, expressing human papillomavirus (HPV) and human immunodeficiency virus (HIV) antigens, induced antigen-specific vaginal immunoglobulin A (IgA) and serum IgG. Local and systemic immune responses also were detected in monkeys immunized intravaginally with recombinant S. gordonii.

CONCLUSION

The results obtained indicated that the approach of using colonizing Gram-positive bacteria as live vectors has a great potential for the development of vaccines against STDs.

Commensal bacteria as vectors for mucosal vaccines against sexually transmitted diseases: vaginal colonization with recombinant streptococci induces local and systemic antibodies in mice

There is a need to develop vaccines to control the spread of sexually transmitted diseases (STDs). Novel immunization strategies that elicit a mucosal immune response in the genital tract, may show improved protection by preventing or at least limiting entry of the pathogenic micro-organism. However, it has proven difficult to obtain a local immune response in the vaginal mucosa. Our approach is based on the use of recombinant bacteria capable of colonizing mucosal surfaces as live vaccine vectors. The human commensal Streptococcus gordonii, engineered to express the E7 protein of human papillomavirus type 16, was used for intravaginal immunization of mice. A single inoculum of recombinant bacteria was sufficient to establish colonization of the murine vagina and therefore induce papillomavirus-specific vaginal IgA and serum IgG. Evidence that mucosal colonization with recombinant commensal bacteria can induce a local immune response in the female genital tract represents a significant step toward the development of new vaccines against STDs.