Lipoprotein e (P4) of Haemophilus influenzae: role in heme utilization and pathogenesis

The Peptide Methionine Sulfoxide Reductase (MsrAB) of Haemophilus influenzae Repairs Oxidatively Damaged Outer Membrane and Periplasmic Proteins Involved in Nutrient Acquisition and Virulence

Sulfoxide-damage repair mechanisms are emerging as essential for the virulence of bacterial pathogens, and in the human respiratory pathogen Haemophilus influenzae the periplasmic MsrAB peptide methionine sulfoxide reductase is necessary for resistance to reactive chlorine species such as hypochlorite. Additionally, this enzyme has a role in modulating the host immune response to infection. Here, we have analysed the enzymatic properties of MsrAB, which revealed that both domains of the protein are catalytically active, with the turnover number of the MsrA domain being 50% greater than that for the MsrB domain. MsrAB was active with small molecular sulfoxides as well as oxidised calmodulin, and maximal activity was observed at 30°C, a temperature close to that found in the natural niche of H. influenzae, the nasopharynx. Analyses of differential methionine oxidation identified 29 outer membrane and periplasmic proteins that are likely substrates for MsrAB. These included the LldD lactate dehydrogenase and the lipoprotein eP4 that is involved in NAD and hemin metabolism in H. influenzae. Subsequent experiments showed that H. influenzae MsrAB can repair oxidative damage to methionines in purified eP4 with up to 100% efficiency. Our work links MsrAB to the maintenance of different adhesins and essential metabolic processes in the H. influenzae, such as NAD metabolism and access to L-lactate, which is a key growth substrate for H. influenzae during infection.

A bacterial vaccine polypeptide protective against nontypable Haemophilus influenzae

Nontypeable strains of Haemophilus influenzae (NTHi) are one of the most common cause of otitis media and the most frequent infection associated with exacerbations of chronic obstructive pulmonary disease; there is currently no vaccine in the U.S. to prevent NTHi. Using bioinformatics and structural vaccinology, we previously identified several NTHi species-conserved and sequence-conserved peptides that mediate passive protection in the rat model of infection. Using these, and similar peptides, we designed Hi Poly 1, a Bacterial Vaccine Polypeptide, comprising 9 unique peptides from 6 different surface proteins. Recombinant Hi Poly 1 was purified by affinity chromatography. Forty chinchillas were immunized three times with 200 µg of Hi Poly 1 with alum adjuvant; similarly, 41 controls were immunized with adjuvant alone. The average Log₂ IgG titer among immunized animals was 17.04, and IgG antibodies against each component peptide were detected. In the infant rat model, antisera from immunized chinchillas provided significant passive protection compared to PBS (p = 0.01) and pre-immune sera (p = 0.03). In the established chinchilla model of NTHi otitis media, the vaccinated group cleared infection faster than the control group as indicated by significantly decreased positive findings on video-otoscopy (p < 0.0001) and tympanometry (p = 0.0002) on day 7, and for middle ear fluid obtained by aspiration (p = 0.0001) on day 10 post-infection. Using 12 representative NTHi strains in a Live-Cell ELISA, greater antibody binding to each strain was detected with post Hi Poly 1 than the pre-immune chinchilla antisera. The data from this proof-of-principle study demonstrate the effectiveness of Hi Poly 1 against the NTHi in two relevant preclinical models of bacteremia and otitis media as well as surface antibody binding across the species. The Bacterial Vaccine Polypeptide approach to a vaccine against NTHi also serves as a paradigm for development of similar vaccines to protect against other bacteria.

Heme Synthesis and Acquisition in Bacterial Pathogens

Bacterial pathogens require the iron-containing cofactor heme to cause disease. Heme is essential to the function of hemoproteins, which are involved in energy generation by the electron transport chain, detoxification of host immune effectors, and other processes. During infection, bacterial pathogens must synthesize heme or acquire heme from the host; however, host heme is sequestered in high-affinity hemoproteins. Pathogens have evolved elaborate strategies to acquire heme from host sources, particularly hemoglobin, and both heme acquisition and synthesis are important for pathogenesis. Paradoxically, excess heme is toxic to bacteria and pathogens must rely on heme detoxification strategies. Heme is a key nutrient in the struggle for survival between host and pathogen, and its study has offered significant insight into the molecular mechanisms of bacterial pathogenesis.

Developing a vaccine to prevent otitis media caused by nontypeable Haemophilus influenzae

Nontypeable Haemophilus influenzae (NTHi) is a predominant organism of the upper respiratory nasopharyngeal microbiota. Its disease spectrum includes otitis media, sinusitis, non-bacteremic pneumonia and invasive infections. Protein-based vaccines to prevent NTHi infections are needed to alleviate these infections in children and vulnerable populations such as the elderly and those with chronic obstructive pulmonary disease (COPD). One NTHi protein is included in a pneumococcal conjugate vaccine and has been shown to provide efficacy. Our lab has been interested in understanding the immunogenicity of NTHi vaccine candidates P6, protein D and OMP26 for preventing acute otitis media in young children. We expect that continued investigation and progress in the development of an efficacious protein based vaccine against NTHi infections is achievable in the near future.

Haemophilus influenzae P4 Interacts With Extracellular Matrix Proteins Promoting Adhesion and Serum Resistance

Interaction with the extracellular matrix (ECM) is one of the successful colonization strategies employed by nontypeable Haemophilus influenzae (NTHi). Here we identified Haemophilus lipoprotein e (P4) as a receptor for ECM proteins. Purified recombinant P4 displayed a high binding affinity for laminin (Kd = 9.26 nM) and fibronectin (Kd = 10.19 nM), but slightly less to vitronectin (Kd = 16.51 nM). A P4-deficient NTHi mutant showed a significantly decreased binding to these ECM components. Vitronectin acquisition conferred serum resistance to both P4-expressing NTHi and Escherichia coli transformants. P4-mediated bacterial adherence to pharynx, type II alveolar, and bronchial epithelial cells was mainly attributed to fibronectin. Importantly, a significantly reduced bacterial infection was observed in the middle ear of the Junbo mouse model when NTHi was devoid of P4. In conclusion, our data provide new insight into the role of P4 as an important factor for Haemophilus colonization and subsequent respiratory tract infection.

Expeditious screening of candidate proteins for microbial vaccines

Advancements in high-throughput "omics" technologies have revolutionized the way vaccine candidates are identified. Now every surface expressed protein that an organism produces can be identified in silico and possibly made available for the rapid development of recombinant/subunit vaccines. However, evaluating the antigenicity of a large number of candidate proteins is an immense challenge, typically requiring cloning of several hundred candidates followed by immunogenicity screening. Here we report the development of a rapid, high-throughput method for screening candidate proteins for vaccines. This method involves utilizing a coupled, cell-free transcription-translation system to screen tagged proteins that are captured at the C-termini using appropriate ligand coated wells in 96 well ELISA plates. The template DNA for the cell-free expression is generated by two sequential PCRs and includes gene coding sequences, promoter, terminator, other necessary cis-acting elements and appropriate tag sequences. The process generates expressible candidate proteins containing two different peptide tags at the N- and the C-termini of the protein molecules. Proteins are screened in parallel for their quantity and immunoreactivity with N-terminal tag antibodies and antisera raised against the pathogen of interest, respectively. Normalization against the total detectable bound protein in the control wells allows for the identification of highly immunoreactive candidates. For this study we selected 30 representatives of >300 potential candidate proteins from Mannheimia haemolytica, a bacterial agent of pneumonia in feedlot cattle for expression with N-terminal Strep-II and C-terminal His(x6)-tag and evaluated their relative immunoreactivities using Strep-tactin-HRP and rabbit antisera generated against M. haemolytica. Using this system we were able to swiftly and quantitatively analyze and rank the suitability of proteins to identify potentially viable vaccine candidates, with the majority of the high ranking candidates being associated with virulence and pathogenicity. The system is adaptable to any bacterial target and presents an alternative to conventional laborious cloning, expression and screening procedures.

Concurrent host-pathogen gene expression in the lungs of pigs challenged with Actinobacillus pleuropneumoniae

Background

Actinobacillus pleuropneumoniae causes pleuropneumonia in pigs, a disease which is associated with high morbidity and mortality, as well as impaired animal welfare. To obtain in-depth understanding of this infection, the interplay between virulence factors of the pathogen and defense mechanisms of the porcine host needs to be elucidated. However, research has traditionally focused on either bacteriology or immunology; an unbiased picture of the transcriptional responses can be obtained by investigating both organisms in the same biological sample.

Results

Host and pathogen responses in pigs experimentally infected with A. pleuropneumoniae were analyzed by high-throughput RT-qPCR. This approach allowed concurrent analysis of selected genes encoding proteins known or hypothesized to be important in the acute phase of this infection. The expression of 17 bacterial and 31 porcine genes was quantified in lung samples obtained within the first 48 hours of infection. This provided novel insight into the early time course of bacterial genes involved in synthesis of pathogen-associated molecular patterns (lipopolysaccharide, peptidoglycan, lipoprotein) and genes involved in pattern recognition (TLR4, CD14, MD2, LBP, MYD88) in response to A. pleuropneumoniae. Significant up-regulation of proinflammatory cytokines such as IL1B, IL6, and IL8 was observed, correlating with protein levels, infection status and histopathological findings. Host genes encoding proteins involved in iron metabolism, as well as bacterial genes encoding exotoxins, proteins involved in adhesion, and iron acquisition were found to be differentially expressed according to disease progression. By applying laser capture microdissection, porcine expression of selected genes could be confirmed in the immediate surroundings of the invading pathogen.

Conclusions

Microbial pathogenesis is the product of interactions between host and pathogen. Our results demonstrate the applicability of high-throughput RT-qPCR for the elucidation of dual-organism gene expression analysis during infection. We showed differential expression of 12 bacterial and 24 porcine genes during infection and significant correlation of porcine and bacterial gene expression. This is the first study investigating the concurrent transcriptional response of both bacteria and host at the site of infection during porcine respiratory infection.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1557-6) contains supplementary material, which is available to authorized users.

Otitis media associated polymorphisms in the hemin receptor HemR of nontypeable Haemophilus influenzae

Nontypeable Haemophilus influenzae (NTHi) colonize the human pharynx asymptomatically, and are also an important cause of otitis media (OM). Previous studies have demonstrated that some genes are more prevalent in OM-causing NTHi strains than in commensal strains, suggesting a role in virulence. These studies, however, are unable to investigate the possible associations between gene polymorphisms and disease. This study examined amino acid polymorphisms and sequence diversity in a potential virulence gene, the hemin receptor hemR, from a previously characterized NTHi strain collection containing both commensal and OM organisms to identify possible associations between the polymorphisms and otitis media. The full open reading frame of hemR was sequenced from a total of 146 NTHi isolates, yielding a total of 47 unique HemR amino acid sequences. The predicted structure of HemR showed substantial similarity to a class of monomeric TonB dependent, ligand-gated channels involved in iron acquisition in other gram negative bacteria. Fifteen amino acid polymorphisms were significantly more prevalent at the 90% confidence level among commensal compared to OM isolates. Upon controlling for the confounding effect of population structure, over half of the polymorphism-otitis media relationships lost statistical significance, emphasizing the importance of assessing the effect of population structure in association studies. The seven polymorphisms that retained significance were dispersed throughout the protein in various functional and structural domains, including the signal peptide, N-terminal plug domain, and intra- and extracellular loops. The alternate amino acid of only one of these seven polymorphisms was more common among OM isolates, demonstrating a strong trend toward the consensus sequence among disease causing NTHi. We hypothesize that variability at these positions in HemR may result in a reduced ability to acquire iron, rendering NTHi with such versions of the gene less fit for survival in the middle ear environment.

Outer membrane protein P4 is not required for virulence in the human challenge model of Haemophilus ducreyi infection

Background

Bacterial lipoproteins often play important roles in pathogenesis and can stimulate protective immune responses. Such lipoproteins are viable vaccine candidates. Haemophilus ducreyi, which causes the sexually transmitted disease chancroid, expresses a number of lipoproteins during human infection. One such lipoprotein, OmpP4, is homologous to the outer membrane lipoprotein e (P4) of H. influenzae. In H. influenzae, e (P4) stimulates production of bactericidal and protective antibodies and contributes to pathogenesis by facilitating acquisition of the essential nutrients heme and nicotinamide adenine dinucleotide (NAD). Here, we tested the hypothesis that, like its homolog, H. ducreyi OmpP4 contributes to virulence and stimulates production of bactericidal antibodies.

Results

We determined that OmpP4 is broadly conserved among clinical isolates of H. ducreyi. We next constructed and characterized an isogenic ompP4 mutant, designated 35000HPompP4, in H. ducreyi strain 35000HP. To test whether OmpP4 was necessary for virulence in humans, eight healthy adults were experimentally infected. Each subject was inoculated with a fixed dose of 35000HP on one arm and three doses of 35000HPompP4 on the other arm. The overall parent and mutant pustule formation rates were 52.4% and 47.6%, respectively (P = 0.74). These results indicate that expression of OmpP4 in not necessary for H. ducreyi to initiate disease or progress to pustule formation in humans. Hyperimmune mouse serum raised against purified, recombinant OmpP4 did not promote bactericidal killing of 35000HP or phagocytosis by J774A.1 mouse macrophages in serum bactericidal and phagocytosis assays, respectively.

Conclusions

Our data suggest that, unlike e (P4), H. ducreyi OmpP4 is not a suitable vaccine candidate. OmpP4 may be dispensable for virulence because of redundant mechanisms in H. ducreyi for heme acquisition and NAD utilization.

Shared and distinct mechanisms of iron acquisition by bacterial and fungal pathogens of humans

Iron is the most abundant transition metal in the human body and its bioavailability is stringently controlled. In particular, iron is tightly bound to host proteins such as transferrin to maintain homeostasis, to limit potential damage caused by iron toxicity under physiological conditions and to restrict access by pathogens. Therefore, iron acquisition during infection of a human host is a challenge that must be surmounted by every successful pathogenic microorganism. Iron is essential for bacterial and fungal physiological processes such as DNA replication, transcription, metabolism, and energy generation via respiration. Hence, pathogenic bacteria and fungi have developed sophisticated strategies to gain access to iron from host sources. Indeed, siderophore production and transport, iron acquisition from heme and host iron-containing proteins such as hemoglobin and transferrin, and reduction of ferric to ferrous iron with subsequent transport are all strategies found in bacterial and fungal pathogens of humans. This review focuses on a comparison of these strategies between bacterial and fungal pathogens in the context of virulence and the iron limitation that occurs in the human body as a mechanism of innate nutritional defense.

Haemophilus influenzae OxyR: characterization of its regulation, regulon and role in fitness

To prevent damage by reactive oxygen species, many bacteria have evolved rapid detection and response systems, including the OxyR regulon. The OxyR system detects reactive oxygen and coordinates the expression of numerous defensive antioxidants. In many bacterial species the coordinated OxyR-regulated response is crucial for in vivo survival. Regulation of the OxyR regulon of Haemophilus influenzae was examined in vitro, and significant variation in the regulated genes of the OxyR regulon among strains of H. influenzae was observed. Quantitative PCR studies demonstrated a role for the OxyR-regulated peroxiredoxin/glutaredoxin as a mediator of the OxyR response, and also indicated OxyR self-regulation through a negative feedback loop. Analysis of transcript levels in H. influenzae samples derived from an animal model of otitis media demonstrated that the members of the OxyR regulon were actively upregulated within the chinchilla middle ear. H. influenzae mutants lacking the oxyR gene exhibited increased sensitivity to challenge with various peroxides. The impact of mutations in oxyR was assessed in various animal models of H. influenzae disease. In paired comparisons with the corresponding wild-type strains, the oxyR mutants were unaffected in both the chinchilla model of otitis media and an infant model of bacteremia. However, in weanling rats the oxyR mutant was significantly impaired compared to the wild-type strain. In contrast, in all three animal models when infected with a mixture of equal numbers of both wild-type and mutant strains the mutant strain was significantly out competed by the wild-type strain. These findings clearly establish a crucial role for OxyR in bacterial fitness.

A functional tonB gene is required for both virulence and competitive fitness in a chinchilla model of Haemophilus influenzae otitis media

Background

Haemophilus influenzae requires heme for aerobic growth and possesses multiple mechanisms to obtain this essential nutrient.

Methods

An insertional mutation in tonB was constructed and the impact of the mutation on virulence and fitness in a chinchilla model of otitis media was determined. The tonB insertion mutant strain was significantly impacted in both virulence and fitness as compared to the wildtype strain in this model.

Conclusions

The tonB gene of H. influenzae is required for the establishment and maintenance of middle ear infection in this chinchilla model of bacterial disease.

Transcriptional portrait of Actinobacillus pleuropneumoniae during acute disease--potential strategies for survival and persistence in the host

Background

Gene expression profiles of bacteria in their natural hosts can provide novel insight into the host-pathogen interactions and molecular determinants of bacterial infections. In the present study, the transcriptional profile of the porcine lung pathogen Actinobacillus pleuropneumoniae was monitored during the acute phase of infection in its natural host.

Methodology/Principal Findings

Bacterial expression profiles of A. pleuropneumoniae isolated from lung lesions of 25 infected pigs were compared in samples taken 6, 12, 24 and 48 hours post experimental challenge. Within 6 hours, focal, fibrino hemorrhagic lesions could be observed in the pig lungs, indicating that A. pleuropneumoniae had managed to establish itself successfully in the host. We identified 237 differentially regulated genes likely to encode functions required by the bacteria for colonization and survival in the host. This group was dominated by genes involved in various aspects of energy metabolism, especially anaerobic respiration and carbohydrate metabolism. Remodeling of the bacterial envelope and modifications of posttranslational processing of proteins also appeared to be of importance during early infection. The results suggested that A. pleuropneumoniae is using various strategies to increase its fitness, such as applying Na+ pumps as an alternative way of gaining energy. Furthermore, the transcriptional data provided potential clues as to how A. pleuropneumoniae is able to circumvent host immune factors and survive within the hostile environment of host macrophages. This persistence within macrophages may be related to urease activity, mobilization of various stress responses and active evasion of the host defenses by cell surface sialylation.

Conclusions/Significance

The data presented here highlight the importance of metabolic adjustments to host conditions as virulence factors of infecting microorganisms and help to provide insight into the mechanisms behind the efficient colonization and persistence of A. pleuropneumoniae during acute disease.

Regulation of virulence gene expression resulting from Streptococcus pneumoniae and nontypeable Haemophilus influenzae interactions in chronic disease

Chronic rhinosinusitis (CRS) is a common inflammatory disease of the sinonasal cavity mediated, in part, by polymicrobial communities of bacteria. Recent molecular studies have confirmed the importance of Streptococcus pneumoniae and nontypeable Haemophilus influenzae (NTHi) in CRS. Here, we hypothesize that interaction between S. pneumoniae and NTHi mixed-species communities cause a change in bacterial virulence gene expression. We examined CRS as a model human disease to validate these polymicrobial interactions. Clinical strains of S. pneumoniae and NTHi were grown in mono- and co-culture in a standard biofilm assay. Reverse transcriptase real-time PCR (RTqPCR) was used to measure gene expression of key virulence factors. To validate these results, we investigated the presence of the bacterial RNA transcripts in excised human tissue from patients with CRS. Consequences of physical or chemical interactions between microbes were also investigated. Transcription of NTHi type IV pili was only expressed in co-culture in vitro, and expression could be detected ex vivo in diseased tissue. S. pneumoniae pyruvate oxidase was up-regulated in co-culture, while pneumolysin and pneumococcal adherence factor A were down-regulated. These results were confirmed in excised human CRS tissue. Gene expression was differentially regulated by physical contact and secreted factors. Overall, these data suggest that interactions between H. influenzae and S. pneumoniae involve physical and chemical mechanisms that influence virulence gene expression of mixed-species biofilm communities present in chronically diseased human tissue. These results extend previous studies of population-level virulence and provide novel insight into the importance of S. pneumoniae and NTHi in CRS.

Identification of a siderophore utilization locus in nontypeable Haemophilus influenzae

BACKGROUND

Haemophilus influenzae has an absolute aerobic growth requirement for either heme, or iron in the presence of protoporphyrin IX. Both iron and heme in the mammalian host are strictly limited in their availability to invading microorganisms. Many bacterial species overcome iron limitation in their environment by the synthesis and secretion of small iron binding molecules termed siderophores, which bind iron and deliver it into the bacterial cell via specific siderophore receptor proteins on the bacterial cell surface. There are currently no reports of siderophore production or utilization by H. influenzae.

RESULTS

Comparative genomics revealed a putative four gene operon in the recently sequenced nontypeable H. influenzae strain R2846 that encodes predicted proteins exhibiting significant identity at the amino acid level to proteins involved in the utilization of the siderophore ferrichrome in other bacterial species. No siderophore biosynthesis genes were identified in the R2846 genome. Both comparative genomics and a PCR based analysis identified several additional H. influenzae strains possessing this operon. In growth curve assays strains containing the genes were able to utilize ferrichrome as an iron source. H. influenzae strains lacking the operon were unable to obtain iron from ferrichrome. An insertional mutation in one gene of the operon abrogated the ability of strains to utilize ferrichrome. In addition transcription of genes in the identified operon were repressible by high iron/heme levels in the growth media.

CONCLUSIONS

We have identified an iron/heme-repressible siderophore utilization locus present in several nontypeable H. influenzae strains. The same strains do not possess genes encoding proteins associated with siderophore synthesis. The siderophore utilization locus may enable the utilization of siderophores produced by other microorganisms in the polymicrobial environmental niche of the human nasopharynx colonized by H. influenzae. This is the first report of siderophore utilization by H. influenzae.

Characterization of the Haemophilus influenzae tehB gene and its role in virulence

The Haemophilus influenzae ORF designated HI1275 in the Rd KW20 genomic sequence encodes a putative S-adenosyl methyltransferase with significant similarity to tellurite-resistance determinants (tehB) in other species. While the H. influenzae tehB can complement an Escherichia coli tehB mutation, thus restoring tellurite resistance, its role in H. influenzae is unknown. In a previous study defining the iron and haem modulon of H. influenzae, we showed that transcription of this gene in H. influenzae Rd KW20 increases during growth in iron- and haem-restricted media. Since iron and haem uptake genes, and other known virulence factors, constitute the majority of the iron- and haem-regulated gene set, we postulated that tehB may play a role in nutrient acquisition and/or the virulence of H. influenzae. A tehB mutant was constructed in the H. influenzae type b strain 10810 and was evaluated for growth defects in various supplemented media, as well as for its ability to cause infection in rat models of infection. Deletion of tehB leads to an increase in sensitivity both to tellurite and to the oxidizing agents cumene hydroperoxide, tert-butyl hydroperoxide and hydrogen peroxide. The tehB mutant additionally showed a significantly reduced ability to utilize free haem as well as several haem-containing moieties including haem-human serum albumin, haemoglobin and haemoglobin-haptoglobin. Examination of the regulation kinetics indicated that transcription of tehB was independent of both tellurite exposure and oxidative stress. Paired comparisons of the tehB mutant and the wild-type H. influenzae strain 10810 showed that tehB is required for wild-type levels of infection in rat models of H. influenzae invasive disease. To our knowledge this is the first report of a role for tehB in virulence in any bacterial species. These data demonstrate that H. influenzae tehB plays a role in both resistance to oxidative damage and haem uptake/utilization, protects H. influenzae from tellurite exposure, and is important for virulence of this organism in an animal model of invasive disease.

The heme-binding protein (HbpA) of Haemophilus influenzae as a virulence determinant

Haemophilus influenzae has an absolute growth requirement for heme and the heme-binding lipoprotein (HbpA) and has been implicated in the utilization of this essential nutrient. We constructed an insertional mutation of hbpA in a type b and a nontypeable H. influenzae strain. In the type b strain, the hbpA mutant was impaired in utilization of heme complexed to either hemopexin or to albumin and in the utilization of low levels of heme but not in the utilization of heme at high levels or of hemoglobin or hemoglobin-haptoglobin complexes. In contrast, the hbpA mutant derivative of the nontypeable strain was impaired in utilization of all tested heme sources. We further examined the impact of the hbpA mutation in animal models of H. influenzae disease. The hbpA mutant of the nontypeable strain was indistinguishable from the wild-type strain in the chinchilla model of otitis media. The hbpA mutant derivative of the type b strain caused bacteremia as well as the wild-type strain in 5-day old infant rats. However, in 30-day old rats the hbpA caused significantly lower rates of bacteremia than the wild-type strain indicating a role for hbpA and heme acquisition in virulence in this model of H. influenzae disease. In conclusion, HbpA is important for heme utilization by multiple H. influenzae strains and is a virulence determinant in a model of H. influenzae invasive disease.

Use of in vivo-induced antigen technology (IVIAT) for the identification of Streptococcus suis serotype 2 in vivo-induced bacterial protein antigens

Background

Streptococcus suis serotype 2 (SS2) is a zoonotic agent that causes death and disease in both humans and swine. A better understanding of SS2-host molecular interactions is crucial for understanding SS2 pathogenesis and immunology. Conventional genetic and biochemical approaches used to study SS2 virulence factors are unable to take into account the complex and dynamic environmental stimuli associated with the infection process. In this study, in vivo-induced antigen technology (IVIAT), an immunoscreening technique, was used to identify the immunogenic bacterial proteins that are induced or upregulated in vivo during SS2 infection.

Results

Convalescent-phase sera from pigs infected with SS2 were pooled, adsorbed against in vitro antigens, and used to screen SS2 genomic expression libraries. Upon analysis of the identified proteins, we were able to assign a putative function to 40 of the 48 proteins. These included proteins implicated in cell envelope structure, regulation, molecule synthesis, substance and energy metabolism, transport, translation, and those with unknown functions. The in vivo-induced changes in the expression of 10 of these 40 genes were measured using real-time reverse transcription (RT)-PCR, revealing that the expression of 6 of the 10 genes was upregulated in the in vivo condition. The strain distribution of these 10 genes was analyzed by PCR, and they were found in the most virulent SS2 strains. In addition, protein sequence alignments of the newly identified proteins demonstrate that three are putative virulence-associated proteins.

Conclusion

Collectively, our results suggest that these in vivo-induced or upregulated genes may contribute to SS2 disease development. We hypothesize that the identification of factors specifically induced or upregulated during SS2 infection will aid in our understanding of SS2 pathogenesis and may contribute to the control SS2 outbreaks. In addition, the proteins identified using IVIAT may be useful potential vaccine candidates or virulence markers.

The iron/heme regulated genes of Haemophilus influenzae: comparative transcriptional profiling as a tool to define the species core modulon

BACKGROUND

Haemophilus influenzae requires heme for aerobic growth and possesses multiple mechanisms to obtain this essential nutrient. Although an understanding of the heme acquisition mechanisms of H. influenzae is emerging, significant gaps in our knowledge remain. Unresolved issues include the identities of all genes exhibiting altered transcription in response to iron and heme availability, the fraction of such genes functioning in iron/heme acquisition, and the heterogeneity of this gene set among clinical isolates. Previously we utilized H. influenzae strain Rd KW20 to demonstrate the utility of transcriptional profiling in defining the genes exhibiting altered transcription in response to environmental iron and heme levels. The current study expands upon those observations by determining the iron/heme modulons of two clinical isolates, the type b isolate 10810 and the nontypeable isolate R2866. These data are used to begin to define the core iron/heme modulon of the species.

RESULTS

Microarray studies were performed to compare gene expression on transition from iron/heme-restricted to iron/heme-replete conditions for each isolate. Of 1820 ORFs on the array corresponding to R2866 genes, 363 were significantly differentially expressed: 233 were maximally transcribed under iron/heme-replete conditions and 130 under iron/heme-restricted conditions. Of the 1883 ORFs representing genes of strain 10810, 353 were significantly differentially transcribed: 150 were preferentially transcribed under iron/heme-replete conditions and 203 under iron/heme-restricted conditions. Comparison of the data sets indicated that 163 genes exhibited similar regulation in both isolates and that 74 of these exhibited similar patterns of regulation in Rd KW20. These comprise the putative core iron/heme modulon.

CONCLUSION

This study provides evidence for a conserved core of H. influenzae genes the transcription of which is altered by the availability of iron and/or heme in the growth environment. Elucidation of this modulon provides a means to identify genes with unrecognized roles in iron/heme acquisition or homeostasis, unanticipated responsiveness to environmental levels of the micronutrients or potential roles in virulence. Defining these core genes is also of potential importance in identifying targets for therapeutic and vaccine designs since products of these genes are likely to be preferentially expressed during growth in iron/heme restricted sites of the human body.

The periplasmic disulfide oxidoreductase DsbA contributes to Haemophilus influenzae pathogenesis

Haemophilus influenzae is an obligate human pathogen that persistently colonizes the nasopharynx and causes disease when it invades the bloodstream, lungs, or middle ear. Proteins that mediate critical interactions with the host during invasive disease are likely to be secreted. Many secreted proteins require addition of disulfide bonds by the DsbA disulfide oxidoreductase for activity or stability. In this study, we evaluated the role in H. influenzae pathogenesis of DsbA, as well as HbpA, a substrate of DsbA. Mutants of H. influenzae Rd and type b strain Eagan having nonpolar deletions of dsbA were attenuated for bacteremia in animal models, and complemented strains exhibited virulence equivalent to that of the parental strains. Comparison of predicted secreted proteins in H. influenzae to known DsbA substrates in other species revealed several proteins that could contribute to the role of dsbA in virulence. One candidate, the heme transport protein, HbpA, was examined because of the importance of exogenous heme for aerobic growth of H. influenzae. The presence of a dsbA-dependent disulfide bond in HbpA was verified by an alkylation protection assay, and HbpA was less abundant in a dsbA mutant. The hbpA mutant exhibited reduced bacteremia in the mouse model, and complementation restored its in vivo phenotype to that of the parental strain. These results indicate that dsbA is required in vivo and that HbpA and additional DsbA-dependent factors are likely to participate in H. influenzae pathogenesis.

Catalase as a source of both X- and V-factor for Haemophilus influenzae

Haemophilus influenzae requires two growth factors, designated factor X (porphyrin) and factor V (NAD). Mammalian catalases contain both bound heme and NADPH. This study shows that catalase can supply both factors X and V to H. influenzae in vitro, thus representing a potential in vivo source of these essential growth factors.