Aerobic growth deficient Haemophilus influenzae mutants are non-virulent: implications on metabolism

Access to highly specialized growth substrates and production of epithelial immunomodulatory metabolites determine survival of Haemophilus influenzae in human airway epithelial cells

Haemophilus influenzae (Hi) infections are associated with recurring acute exacerbations of chronic respiratory diseases in children and adults including otitis media, pneumonia, chronic obstructive pulmonary disease and asthma. Here, we show that persistence and recurrence of Hi infections are closely linked to Hi metabolic properties, where preferred growth substrates are aligned to the metabolome of human airway epithelial surfaces and include lactate, pentoses, and nucleosides, but not glucose that is typically used for studies of Hi growth in vitro. Enzymatic and physiological investigations revealed that utilization of lactate, the preferred Hi carbon source, required the LldD L-lactate dehydrogenase (conservation: 98.8% of strains), but not the two redox-balancing D-lactate dehydrogenases Dld and LdhA. Utilization of preferred substrates was directly linked to Hi infection and persistence. When unable to utilize L-lactate or forced to rely on salvaged guanine, Hi showed reduced extra- and intra-cellular persistence in a murine model of lung infection and in primary normal human nasal epithelia, with up to 3000-fold attenuation observed in competitive infections. In contrast, D-lactate dehydrogenase mutants only showed a very slight reduction compared to the wild-type strain. Interestingly, acetate, the major Hi metabolic end-product, had anti-inflammatory effects on cultured human tissue cells in the presence of live but not heat-killed Hi, suggesting that metabolic endproducts also influence HI-host interactions. Our work provides significant new insights into the critical role of metabolism for Hi persistence in contact with host cells and reveals for the first time the immunomodulatory potential of Hi metabolites.

Haemophilus influenzae Glucose Catabolism Leading to Production of the Immunometabolite Acetate Has a Key Contribution to the Host Airway-Pathogen Interplay

Chronic obstructive pulmonary disease (COPD) is characterized by abnormal inflammatory responses and impaired airway immunity, which provides an opportunistic platform for nontypeable Haemophilus influenzae (NTHi) infection. Clinical evidence supports that the COPD airways present increased concentrations of glucose, which may facilitate proliferation of pathogenic bacteria able to use glucose as a carbon source. NTHi metabolizes glucose through respiration-assisted fermentation, leading to the excretion of acetate, formate, and succinate. We hypothesized that such specialized glucose catabolism may be a pathoadaptive trait playing a pivotal role in the NTHi airway infection. To find out whether this is true, we engineered and characterized bacterial mutant strains impaired to produce acetate, formate, or succinate by inactivating the ackA, pflA, and frdA genes, respectively. While the inactivation of the pflA and frdA genes only had minimal physiological effects, the inactivation of the ackA gene affected acetate production and led to reduced bacterial growth, production of lactate under low oxygen tension, and bacterial attenuation in vivo. Moreover, bacterially produced acetate was able to stimulate the expression of inflammatory genes by cultured airway epithelial cells. These results back the notion that the COPD lung supports NTHi growth on glucose, enabling production of fermentative end products acting as immunometabolites at the site of infection. Thus, glucose catabolism may contribute not only to NTHi growth but also to bacterially driven airway inflammation. This information has important implications for developing nonantibiotic antimicrobials, given that airway glucose homeostasis modifying drugs could help prevent microbial infections associated with chronic lung disease.

Comprehensive Proteomic and Metabolomic Signatures of Nontypeable Haemophilus influenzae-Induced Acute Otitis Media Reveal Bacterial Aerobic Respiration in an Immunosuppressed Environment

A thorough understanding of the molecular details of the interactions between bacteria and host are critical to ultimately prevent disease. Recent technological advances allow simultaneous analysis of host and bacterial protein and metabolic profiles from a single small tissue sample to provide insight into pathogenesis. We used the chinchilla model of human otitis media to determine, for the first time, the most expansive delineation of global changes in protein and metabolite profiles during an experimentally induced disease. After 48 h of infection with nontypeable Haemophilus influenzae, middle ear tissue lysates were analyzed by high-resolution quantitative two-dimensional liquid chromatography-tandem mass spectrometry. Dynamic changes in 105 chinchilla proteins and 66 metabolites define the early proteomic and metabolomic signature of otitis media. Our studies indicate that establishment of disease coincides with actin morphogenesis, suppression of inflammatory mediators, and bacterial aerobic respiration. We validated the observed increase in the actin-remodeling complex, Arp2/3, and experimentally showed a role for Arp2/3 in nontypeable Haemophilus influenzae invasion. Direct inhibition of actin branch morphology altered bacterial invasion into host epithelial cells, and is supportive of our efforts to use the information gathered to modify outcomes of disease. The twenty-eight nontypeable Haemophilus influenzae proteins identified participate in carbohydrate and amino acid metabolism, redox homeostasis, and include cell wall-associated metabolic proteins. Quantitative characterization of the molecular signatures of infection will redefine our understanding of host response driven developmental changes during pathogenesis. These data represent the first comprehensive study of host protein and metabolite profiles in vivo in response to infection and show the feasibility of extensive characterization of host protein profiles during disease. Identification of novel protein targets and metabolic biomarkers will advance development of therapeutic and diagnostic options for treatment of disease.

Metabolic versatility in Haemophilus influenzae: a metabolomic and genomic analysis

Haemophilus influenzae is a host adapted human pathogen known to contribute to a variety of acute and chronic diseases of the upper and lower respiratory tract as well as the middle ear. At the sites of infection as well as during growth as a commensal the environmental conditions encountered by H. influenzae will vary significantly, especially in terms of oxygen availability, however, the mechanisms by which the bacteria can adapt their metabolism to cope with such changes have not been studied in detail. Using targeted metabolomics the spectrum of metabolites produced during growth of H. influenzae on glucose in RPMI-based medium was found to change from acetate as the main product during aerobic growth to formate as the major product during anaerobic growth. This change in end-product is likely caused by a switch in the major route of pyruvate degradation. Neither lactate nor succinate or fumarate were major products of H. influenzae growth under any condition studied. Gene expression studies and enzyme activity data revealed that despite an identical genetic makeup and very similar metabolite production profiles, H. influenzae strain Rd appeared to favor glucose degradation via the pentose phosphate pathway, while strain 2019, a clinical isolate, showed higher expression of enzymes involved in glycolysis. Components of the respiratory chain were most highly expressed during microaerophilic and anaerobic growth in both strains, but again clear differences existed in the expression of genes associated e.g., with NADH oxidation, nitrate and nitrite reduction in the two strains studied. Together our results indicate that H. influenzae uses a specialized type of metabolism that could be termed “respiration assisted fermentation” where the respiratory chain likely serves to alleviate redox imbalances caused by incomplete glucose oxidation, and at the same time provides a means of converting a variety of compounds including nitrite and nitrate that arise as part of the host defence mechanisms.

The role of the RNA chaperone Hfq in Haemophilus influenzae pathogenesis

Background

The RNA binding protein Hfq of Haemophilus influenzae is highly homologous to Hfq from other bacterial species. In many of these other bacteria, Hfq affects the expression of a broad range of genes and enhances the ability to respond to stressful environments. However, the role of Hfq in H. influenzae is unknown.

Results

Deletion mutants of hfq were generated in the nontypeable H. influenzae strains R2866 and 86-028NP to assess the role of Hfq in these well characterized but genotypically and phenotypically divergent clinical isolates. A deletion mutation of hfq had no effect on growth of H. influenzae in nutrient rich media and had no effect on survival in several stressful conditions in vitro. However, the mutation resulted in a reduced ability to utilize heme from hemoglobin. The mutant and wild type strains were assessed for virulence and competitive fitness in models of invasive disease and otitis media. In the chinchilla model of otitis media, the hfq mutant of 86-028NP exhibited impaired competitive fitness when compared to its wild type progenitor but exhibited no apparent defect in virulence. In the infant rat model, deletion of hfq in R2866 resulted in reduced bacterial titers in blood and a shorter duration of infection when compared to the wild type strain in the competitive fitness study.

Conclusion

We conclude that Hfq is involved in the utilization of essential nutrients and facilitates infection by H. influenzae.

Haemophilus influenzae and oxidative stress

Haemophilus influenzae is a commensal of the human upper respiratory tract. H. influenzae can, however, move out of its commensal niche and cause multiple respiratory tract diseases. Such diseases include otitis media in young children, as well as exacerbations of chronic obstructive pulmonary disease (COPD), sinusitis, conjunctivitis, and bronchitis. During the course of colonization and infection, H. influenzae must withstand oxidative stress generated by multiple reactive oxygen species produced endogenously, by other co-pathogens and by host cells. H. influenzae has, therefore, evolved multiple mechanisms that protect the cell against oxygen-generated stresses. In this review, we will describe these systems relative to the well-described systems in Escherichia coli. Moreover, we will compare how H. influenzae combats the effect of oxidative stress as a necessary phenotype for its roles as both a successful commensal and pathogen.

Exometabolome analysis identifies pyruvate dehydrogenase as a target for the antibiotic triphenylbismuthdichloride in multiresistant bacterial pathogens

The desperate need for new therapeutics against notoriously antibiotic-resistant bacteria has led to a quest for novel antibacterial target structures and compounds. Moreover, defining targets and modes of action of new antimicrobial compounds remains a major challenge with standard technologies. Here we characterize the antibacterial properties of triphenylbismuthdichloride (TPBC), which has recently been successfully used against device-associated infections. We demonstrate that TPBC has potent antimicrobial activity against many bacterial pathogens. Using an exometabolome profiling approach, a unique TPBC-mediated change in the metabolites of Staphylococcus aureus was identified, indicating that TPBC blocks bacterial pyruvate catabolism. Enzymatic studies showed that TPBC is a highly efficient, uncompetitive inhibitor of the bacterial pyruvate dehydrogenase complex. Our study demonstrates that metabolomics approaches can offer new avenues for studying the modes of action of antimicrobial compounds, and it indicates that inhibition of the bacterial pyruvate dehydrogenase complex may represent a promising strategy for combating multidrug-resistant bacteria.

Triazaspirodimethoxybenzoyls as selective inhibitors of mycobacterial lipoamide dehydrogenase

Mycobacterium tuberculosis (Mtb) remains the leading single cause of death from bacterial infection. Here we explored the possibility of species-selective inhibition of lipoamide dehydrogenase (Lpd), an enzyme central to Mtb's intermediary metabolism and antioxidant defense. High-throughput screening of combinatorial chemical libraries identified triazaspirodimethoxybenzoyls as high-nanomolar inhibitors of Mtb's Lpd that were noncompetitive versus NADH, NAD(+), and lipoamide and >100-fold selective compared to human Lpd. Efficacy required the dimethoxy and dichlorophenyl groups. The structure of an Lpd-inhibitor complex was resolved to 2.42 A by X-ray crystallography, revealing that the inhibitor occupied a pocket adjacent to the Lpd NADH/NAD(+) binding site. The inhibitor did not overlap with the adenosine moiety of NADH/NAD(+) but did overlap with positions predicted to bind the nicotinamide rings in NADH and NAD(+) complexes. The dimethoxy ring occupied a deep pocket adjacent to the FAD flavin ring where it would block coordination of the NADH nicotinamide ring, while the dichlorophenyl group occupied a more exposed pocket predicted to coordinate the NAD(+) nicotinamide. Several residues that are not conserved between the bacterial enzyme and its human homologue were predicted to contribute both to inhibitor binding and to species selectivity, as confirmed for three residues by analysis of the corresponding mutant Mtb Lpd proteins. Thus, nonconservation of residues lining the electron-transfer tunnel in Mtb Lpd can be exploited for development of species-selective Lpd inhibitors.

Transposon insertion in a serine-specific minor tRNA coding sequence affects intraperitoneal survival of Haemophilus influenzae in the infant rat model

Due to its lifestyle as a commensal and occasional pathogen in the upper and lower respiratory tracts of humans, Haemophilus influenzae needs to protect itself from endogenously and exogenously generated reactive oxygen species. To better understand the oxygen radical resistance and to investigate a correlation with virulence, randomly generated paraquat-sensitive H. influenzae transposon mutants were analyzed in an infant rat model of infection. Among 25 different paraquat-sensitive mutants only one mutant harbouring a Tn-insertion within the tRNA-Ser1 gene specific for the rare serine codon UCC, was highly attenuated for intraperitoneal infectivity. Compared to the wild-type strain, the tRNA-Ser1 mutant was also more sensitive to neutrophil-mediated killing, deficient for DNA transformation but showed similar growth rates under laboratory conditions. However, by comparative analysis using an oxyR mutant strain, we could show that neutrophil-mediated killing might not be relevant for intraperitoneal infectivity. Therefore, the increased ROS sensitivity observed for tRNA-Ser1 mutant may not be directly responsible for the observed virulence deficiency in the intraperitoneal infection. We speculate that a reduced translation efficiency of several UCC containing mRNAs results in a delay of protein synthesis and consequently in the loss of cellular mechanisms which are necessary for ROS resistance and virulence.

The heat-shock protein ClpB of Francisella tularensis is involved in stress tolerance and is required for multiplication in target organs of infected mice

Intracellular bacterial pathogens generally express chaperones such as Hsp100s during multiplication in host cells, allowing them to survive potentially hostile conditions. Francisella tularensis is a highly infectious bacterium causing the zoonotic disease tularaemia. The ability of F. tularensis to multiply and survive in macrophages is considered essential for its virulence. Although previous mutant screens in Francisella have identified the Hsp100 chaperone ClpB as important for intracellular survival, no detailed study has been performed. We demonstrate here that ClpB of F. tularensis live vaccine strain (LVS) is important for resistance to cellular stress. Promoter analysis shows that the transcriptional start is preceded by a sigma32-like promoter sequence and we demonstrate that expression of clpB is induced by heat shock. This indicates that expression of clpB is dependent on the heat-shock response mediated by sigma32, the only alternative sigma-factor present in Francisella. Our studies demonstrate that ClpB contributes to intracellular multiplication in vitro, but is not essential. However, ClpB is absolutely required for Francisella to replicate in target organs and induce disease in mice. Proteomic analysis of membrane-enriched fractions shows that five proteins are recovered at lower levels in the mutant strain. The crucial role of ClpB for in vivo persistence of Francisella may be linked to its assumed function in reactivation of aggregated proteins under in vivo stress conditions.

Pulsed-field gel electrophoresis, pertactin, pertussis toxin S1 subunit polymorphisms, and surfaceome analysis of vaccine and clinical Bordetella pertussis strains

To add new insight to our previous work on the molecular epidemiology of Bordetella pertussis in Argentina, the prn and ptxS1 gene sequences and pulsed-field gel electrophoresis (PFGE) profiles of 57 clinical isolates obtained during two periods, 1969 to 1989 and 1997 to 2006, were analyzed. Non-vaccine-type ptxS1A was detected in isolates obtained since 1969. From 1989 on, a shift of predominance from the vaccine prn1 type to the nonvaccine prn2 type was observed. This was also reflected in a transition of PFGE group IV to group VI. These results show that nonvaccine B. pertussis strains are currently circulating. To analyze whether the observed genomic divergences between vaccine strains and clinical isolates have functional implications, protection assays using the intranasal mouse challenge model were performed. For such experiments, the clinical isolate B. pertussis 106 was selected as representative of circulating bacteria, since it came from the major group of the PFGE dendrogram (PFGE group VI). Groups of mice were immunized either with diphtheria-tetanus-whole-cell pertussis vaccine (ptxS1B prn1) or a vaccine prepared by us containing B. pertussis 106. Immunized mice were then challenged with a B. pertussis vaccine strain (Tohama, harboring ptxS1B and prn1) or the clinical isolate B. pertussis 106 (ptxS1A prn2). An adequate bacterial-elimination rate was observed only when mice were immunized and challenged with the same kind of strain. For further characterization, comparative proteomic profiling of enriched membrane proteins was done using three vaccine strains and the selected B. pertussis 106 clinical isolate. By matrix-assisted laser desorption ionization-time of flight mass spectrometry analysis, a total of 54 proteins were identified. This methodology allowed us to detect differing proteins among the four strains studied and, in particular, to distinguish the three vaccine strains from each other, as well as the vaccine strains from the clinical isolate. The differing proteins observed have cellular roles associated with amino acid and carbohydrate transport and metabolism. Some of them have been proposed as novel vaccine candidate proteins for other pathogens. Overall, the global strategy described here is presented as a good tool for the development of next-generation acellular vaccines.

Pleiotropic role of quorum-sensing autoinducer 2 in Photorhabdus luminescens

Bacterial virulence is an integrative process that may involve quorum sensing. In this work, we compared by global expression profiling the wild-type entomopathogenic Photorhabdus luminescens subsp. laumondii TT01 to a luxS-deficient mutant unable to synthesize the type 2 quorum-sensing inducer AI-2. AI-2 was shown to regulate more than 300 targets involved in most compartments and metabolic pathways of the cell. AI-2 is located high in the hierarchy, as it controls the expression of several transcriptional regulators. The regulatory effect of AI-2 appeared to be dose dependent. The luxS-deficient strain exhibited decreased biofilm formation and increased type IV/V pilus-dependent twitching motility. AI-2 activated its own synthesis and transport. It also modulated bioluminescence by regulating the synthesis of spermidine. AI-2 was further shown to increase oxidative stress resistance, which is necessary to overcome part of the innate immune response of the host insect involving reactive oxygen species. Finally, we showed that the luxS-deficient strain had attenuated virulence against the lepidopteran Spodoptera littoralis. We concluded that AI-2 is involved mainly in early steps of insect invasion in P. luminescens.

Complex role of hemoglobin and hemoglobin-haptoglobin binding proteins in Haemophilus influenzae virulence in the infant rat model of invasive infection

Haemophilus influenzae requires an exogenous heme source for aerobic growth in vitro. Hemoglobin or hemoglobin-haptoglobin satisfies this requirement. Heme acquisition from hemoglobin-haptoglobin is mediated by proteins encoded by hgp genes. Both Hgps and additional proteins, including those encoded by the hxu operon, provide independent pathways for hemoglobin utilization. Recently we showed that deletion of the set of three hgp genes from a nontypeable strain (86-028NP) of H. influenzae attenuated virulence in the chinchilla otitis media model of noninvasive disease. The present study was undertaken to investigate the role of the hgp genes in virulence of the wild-type serotype b clinical isolate HI689 in the infant rat model of hematogenous meningitis, an established model of invasive disease requiring aerobic growth. Bacteremia of high titer and long duration (>14 days) and histopathologically confirmed meningitis occurred in >95% of infant rats challenged at 5 days of age with strain HI689. While mutations disrupting either the Hgp- or Hxu-mediated pathway of heme acquisition had no effect on virulence in infant rats, an isogenic mutant deficient for both pathways was unable to sustain bacteremia or produce meningitis. In contrast, mutations disrupting either pathway decreased the limited ability of H. influenzae to initiate and sustain bacteremia in weanling rats. Biochemical and growth studies also indicated that infant rat plasma contains multiple heme sources that change with age. Taken together, these data indicate that both the hgp genes and the hxuC gene are virulence determinants in the rat model of human invasive disease.

Identification of a virulence-associated determinant, dihydrolipoamide dehydrogenase (lpd), in Mycoplasma gallisepticum through in vivo screening of transposon mutants

To effectively analyze Mycoplasma gallisepticum for virulence-associated determinants, the ability to create stable genetic mutations is essential. Global M. gallisepticum mutagenesis is currently limited to the use of transposons. Using the gram-positive transposon Tn4001mod, a mutant library of 110 transformants was constructed and all insertion sites were mapped. To identify transposon insertion points, a unique primer directed outward from the end of Tn4001mod was used to sequence flanking genomic regions. By comparing sequences obtained in this manner to the annotated M. gallisepticum genome, the precise locations of transposon insertions were discerned. After determining the transposon insertion site for each mutant, unique reverse primers were synthesized based on the specific sequences, and PCR was performed. The resultant amplicons were used as unique Tn4001mod mutant identifiers. This procedure is referred to as signature sequence mutagenesis (SSM). SSM permits the comprehensive screening of the M. gallisepticum genome for the identification of novel virulence-associated determinants from a mixed mutant population. To this end, chickens were challenged with a pool of 27 unique Tn4001mod mutants. Two weeks postinfection, the birds were sacrificed, and organisms were recovered from respiratory tract tissues and screened for the presence or absence of various mutants. SSM is a negative-selection screening technique whereby those mutants possessing transposon insertions in genes essential for in vivo survival are not recovered from the host. We have identified a virulence-associated gene encoding dihydrolipoamide dehydrogenase (lpd). A transposon insertion in the middle of the coding sequence resulted in diminished biologic function and reduced virulence of the mutant designated Mg 7.

Hydrogen peroxide scavenging is not a virulence determinant in the pathogenesis of Haemophilus influenzae type b strain Eagan

BACKGROUND

A potentially lethal flux of hydrogen peroxide (H2O2) is continuously generated during aerobic metabolism. It follows that aerobic organisms have equipped themselves with specific H2O2 dismutases and H2O2 reductases, of which catalase and the alkyl hydroperoxide reductase (AhpR) are the best-studied prokaryotic members. The sequenced Haemophilus influenzae Rd genome reveals one catalase, designated HktE, and no AhpR. However, Haemophilus influenzae type b strain Eagan (Hib), a causative agent of bacterial sepsis and meningitis in young children, disrupted in its hktE gene is not attenuated in virulence, and retains the ability to rapidly scavenge H2O2. This redundancy in H2O2-scavenging is accounted for by peroxidatic activity which specifically uses glutathione as the reducing substrate.

RESULTS

We show here that inside acatalasaemic H. influenzae all of the residual peroxidatic activity is catalyzed by PGdx, a hybrid peroxiredoxin-glutaredoxin glutathione-dependent peroxidase. In vitro kinetic assays on crude hktE- pgdx- H. influenzae Rd extracts revealed the presence of NAD(P)H:peroxide oxidoreductase activity, which, however, appears to be physiologically insignificant because of its low affinity for H2O2 (Km = 1.1 mM). Hydroperoxidase-deficient hktE- pgdx- H. influenzae Rd showed a slightly affected aerobic growth phenotype in rich broth, while, in chemically defined medium, growth was completely inhibited by aerobic conditions, unless the medium contained an amino acid/vitamin supplement. To study the role of PGdx in virulence and to assess the requirement of H2O2-scavenging during the course of infection, both a pgdx single mutant and a pgdx/hktE double mutant of Hib were assayed for virulence in an infant rat model. The ability of both mutant strains to cause bacteremia was unaffected.

CONCLUSION

Catalase (HktE) and a sole peroxidase (PGdx) account for the majority of scavenging of metabolically generated H2O2 in the H. influenzae cytoplasm. Growth experiments with hydroperoxidase-deficient hktE- pgdx- H. influenzae Rd suggest that the cytotoxicity inflicted by the continuous accumulation of H2O2 during aerobic growth brings about bacteriostasis rather than bacterial killing. Finally, H2O2-scavenging is not a determinant of Hib virulence in the infant rat model of infection.

Immunoproteomics of membrane proteins of Shigella flexneri 2a 2457T

AIM

To screen the immunogenic membrane proteins of Shigella flexneri 2a 2457T.

METHODS

The routine two-dimensional polyacrylamide gel electrophoresis (2-DE) and Western blotting were combined to screen immunogenic proteins of S. flexneri 2a 2457T. Serum was gained from rabbits immunized with the same bacteria. Immunogenic spots were cut out from the polyacrylamide gel and digested by trypsin in-gel. Matrix-assisted laser desorption/ionization time of flight-mass spectrometry (MALDI-TOF-MS) was performed to determine the molecular weight of peptides. Electrospray ionization (ESI-MS/MS) was performed to determine the sequences of the interesting peptides.

RESULTS

A total of 20 spots were successfully identified from Coomassie brilliant blue stained gels representing 13 protein entries, 5 known antigens and 8 novel antigens. A hypothetical protein (YaeT) was detected, which might be a candidate target of vaccine.

CONCLUSION

Membrane proteins of S. flexneri 2a 2457T were successfully observed by 2-DE. Several known and novel antigens were identified by mass spectrum.

Coupling of NAD+ biosynthesis and nicotinamide ribosyl transport: characterization of NadR ribonucleotide kinase mutants of Haemophilus influenzae

Previously, we characterized a pathway necessary for the processing of NAD+ and for uptake of nicotinamide riboside (NR) in Haemophilus influenzae. Here we report on the role of NadR, which is essential for NAD+ utilization in this organism. Different NadR variants with a deleted ribonucleotide kinase domain or with a single amino acid change were characterized in vitro and in vivo with respect to cell viability, ribonucleotide kinase activity, and NR transport. The ribonucleotide kinase mutants were viable only in a nadV+ (nicotinamide phosphoribosyltransferase) background, indicating that the ribonucleotide kinase domain is essential for cell viability in H. influenzae. Mutations located in the Walker A and B motifs and the LID region resulted in deficiencies in both NR phosphorylation and NR uptake. The ribonucleotide kinase function of NadR was found to be feedback controlled by NAD+ under in vitro conditions and by NAD+ utilization in vivo. Taken together, our data demonstrate that the NR phosphorylation step is essential for both NR uptake across the inner membrane and NAD+ synthesis and is also involved in controlling the NAD+ biosynthesis rate.

Environmental and genetic regulation of the phosphorylcholine epitope of Haemophilus influenzae lipooligosaccharide

In response to environmental signals in the host, bacterial pathogens express factors required during infection and repress those that interfere with specific stages of this process. Signalling pathways controlling virulence factors of the human respiratory pathogen, Haemophilus influenzae, are predominantly unknown. The lipooligosaccharide (LOS) outer core represents a prototypical virulence trait of H. influenzae that enhances virulence but also provides targets for innate and adaptive immunity. We report regulation of the display of the virulence-associated phosphorylcholine (PC) epitope on the LOS in response to environmental conditions. PC display is optimal under microaerobic conditions and markedly decreased under conditions of high culture aeration. Gene expression analysis using a DNA microarray was performed to begin to define the metabolic state of the cell under these conditions and to identify genes potentially involved in PC epitope modulation. Global gene expression profiling detected changes in redox responsive genes and in genes of carbohydrate metabolism. The effects on carbohydrate metabolism led us to examine the role of the putative H. influenzae homologue of csrA, a regulator of glycolysis and gluconeogenesis in Escherichia coli. A mutant containing an in-frame deletion of the H. influenzae csrA gene showed increased PC epitope levels under aerobic conditions. Furthermore, deletion of csrA elevated mRNA expression of galU, an essential virulence gene that is critical in generating sugar precursors needed for polysaccharide formation and LOS outer core synthesis. Growth conditions predicted to alter the redox state of the culture modulated the PC epitope and galU expression as well. The results are consistent with a multifactorial mechanism of control of LOS-PC epitope display involving csrA and environmental signals that coordinately regulate biosynthetic and metabolic genes controlling the LOS structure.

PnuC and the utilization of the nicotinamide riboside analog 3-aminopyridine in Haemophilus influenzae

The utilization pathway for the uptake of NAD and nicotinamide riboside was previously characterized for Haemophilus influenzae. We now report on the cellular location, topology, and substrate specificity of PnuC. pnuC of H. influenzae is only distantly related to pnuC of Escherichia coli and Salmonella enterica serovar Typhimurium. When E. coli PnuC was expressed in an H. influenzae pnuC mutant, it was able to take up only nicotinamide riboside and not nicotinamide mononucleotide. Therefore, we postulated that PnuC transporters in general possess specificity for nicotinamide riboside. Earlier studies showed that 3-aminopyridine derivatives (e.g., 3-aminopyridine adenine dinucleotide) are inhibitory for H. influenzae growth. By testing characterized strains with mutations in the NAD utilization pathway, we show that 3-aminopyridine riboside is inhibitory to H. influenzae and is taken up by the NAD-processing and nicotinamide riboside route. 3-Aminopyridine riboside is utilized effectively in a pnuC+ background. In addition, we demonstrate that 3-aminopyridine adenine dinucleotide resynthesis is produced by NadR. 3-Aminopyridine riboside-resistant H. influenzae isolates were characterized, and mutations in nadR could be detected. We also tested other species of the family Pasteurellaceae, Pasteurella multocida and Actinobacillus actinomycetemcomitans, and found that 3-aminopyridine riboside does not act as a growth inhibitor; hence, 3-aminopyridine riboside represents an anti-infective agent with a very narrow host range.

Physiological characterization of Haemophilus influenzae Rd deficient in its glutathione-dependent peroxidase PGdx

The chimeric peroxidase PGdx of Haemophilus influenzae Rd belongs to a recently identified family of thiol peroxidases capable of reducing hydrogen peroxide as well as alkylhydroperoxides by means of glutathione redox cycling. In the present study, we constructed a H. influenzae Rd strain, deficient in its PGdx encoding gene (open reading frame HI0572). The mutant was shown by disk inhibition and liquid culture growth assays to exhibit increased susceptibility to organic hydroperoxides. The hampered growth was restored by complementing the interrupted gene on the genome with a replicating plasmid bearing an intact copy of the gene, hereby rejecting the possible influences of polar effects. Elevated levels of hydrogen peroxide scavenging activity, due to the catalase HktE, were measured in the absence of a functional pgdx gene rendering the mutant more resilient against hydrogen peroxide. On the other hand, after initiation of the stationary phase, aerobic cultures of the pgdx mutant were practically devoid of living cells, whereas wild-type counterparts retained viability. This observed feature was alleviated by complementation with the functional gene or with the addition of catalase.