A riboflavin auxotroph of Actinobacillus pleuropneumoniae is attenuated in swine

Role of riboflavin biosynthesis gene duplication and transporter in Aeromonas salmonicida virulence in marine teleost fish

Active flavins derived from riboflavin (vitamin B₂) are essential for life. Bacteria biosynthesize riboflavin or scavenge it through uptake systems, and both mechanisms may be present. Because of riboflavin's critical importance, the redundancy of riboflavin biosynthetic pathway (RBP) genes might be present. Aeromonas salmonicida, the aetiological agent of furunculosis, is a pathogen of freshwater and marine fish, and its riboflavin pathways have not been studied. This study characterized the A. salmonicida riboflavin provision pathways. Homology search and transcriptional orchestration analysis showed that A. salmonicida has a main riboflavin biosynthetic operon that includes ribD, ribE1, ribBA, and ribH genes. Outside the main operon, putative duplicated genes ribA, ribB and ribE, and a ribN riboflavin importer encoding gene, were found. Monocistronic mRNA ribA, ribB and ribE2 encode for their corresponding functional riboflavin biosynthetic enzyme. While the product of ribBA conserved the RibB function, it lacked the RibA function. Likewise, ribN encodes a functional riboflavin importer. Transcriptomics analysis indicated that external riboflavin affected the expression of a relatively small number of genes, including a few involved in iron metabolism. ribB was downregulated in response to external riboflavin, suggesting negative feedback. Deletion of ribA, ribB and ribE1 showed that these genes are required for A. salmonicida riboflavin biosynthesis and virulence in Atlantic lumpfish (Cyclopterus lumpus). A. salmonicida riboflavin auxotrophic attenuated mutants conferred low protection to lumpfish against virulent A. salmonicida. Overall, A. salmonicida has multiple riboflavin endowment forms, and duplicated riboflavin provision genes are critical for A. salmonicida infection.

Deletion of the znuA virulence factor attenuates Actinobacillus pleuropneumoniae and confers protection against homologous or heterologous strain challenge

The znuA gene is known to be important for growth and survival in Escherichia coli, Haemophilus spp., Neisseria gonorrhoeae, and Pasteurella multocida under low Zn(2+) conditions. This gene is also present in Actinobacillus pleuropneumoniae serotype 1; therefore, the aim of this study was to investigate the existence of a similar role for the znuA gene in the growth and virulence of this organism. A precisely defined ΔznuA deletion mutant of A. pleuropneumoniae was constructed based on the sequence of the wild-type SLW01 using transconjugation and counterselection. This mutation was found to be lethal in low-Zn(2+) medium. Furthermore, the ΔznuA mutant strain exhibited attenuated virulence (≥22-fold) as well as reduced mortality and morbidity in a murine (Balb/C) model of infection. The majority of the bacteria were cleared from the lungs within 2 weeks. The ΔznuA mutant strain caused no adverse effects in pigs at doses of up to 1.0×10(9) CFU/mL. The ΔznuA mutant strain induced a significant immune response and conferred 80% and 100% protection on immunised pigs against challenge with A. pleuropneumoniae strains belonging to homologous or heterologous serovars, respectively, compared to the blank controls. The data obtained in this study indicate the potential of the mutant ΔznuA strain for development as a live vaccine capable of inducing reliable cross-serovar protection following intratracheal immunisation.

The RNA chaperone Hfq promotes fitness of Actinobacillus pleuropneumoniae during porcine pleuropneumonia

Actinobacillus pleuropneumoniae is the etiological agent of porcine pleuropneumonia, an economically important disease of pigs. The hfq gene in A. pleuropneumoniae, encoding the RNA chaperone and posttranscriptional regulator Hfq, is upregulated during infection of porcine lungs. To investigate the role of this in vivo-induced gene in A. pleuropneumoniae, an hfq mutant strain was constructed. The hfq mutant was defective in biofilm formation on abiotic surfaces. The level of pgaC transcript, encoding the biosynthesis of poly-β-1,6-N-acetylglucosamine (PNAG), a major biofilm matrix component, was lower and PNAG content was 10-fold lower in the hfq mutant than in the wild-type strain. When outer membrane proteins were examined, cysteine synthase, implicated in resistance to oxidative stress and tellurite, was not found at detectable levels in the absence of Hfq. The hfq mutant displayed enhanced sensitivity to superoxide generated by methyl viologen and tellurite. These phenotypes were readily reversed by complementation with the hfq gene expressed from its native promoter. The role of Hfq in the fitness of A. pleuropneumoniae was assessed in a natural host infection model. The hfq mutant failed to colonize porcine lungs and was outcompeted by the wild-type strain (median competitive index of 2 × 10(-5)). Our data demonstrate that the in vivo-induced gene hfq is involved in the regulation of PNAG-dependent biofilm formation, resistance to superoxide stress, and the fitness and virulence of A. pleuropneumoniae in pigs and begin to elucidate the role of an in vivo-induced gene in the pathogenesis of pleuropneumonia.

Type IV fimbrial subunit protein ApfA contributes to protection against porcine pleuropneumonia

Porcine pleuropneumonia caused by Actinobacillus pleuropneumoniae accounts for serious economic losses in the pig farming industry worldwide. We examined here the immunogenicity and protective efficacy of the recombinant type IV fimbrial subunit protein ApfA as a single antigen vaccine against pleuropneumonia, or as a component of a multi-antigen preparation comprising five other recombinant antigens derived from key virulence factors of A. pleuropneumoniae (ApxIA, ApxIIA, ApxIIIA, ApxIVA and TbpB). Immunization of pigs with recombinant ApfA alone induced high levels of specific serum antibodies and provided partial protection against challenge with the heterologous A. pleuropneumoniae serotype 9 strain. This protection was higher than that engendered by vaccination with rApxIVA or rTbpB alone and similar to that observed after immunization with the tri-antigen combination of rApxIA, rApxIIA and rApxIIIA. In addition, rApfA improved the vaccination potential of the penta-antigen mixture of rApxIA, rApxIIA, rApxIIIA, rApxIVA and rTbpB proteins, where the hexa-antigen vaccine containing rApfA conferred a high level of protection on pigs against the disease. Moreover, when rApfA was used for vaccination alone or in combination with other antigens, such immunization reduced the number of pigs colonized with the challenge strain. These results indicate that ApfA could be a valuable component of an efficient subunit vaccine for the prevention of porcine pleuropneumonia.

An atypical riboflavin pathway is essential for Brucella abortus virulence

Brucellosis is a worldwide zoonosis that affects livestock and humans and is caused by closely related Brucella spp., which are adapted to intracellular life within cells of a large variety of mammals. Brucella can be considered a furtive pathogen that infects professional and non-professional phagocytes. In these cells Brucella survives in a replicative niche, which is characterized for having a very low oxygen tension and being deprived from nutrients such as amino acids and vitamins. Among these vitamins, we have focused on riboflavin (vitamin B2). Flavin metabolism has been barely implicated in bacterial virulence. We have recently described that Brucella and other Rhizobiales bear an atypical riboflavin metabolic pathway. In the present work we analyze the role of the flavin metabolism on Brucella virulence. Mutants on the two lumazine synthases (LS) isoenzymes RibH1 and RibH2 and a double RibH mutant were generated. These mutants and different complemented strains were tested for viability and virulence in cells and in mice. In this fashion we have established that at least one LS must be present for B. abortus survival and that RibH2 and not RibH1 is essential for intracellular survival due to its LS activity in vivo. In summary, we show that riboflavin biosynthesis is essential for Brucella survival inside cells or in mice. These results highlight the potential use of flavin biosynthetic pathway enzymes as targets for the chemotherapy of brucellosis.

Construction and characterization of an Actinobacillus pleuropneumoniae serotype 2 mutant lacking the Apx toxin secretion protein genes apxIIIB and apxIIID

Apx toxins have been identified as important virulence factors of Actinobacillus pleuropneumoniae, the etiologic agent of porcine pleuropneumonia. In some A. pleuropneumoniae serotypes, Apx toxins are secreted by the cell membrane proteins encoded by apxIIIB and apxIIID genes. In an effort to develop a live vaccine strain against A. pleuropneumoniae, we inactivated the apxIIIB and apxIIID genes in A. pleuropneumoniae 1536, a serotype 2 strain, resulting in the DeltaapxIIIB/DapxIIID mutant strain (1536DeltaBDeltaD). Immunization of pigs with live 1536DeltaBDeltaD A. pleuropneumoniae conferred protection against homologous challenge with wild-type A. pleuropneumoniae 1536. Thus, impaired Apx toxin secretion may decrease the virulence of A. pleuropneumoniae and may be an effective strategy for the development of a live-attenuated A. pleuropneumoniae vaccine.

Branched-chain amino acids are required for the survival and virulence of Actinobacillus pleuropneumoniae in swine

In Actinobacillus pleuropneumoniae, which causes porcine pleuropneumonia, ilvI was identified as an in vivo-induced (ivi) gene and encodes the enzyme acetohydroxyacid synthase (AHAS) required for branched-chain amino acid (BCAA) biosynthesis. ilvI and 7 of 32 additional ivi promoters were upregulated in vitro when grown in chemically defined medium (CDM) lacking BCAA. Based on these observations, we hypothesized that BCAA would be found at limiting concentrations in pulmonary secretions and that A. pleuropneumoniae mutants unable to synthesize BCAA would be attenuated in a porcine infection model. Quantitation of free amino acids in porcine pulmonary epithelial lining fluid showed concentrations of BCAA ranging from 8 to 30 micromol/liter, which is 10 to 17% of the concentration in plasma. The expression of both ilvI and lrp, a global regulator that is required for ilvI expression, was strongly upregulated in CDM containing concentrations of BCAA similar to those found in pulmonary secretions. Deletion-disruption mutants of ilvI and lrp were both auxotrophic for BCAA in CDM and attenuated compared to wild-type A. pleuropneumoniae in competitive index experiments in a pig infection model. Wild-type A. pleuropneumoniae grew in CDM+BCAA but not in CDM-BCAA in the presence of sulfonylurea AHAS inhibitors. These results clearly demonstrate that BCAA availability is limited in the lungs and support the hypothesis that A. pleuropneumoniae, and potentially other pulmonary pathogens, uses limitation of BCAA as a cue to regulate the expression of genes required for survival and virulence. These results further suggest a potential role for AHAS inhibitors as antimicrobial agents against pulmonary pathogens.

Ohr, an in vivo-induced gene in Actinobacillus pleuropneumoniae, is located on a genomic island and requires glutathione-S-transferase for activity

Actinobacillus pleuropneumoniae is the causative agent of severe necrotizing pneumonia in swine. Previously, we identified the ohr gene encoding organic hydroperoxide reductase as specifically induced during infection of pigs, induced in vitro by organic peroxides but not other oxygen radicals, and present in A. pleuropneumoniae serotypes 1, 9 and 11 but not in other serotypes (Shea & Mulks, 2002). Through analysis of flanking genomic sequence, we identify a homologue of gst, which encodes glutathione-S-transferase, immediately downstream of ohr and demonstrate that ohr-gst confers low but uninducible Ohr activity to serotype 5. We further identify a genomic island of 9.3 kb, flanked by lysR and araC homologues, in serotypes 1, 9 and 11, which contains ohr and gst. In serotypes 2-8, 10 and 12, this region of the genome contains a 1.1-kb islet with a putative transposase flanked by lysR and araC.

Identification of two proteins that interact with the Erp virulence factor from Mycobacterium tuberculosis by using the bacterial two-hybrid system

Background

The exported repetitive protein (erp) gene encodes a secreted 36-kDa protein with a central domain containing several proline-glycine-leucine-threonine-serine (PGLTS) repeats. It has been demonstrated that erp is a virulence-associated factor since the disruption of this gene impairs the growth of Mycobacterium bovis and Mycobacterium tuberculosis in mice.

Results

In order to elucidate the function of Erp we searched for Erp-binding proteins from M. tuberculosis by using a bacterial two-hybrid system. Our results indicate that Erp interacts specifically with two putative membrane proteins, Rv1417 and Rv2617c. Further analysis revealed that the latter two interact with each other, indicating that Rv1417, Rv2617c and Erp are connected through multiple interactions. While Rv1417 is disseminated in several Actinomycetales genera, orthologues of Rv2617c are exclusively present in members of the M. tuberculosis complex (MTC). The central and amino-terminal regions of Erp were determined to be involved in the interaction with Rv1417 and Rv2627c. Erp forms from Mycobacterium smegmatis and Mycobacterium leprae were not able to interact with Rv2617c in two-hybrid assays. Immunolocalization experiments showed that Rv1417 and Rv2617c are found on the cell membrane and Erp on the bacterial cell wall. Finally, comparative genomics and expression studies revealed a possible role of Rv1417 in riboflavin metabolism.

Conclusion

We identified interactive partners of Erp, an M. tuberculosis protein involved in virulence, which will be the focus of future investigation to decipher the function of the Erp family protein.

Nucleotide biosynthesis is critical for growth of bacteria in human blood

Proliferation of bacterial pathogens in blood represents one of the most dangerous stages of infection. Growth in blood serum depends on the ability of a pathogen to adjust metabolism to match the availability of nutrients. Although certain nutrients are scarce in blood and need to be de novo synthesized by proliferating bacteria, it is unclear which metabolic pathways are critical for bacterial growth in blood. In this study, we identified metabolic functions that are essential specifically for bacterial growth in the bloodstream. We used two principally different but complementing techniques to comprehensively identify genes that are required for the growth of Escherichia coli in human serum. A microarray-based and a dye-based mutant screening approach were independently used to screen a library of 3,985 single-gene deletion mutants in all non-essential genes of E. coli (Keio collection). A majority of the mutants identified consistently by both approaches carried a deletion of a gene involved in either the purine or pyrimidine nucleotide biosynthetic pathway and showed a 20- to 1,000-fold drop in viable cell counts as compared to wild-type E. coli after 24 h of growth in human serum. This suggests that the scarcity of nucleotide precursors, but not other nutrients, is the key limitation for bacterial growth in serum. Inactivation of nucleotide biosynthesis genes in another Gram-negative pathogen, Salmonella enterica, and in the Gram-positive pathogen Bacillus anthracis, prevented their growth in human serum. The growth of the mutants could be rescued by genetic complementation or by addition of appropriate nucleotide bases to human serum. Furthermore, the virulence of the B. anthracis purE mutant, defective in purine biosynthesis, was dramatically attenuated in a murine model of bacteremia. Our data indicate that de novo nucleotide biosynthesis represents the single most critical metabolic function for bacterial growth in blood and reveal the corresponding enzymes as putative antibiotic targets for the treatment of bloodstream infections.

Bacterial growth in the bloodstream is a common manifestation of a number of bacterial infections. When growing in blood, bacteria not only have to evade the host's immune response, but also adjust their metabolism to suit availability of nutrients. Although the concentrations of various metabolites in human blood are known, it is difficult to predict which nutrients are abundant and which are scarce. To proliferate in human blood, bacteria need to synthesize metabolites that are present in the limiting concentrations. For that, they need to produce specific enzymes that are, thus, critical for the bacterial growth in the bloodstream. We carried out a comprehensive, genome-wide search for Escherichia coli genes that are essential for growth in human serum. We found that inactivation of nucleotide biosynthesis genes leads to a significant growth defect in human serum not only for E. coli but also for two other pathogens, Salmonella Typhimurium and Bacillus anthracis. The results of this study demonstrate that the limiting amounts of the nucleotide bases in human serum force invading pathogens to rely on de novo nucleotide biosynthesis. Hence, our findings reveal nucleotide biosynthesis enzymes as a possible target for the treatment of bloodstream infections.

Actinobacillus pleuropneumoniaevaccines: from bacterins to new insights into vaccination strategies

With the growing emergence of antibiotic resistance and rising consumer demands concerning food safety, vaccination to prevent bacterial infections is of increasing relevance.Actinobacillus pleuropneumoniaeis the etiological agent of porcine pleuropneumonia, a respiratory disease leading to severe economic losses in the swine industry. Despite all the research and trials that were performed withA. pleuropneumoniaevaccination in the past, a safe vaccine that offers complete protection against all serotypes has yet not reached the market. However, recent advances made in the identification of new potential vaccine candidates and in the targeting of specific immune responses, give encouraging vaccination perspectives. Here, we review past and current knowledge onA. pleuropneumoniaevaccines as well as the newly available genomic tools and vaccination strategies that could be useful in the design of an efficient vaccine againstA. pleuropneumoniaeinfection.

Potential use an Actinobacillus pleuropneumoniae double mutant strain ΔapxIICΔapxIVA as live vaccine that allows serological differentiation between vaccinated and infected animals

Actinobacillus pleuropneumoniae is the causative agent of porcine pleuropneumonia, a highly contagious and often fatal disease. We have previously reported the construction and characterization of a single gene apxIIC deletion mutant HB04C(-) based on A. pleuropneumoniae serovar 7 which produces ApxII toxin and ApxIV. A precisely defined DeltaapxIICDeltaapxIVA double-deletion mutant of A. pleuropneumoniae was constructed based on HB04C(-) by transconjugation and counterselection, and the levels of virulence of the DeltaapxIIC single mutant and DeltaapxIICDeltaapxIVA double mutant were compared in an experimental infection in mice and pigs. The results demonstrated that the DeltaapxIICDeltaapxIVA double mutant strain was less virulent than HB04C(-). Despite attenuation of virulence, the DeltaapxIICDeltaapxIVA double mutant remains immunogenic and conferred a similar level of protective immunity to pigs against challenge with a lethal dose of a heterologous fully virulent standard serovar 1 strain of A. pleuropneumoniae. The results of the virulence study suggest that ApxIV is a critical virulence factor of A. pleuropneumoniae serovar 7 and is able to induce clinical disease, but it not required for efficient vaccination of pigs against A. pleuropneumoniae infection. Two weeks after the booster immunization, animals vaccinated with HB04C(-) were positive in the ApxIVAM-ELISA based on a recombinant GST-fusion protein GST-ApxIVAM as the solid-phase antigen while animals vaccinated with the DeltaapxIICDeltaapxIVA double mutant were negative. These data demonstrate that the double mutant DeltaapxIICDeltaapxIVA can be used as an effective live marker vaccine allowing serological differentiation between vaccinated and infected animals.

A subset of Actinobacillus pleuropneumoniae in vivo induced promoters respond to branched-chain amino acid limitation

Actinobacillus pleuropneumoniae is the causative agent of a necrotizing hemorrhagic pleuropneumonia in swine. In this study, we investigate the possibility that the limitation of branched-chain amino acids is a stimulus that A. pleuropneumoniae will encounter during infection and will respond to by up-regulation of genes involved in branched-chain amino acid biosynthesis and virulence. Actinobacillus pleuropneumoniae genetic loci that are specifically induced during infection were screened in vitro for expression in response to limitation of branched-chain amino acids. Of 32 in vivo induced promoter clones screened in vitro, eight were induced on chemically defined medium without isoleucine, leucine and valine as compared to complete chemically defined medium. We identify the genomic context of each clone and discuss its relevance to branched-chain amino acid limitation and virulence. We conclude that limitation of branched-chain amino acids is a cue for expression of a subset in vivo induced genes, including not only genes involved in the biosynthesis of branched-chain amino acids, but also other genes that are induced during infection of the natural host. These results suggest that limitation of branched-chain amino acids may be one of an array of environmental cues responsible for the induction of virulence-associated genes in A. pleuropneumoniae.

Identification of the Actinobacillus pleuropneumoniae leucine-responsive regulatory protein and its involvement in the regulation of in vivo-induced genes

Actinobacillus pleuropneumoniae is a gram-negative bacterial pathogen that causes a severe hemorrhagic pneumonia in swine. We have previously shown that the limitation of branched-chain amino acids (BCAAs) is a cue that induces the expression of a subset of A. pleuropneumoniae genes identified as specifically induced during infection of the natural host animal by using an in vivo expression technology screen. Leucine-responsive regulatory protein (Lrp) is a global regulator and has been shown in Escherichia coli to regulate many genes, including genes involved in BCAA biosynthesis. We hypothesized that A. pleuropneumoniae contains a regulator similar to Lrp and that this protein is involved in the regulation of a subset of genes important during infection and recently shown to have increased expression in the absence of BCAAs. We report the identification of an A. pleuropneumoniae serotype 1 gene encoding a protein with similarity to amino acid sequence and functional domains of other reported Lrp proteins. We further show that purified A. pleuropneumoniae His6-Lrp binds in vitro to the A. pleuropneumoniae promoter regions for ilvI, antisense cps1AB, lrp, and nqr. A genetically defined A. pleuropneumoniae lrp mutant was constructed using an allelic replacement and sucrose counterselection method. Analysis of expression from the ilvI and antisense cps1AB promoters in wild-type, lrp mutant, and complemented lrp mutant strains indicated that Lrp is required for induction of expression of ilvI under BCAA limitation.

Riboflavin analogs and inhibitors of riboflavin biosynthesis

Flavins are active components of many enzymes. In most cases, riboflavin (vitamin B(2)) as a coenzyme represents the catalytic part of the holoenzyme. Riboflavin is an amphiphatic molecule and allows a large variety of different interactions with the enzyme itself and also with the substrate. A great number of active riboflavin analogs can readily be synthesized by chemical methods and, thus, a large number of possible inhibitors for many different enzyme targets is conceivable. As mammalian and especially human biochemistry depends on flavins as well, the target of the inhibiting flavin analog has to be carefully selected to avoid unwanted effects. In addition to flavoproteins, enzymes, which are involved in the biosynthesis of flavins, are possible targets for anti-infectives. Only a few flavin analogs or inhibitors of flavin biosynthesis have been subjected to detailed studies to evaluate their biological activity. Nevertheless, flavin analogs certainly have the potential to serve as basic structures for the development of novel anti-infectives and it is possible that, in the future, the urgent need for new molecules to fight multiresistant microorganisms will be met.

Construction and characterization of a live, attenuated apxIICA inactivation mutant ofActinobacillus pleuropneumoniaelacking a drug resistance marker

The apxIIC gene of Actinobacillus pleuropneumoniae serotype 7 was inactivated by homologous recombination using a sucrose counter-selectable marker system, resulting in a mutant strain that had no antibiotic resistance marker and expressed an inactivated ApxII toxin. The safety and immunogenicity of the mutant were evaluated in mice. The mutant strain caused no adverse effects in mice at doses up to 2 x 10(9) CFU via the intraperitoneal route while the parental strain induced total mortality at a dose of 2 x 10(7) CFU. Mice vaccinated intraperitoneally with the mutant strain had 100% and 70% protection against homologous (serotype 7) or heterologous (serotype 1, 3) challenge with A. pleuropneumoniae, respectively. The A. pleuropneumoniae mutant strain HB04C- and the counterselection method used in the study show promise in developing effective live vaccines for porcine pleuropneumonia and for other infections diseases of the respiratory system.

Phenotypic mutants of the intracellular actinomycete Rhodococcus equi created by in vivo Himar1 transposon mutagenesis

Rhodococcus equi is a facultative intracellular opportunistic pathogen of immunocompromised people and a major cause of pneumonia in young horses. An effective live attenuated vaccine would be extremely useful in the prevention of R. equi disease in horses. Toward that end, we have developed an efficient transposon mutagenesis system that makes use of a Himar1 minitransposon delivered by a conditionally replicating plasmid for construction of R. equi mutants. We show that Himar1 transposition in R. equi is random and needs no apparent consensus sequence beyond the required TA dinucleotide. The diversity of the transposon library was demonstrated by the ease with which we were able to screen for auxotrophs and mutants with pigmentation and capsular phenotypes. One of the pigmentation mutants contained an insertion in a gene encoding phytoene desaturase, an enzyme of carotenoid biosynthesis, the pathway necessary for production of the characteristic salmon color of R. equi. We identified an auxotrophic mutant with a transposon insertion in the gene encoding a putative dual-functioning GTP cyclohydrolase II-3,4-dihydroxy-2-butanone-4-phosphate synthase, an enzyme essential for riboflavin biosynthesis. This mutant cannot grow in minimal medium in the absence of riboflavin supplementation. Experimental murine infection studies showed that, in contrast to wild-type R. equi, the riboflavin-requiring mutant is attenuated because it is unable to replicate in vivo. The mutagenesis methodology we have developed will allow the characterization of R. equi virulence mechanisms and the creation of other attenuated strains with vaccine potential.

Actinobacillus pleuropneumoniae serotype 1 carrying the defined aroA mutation is fully avirulent in the pig

The aroA gene from Actinobacillus pleuropneumoniae serotype 1 reference strain 4074 was isolated and sequenced. The gene complemented the aroA mutation in Escherichia coli AB2829. A kanamycin resistance cassette was inserted into the aroA gene and the mutant gene was reintroduced into A. pleuropneumoniae by allelic replacement. Intratracheal infection of susceptible pigs with A. pleuropneumoniae aroA caused no signs of respiratory disease or lung lesions in any of the animals at a dose 10(4) times the dose reliably known to induce acute pleuropneumonia; all animals infected with the unaltered control strain developed acute disease. The aroA mutant was rapidly eliminated from the lungs and tonsil of infected animals. The mutant may represent a safely attenuated strain for use in live bacterial vaccination or the delivery of antigen by the intranasal route. However, the residence time of the mutant in the respiratory tract of the pig may be too short for it to be useful in generating a protective immune response.

ohr, Encoding an organic hydroperoxide reductase, is an in vivo-induced gene in Actinobacillus pleuropneumoniae

Actinobacillus pleuropneumoniae is the causative agent of porcine pleuropneumonia, a disease characterized by pulmonary necrosis and hemorrhage caused in part by neutrophil degranulation. In an effort to understand the pathogenesis of this disease, we have developed an in vivo expression technology (IVET) system to identify genes that are specifically up-regulated during infection. One of the genes that we have identified as being induced in vivo is ohr, encoding organic hydroperoxide reductase, an enzyme that could play a role in detoxification of organic hydroperoxides generated during infection. Among the 12 serotypes of A. pleuropneumoniae, ohr was found in only serotypes 1, 9, and 11. This distribution correlated with increased resistance to cumene hydroperoxide, an organic hydroperoxide, but not to hydrogen peroxide or to paraquat, a superoxide generator. Functional assays of Ohr activity demonstrated that A. pleuropneumoniae serotype 1 cultures, but not serotype 5 cultures, were able to degrade cumene hydroperoxide. In A. pleuropneumoniae serotype 1, expression of ohr was induced by cumene hydroperoxide, but not by either hydrogen peroxide or paraquat. In contrast, an ohr gene from serotype 1 cloned into A. pleuropneumoniae serotype 5 was not induced by cumene hydroperoxide or by other forms of oxidative stress, suggesting the presence of a serotype-specific positive regulator of ohr in A. pleuropneumoniae serotype 1.

Attenuation of Actinobacillus pleuropneumoniae by inactivation of aroQ

Actinobacillus pleuropneumoniae is the aetiological agent of porcine pleuropneumonia, a disease resulting in morbidity and mortality of pigs and accordingly economic losses within the swine industry. In order to construct a potential vaccine strain of A. pleuropneumoniae for control of this disease, the aroQ gene, required for the aromatic biosynthetic pathway, was targeted for inactivation. The resulting strain was tested for virulence within pigs. The aroQ gene and an adjacent gene, dapD, were cloned. A recombination cassette, for inactivation of aroQ, was constructed from these cloned genes by inserting an ampicillin resistance gene and this was transformed into A. pleuropneumoniae. Integration of this construct into the chromosomal location of aroQ and disruption of the aroQ/dapD gene arrangement was confirmed through PCR and Southern analysis. The resulting HS25 aroQ mutants were unable to grow in a chemically defined medium and following intratracheal delivery to pigs were only able to induce lung lesions when given at a level 10,000 times greater than that of the parent strain. Complementation with an in trans, functional, aroQ gene restored the ability of the mutant strain to grow in a chemically defined medium and virulence, when tested in pigs, confirming attenuation results from inactivation of aroQ. In conclusion, this work has constructed a defined mutant of A. pleuropneumoniae that is attenuated and may be safely delivered live to pigs.

Identification of an Actinobacillus pleuropneumoniae consensus promoter structure

Actinobacillus pleuropneumoniae promoter-containing clones were isolated from a genomic DNA library constructed in our lVET promoter trap vector pTF86. The promoter-containing clones were identified by their ability to drive expression of the promoterless luxAB genes of Vibrio harveyi. The degree of expression was quantifiable, and only high-expression or "hot" promoters were used for this study. Nine clones were sequenced, and their transcriptional start sites were determined by primer extension. The sequences upstream of the start site were aligned, and a consensus promoter structure for A. pleuropneumoniae was identified. The consensus promoter sequence for A. pleuropneumoniae was found to be TATAAT and TTG/AAA, centered approximately 10 and 35 bp upstream of the transcriptional start site, respectively. A comparison of the A. pleuropneumoniae consensus with other prokaryotic consensus promoters showed that the A. pleuropneumoniae consensus promoter is similar to that found in other eubacteria in terms of sequence, with an identical -10 element and a similar but truncated -35 element. However, the A. pleuropneumoniae consensus promoter is unique in the spacing between the -10 and -35 elements. The promoter spacing was analyzed by site-directed mutagenesis, which demonstrated that optimal spacing for an A. pleuropneumoniae promoter is shorter than the spacing identified for Escherichia coli and Bacillus subtilis promoters.

Biosynthesis of vitamin b2 (riboflavin)

The biosynthesis of one riboflavin molecule requires one molecule of GTP and two molecules of ribulose 5-phosphate as substrates. The imidazole ring of GTP is hydrolytically opened, yielding a 4, 5-diaminopyrimidine which is converted to 5-amino-6-ribitylamino-2, 4(1H,3H)-pyrimidinedione by a sequence of deamination, side chain reduction and dephosphorylation. Condensation of 5-amino-6-ribitylamino-2,4(1H,3H)-pyrimidinedione with 3, 4-dihydroxy-2-butanone 4-phosphate obtained from ribulose 5-phosphate affords 6,7-dimethyl-8-ribityllumazine. Dismutation of the lumazine derivative yields riboflavin and 5-amino-6-ribitylamino-2,4(1H,3H)-pyrimidinedione, which is recycled in the biosynthetic pathway. The structure of the biosynthetic enzyme, 6,7-dimethyl-8-ribityllumazine synthase, has been studied in considerable detail.

Identification of a plasmid-encoded gene from Haemophilus ducreyi which confers NAD independence

Members of the family Pasteurellaceae are classified in part by whether or not they require an NAD supplement for growth on laboratory media. In this study, we demonstrate that this phenotype can be determined by a single gene, nadV, whose presence allows NAD-independent growth of Haemophilus influenzae and Actinobacillus pleuropneumoniae. This gene was cloned from a 5.2-kb plasmid which was previously shown to be responsible for NAD independence in Haemophilus ducreyi. When transformed into A. pleuropneumoniae, this cloned gene allowed NAD-independent growth on complex media and allowed the utilization of nicotinamide in place of NAD on defined media. Sequence analysis revealed an open reading frame of 1,482 bp that is predicted to encode a protein with a molecular mass of 55,619 Da. Compared with the sequence databases, NadV was found to have significant sequence homology to the human pre-B-cell colony-enhancing factor PBEF and to predicted proteins of unknown function identified in the bacterial species Mycoplasma genitalium, Mycoplasma pneumoniae, Shewanella putrefaciens, Synechocystis sp., Deinococcus radiodurans, Pasteurella multocida, and Actinobacillus actinomycetemcomitans. P. multocida and A. actinomycetemcomitans are among the NAD-independent members of the Pasteurellaceae. Homologues of NadV were not found in the sequenced genome of H. influenzae, an NAD-dependent member of the Pasteurellaceae, or in species known to utilize a different pathway for synthesis of NAD, such as Escherichia coli. Sequence alignment of these nine homologues revealed regions and residues of complete conservation that may be directly involved in the enzymatic activity. Identification of a function for this gene in the Pasteurellaceae should help to elucidate the role of its homologues in other species.

Structural and functional analysis of the riboflavin synthesis genes encoding GTP cyclohydrolase II (ribA), DHBP synthase (ribBA), riboflavin synthase (ribC), and riboflavin deaminase/reductase (ribD) from Helicobacter pylori strain P1

The functions of the riboflavin synthesis gene homologues ribA, ribBA, ribC, and ribD from Helicobacter pylori strain P1 were confirmed by complementation of defined Escherichia coli mutant strains. The H. pylori ribBA gene, which is similar to bifunctional ribBA genes of Gram-positive bacteria, fully complemented the ribB mutation and partially restored growth in a ribC mutant. However, ribBA did not complement the ribA mutation in E. coli, thus explaining the presence of the additional separate copy of the ribA gene in the H. pylori chromosome. In E. coli exclusively ribA conferred hemolytic activity and gave rise to production of molecules with fluorescence characteristics similar to flavins, as observed earlier. The E. coli hemolysin ClyA was not involved in causing the hemolytic phenotype. No riboflavin synthesis genes on plasmids conferred iron uptake functions to a siderophore-deficient mutant of E. coli. Marker exchange mutagenesis of the genes in H. pylori was not successful indicating that riboflavin synthesis is essential for basic metabolic functions of the gastric pathogen.

[Cu,Zn]-Superoxide dismutase mutants of the swine pathogen Actinobacillus pleuropneumoniae are unattenuated in infections of the natural host

Actinobacillus pleuropneumoniae, the causative agent of porcine pleuropneumonia, contains a periplasmic Cu- and Zn-cofactored superoxide dismutase ([Cu,Zn]-SOD, or SodC) which has the potential, realized in other pathogens, to promote bacterial survival during infection by dismutating host-defense-derived superoxide. Here we describe the construction of a site-specific, [Cu,Zn]-SOD-deficient A. pleuropneumoniae serotype 1 mutant and show that although the mutant is highly sensitive to the microbicidal action of superoxide in vitro, it remains fully virulent in experimental pulmonary infection in pigs.

A genetically-defined riboflavin auxotroph of Actinobacillus pleuropneumoniae as a live attenuated vaccine

Actinobacillus pleuropneumoniae is a gram negative pleiomorphic rod that is the causative agent of a severe, highly infectious and often fatal pleuropneumonia in swine. We have previously reported the construction of genetically-defined stable riboflavin auxotrophs by replacement of a portion of the APP riboflavin biosynthetic operon (ribGBAH) with an antibiotic cassette encoding resistance to kanamycin, and have demonstrated that such riboflavin auxotrophs are avirulent. In this study, we evaluated riboflavin auxotrophs of A. pleuropneumoniae for their ability to stimulate protective immunity against pleuropneumonia. An initial challenge experiment demonstrated that intramuscular vaccination with a live attenuated serotype 1A rib mutant, in a vaccine formulation that included a limiting amount of exogenous riboflavin, provided better protection against challenge with virulent A. pleuropneumoniae than either intratracheal immunization or intramuscular immunization with live bacteria in the absence of exogenous riboflavin. Subsequent studies in which the vaccine inoculating dose, concentration of exogenous riboflavin, and serotype of the vaccine strain were varied demonstrated that immunization with live avirulent riboflavin auxotrophs could elicit significant protection against experimental challenge with both homologous and heterologous virulent serotypes of A. pleuropneumoniae.

Identification of in vivo induced genes in Actinobacillus pleuropneumoniae

We have developed an in vivo expression technology (IVET) system to identify Actinobacillus pleuropneumoniae gene promoters that are specifically induced in vivo during infection. This system is based upon an avirulent riboflavin-requiring A. pleuropneumoniae mutant and a promoter-trap vector (pTF86) that contains, in sequence, the T4 terminator, a unique Bam HI site, a promoterless copy of the V. harveyi luxAB genes, and a promoterless copy of the B. subtilis ribBAH genes in the E. coli - A. pleuropneumoniae shuttle vector pGZRS19. Sau 3A fragments of A. pleuropneumoniae genomic DNA were cloned into the Bam HI site in pTF86 and transformed into the A. pleuropneumoniae Rib- mutant. Pigs were infected with pools of 300-600 transformants by endobronchial inoculation and surviving bacteria were isolated from the pigs' lungs at 12-16 h post-infection. Infection strongly selected for transformants containing cloned promoters which drove expression of the vector ribBAH genes and allowed survival of the Rib- mutant in vivo. Strains that survived in vivo, but which minimally expressed luciferase activity in vitro, should contain cloned promoters that are specifically induced in vivo. Ten clones, designated iviA-J, were isolated which contain promoters that are induced in vivo during infection. These ivi clones were shown to be induced in the animal by luminescence of infected tissue and by direct assay of bacteria recovered from bronchoalveolar lavage. Four of these clones were putatively identified by amino acid sequence similarity as ilvI, the ilvDA operon, the secE-nusG operon, and the mrp gene. This is the first report of an IVET system for use in the family Pasteurellaceae, as well as the first report of an IVET system utilizing an infection model of pneumonia in the natural host.

Molecular analysis of riboflavin synthesis genes in Bartonella henselae and use of the ribC gene for differentiation of Bartonella species by PCR

The biosynthesis pathway for riboflavin (vitamin B(2)), the precursor of the essential cofactors flavin mononucleotide and flavin adenine dinucleotide, is present in bacteria and plants but is absent in vertebrates. Due to their conservation in bacterial species and their absence in humans, the riboflavin synthesis genes should be well suited either for detection of bacterial DNA in human specimens or for the differentiation of pathogenic bacteria by molecular techniques. A DNA fragment carrying the genes ribD, ribC, and ribE, which encode homologues of riboflavin deaminase (RibD) and subunits of riboflavin synthetase (RibC and RibE), respectively, was isolated from a plasmid-based DNA library of the human pathogen Bartonella henselae by complementation of a ribC mutation in Escherichia coli. Sequence analysis of the ribC gene region in strains of B. henselae, which were previously shown to be genetically different, revealed that the ribC gene is highly conserved at the species level. PCR amplification with primers derived from the ribC locus of B. henselae was used to isolate the corresponding DNA regions in B. bacilliformis, B. clarridgeiae, and B. quintana. Sequence analysis indicated that the riboflavin synthesis genes are conserved and show the same operon-like genetic organization in all four Bartonella species. Primer oligonucleotides designed on the basis of localized differences within the ribC DNA region were successfully used to develop species-specific PCR assays for the differentiation of B. henselae, B. clarridgeiae, B. quintana, and B. bacilliformis. The results obtained indicate that the riboflavin synthesis genes are excellent targets for PCR-directed differentiation of these emerging pathogens. The PCR assays developed should increase our diagnostic potential to differentiate Bartonella species, especially B. henselae and the newly recognized species B. clarridgeiae.

A single-step transconjugation system for the introduction of unmarked deletions into Actinobacillus pleuropneumoniae serotype 7 using a sucrose sensitivity marker

Research on the porcine respiratory tract pathogen Actinobacillus pleuropneumoniae requires the availability of improved genetic tools. Therefore, using the sacB gene of Bacillus subtilis, we developed a sucrose-based counterselection system that allows rapid curing of an Escherichia coli-A. pleuropneumoniae shuttle vector as well as the introduction of unmarked mutations into the A. pleuropneumoniae chromosome. A cassette containing the Tn903 kanamycin resistance determinant (km(r)) and the sacB gene expressed from the A. pleuropneumoniae omlA promoter was introduced by homologous recombination into the ureC gene of A. pleuropneumoniae. The resultant stable plasmid cointegrates were kanamycin-resistant, sucrose-sensitive, and urease-positive. A simple counterselection on sucrose-containing agar plates without an additional transconjugation step allowed the efficient isolation of urease-negative A. pleuropneumoniae mutants that had lost the km(r)-sacB cassette.