Pasteurella multocida exbB, exbD and tonB genes are physically linked but independently transcribed

The dual role of TonB genes in turnerbactin uptake and carbohydrate utilization in the shipworm symbiont Teredinibacter turnerae

IMPORTANCE

This study highlights diversity in iron acquisition and regulation in bacteria. The mechanisms of iron acquisition and its regulation in Teredinibacter turnerae, as well as its connection to cellulose utilization, a hallmark phenotype of T. turnerae, expand the paradigm of bacterial iron acquisition. Two of the four TonB genes identified in T. turnerae exhibit functional redundancy and play a crucial role in siderophore-mediated iron transport. Unlike typical TonB genes in bacteria, none of the TonB genes in T. turnerae are clearly iron regulated. This unusual regulation could be explained by another important finding in this study, namely, that the two TonB genes involved in iron transport are also essential for cellulose utilization as a carbon source, leading to the expression of TonB genes even under iron-rich conditions.

The dual role of TonB genes in turnerbactin uptake and carbohydrate utilization in the shipworm symbiont Teredinibacter turnerae

Teredinibacter turnerae is an intracellular bacterial symbiont that resides in the gills of shipworms, wood-eating bivalve mollusks. This bacterium produces a catechol siderophore, turnerbactin, required for the survival of this bacterium under iron limiting conditions. The turnerbactin biosynthetic genes are contained in one of the secondary metabolite clusters conserved among T. turnerae strains. However, Fe(III)-turnerbactin uptake mechanisms are largely unknown. Here, we show that the first gene of the cluster, fttA a homologue of Fe(III)-siderophore TonB-dependent outer membrane receptor (TBDR) genes is indispensable for iron uptake via the endogenous siderophore, turnerbactin, as well as by an exogenous siderophore, amphi-enterobactin, ubiquitously produced by marine vibrios. Furthermore, three TonB clusters containing four tonB genes were identified, and two of these genes, tonB1b and tonB2, functioned not only for iron transport but also for carbohydrate utilization when cellulose was a sole carbon source. Gene expression analysis revealed that none of the tonB genes and other genes in those clusters were clearly regulated by iron concentration while turnerbactin biosynthesis and uptake genes were up-regulated under iron limiting conditions, highlighting the importance of tonB genes even in iron rich conditions, possibly for utilization of carbohydrates derived from cellulose.

Comparative Genomic Analysis of the Human Pathogen Wohlfahrtiimonas Chitiniclastica Provides Insight Into the Identification of Antimicrobial Resistance Genotypes and Potential Virulence Traits

Recent studies suggest that Wohlfahrtiimonas chitiniclastica may be the cause of several diseases in humans including sepsis and bacteremia making the bacterium as a previously underappreciated human pathogen. However, very little is known about the pathogenicity and genetic potential of W. chitiniclastica; therefore, it is necessary to conduct systematic studies to gain a deeper understanding of its virulence characteristics and treatment options. In this study, the entire genetic repertoire of all publicly available W. chitiniclastica genomes was examined including in silico characterization of bacteriophage content, antibiotic resistome, and putative virulence profile. The pan-genome of W. chitiniclastica comprises 3819 genes with 1622 core genes (43%) indicating a putative metabolic conserved species. Furthermore, in silico analysis indicated presumed resistome expansion as defined by the presence of genome-encoded transposons and bacteriophages. While macrolide resistance genes macA and macB are located within the core genome, additional antimicrobial resistance genotypes for tetracycline (tetH, tetB, and tetD), aminoglycosides (ant(2'')-Ia, aac(6')-Ia,aph(3'')-Ib, aph(3')-Ia, and aph(6)-Id)), sulfonamide (sul2), streptomycin (strA), chloramphenicol (cat3), and beta-lactamase (blaVEB) are distributed among the accessory genome. Notably, our data indicate that the type strain DSM 18708T does not encode any additional clinically relevant antibiotic resistance genes, whereas drug resistance is increasing within the W. chitiniclastica clade. This trend should be monitored with caution. To the best of our knowledge, this is the first comprehensive genome analysis of this species, providing new insights into the genome of this opportunistic human pathogen.

Comparative Proteomic Analysis for a Putative Pyridoxal Phosphate-Dependent Aminotransferase Required for Virulence in Acidovorax citrulli

Acidovorax citrulli (Ac) is the causative agent of bacterial fruit blotch disease in watermelon. Since resistant cultivars have not yet been developed, the virulence factors/mechanisms of Ac need to be characterized. This study reports the functions of a putative pyridoxal phosphate-dependent aminotransferase (PpdaAc) that transfers amino groups to its substrates and uses pyridoxal phosphate as a coenzyme. It was observed that a ppdaAc knockout mutant had a significantly reduced virulence in watermelon when introduced via germinated-seed inoculation as well as leaf infiltration. Comparative proteomic analysis predicted the cellular mechanisms related to PpdaAc. Apart from causing virulence, the PpdaAc may have significant roles in energy production, cell membrane, motility, chemotaxis, post-translational modifications, and iron-related mechanisms. Therefore, it is postulated that PpdaAc may possess pleiotropic effects. These results provide new insights into the functions of a previously unidentified PpdaAc in Ac.

Analysis of six tonB gene homologs in Bacteroides fragilis revealed that tonB3 is essential for survival in experimental intestinal colonization and intra-abdominal infection

The opportunistic, anaerobic pathogen and commensal of the human large intestinal tract, Bacteroides fragilis strain 638R, contains six predicted TonB proteins, termed TonB1-6, four ExbBs orthologs, ExbB1-4, and five ExbDs orthologs, ExbD1-5. The inner membrane TonB/ExbB/ExbD complex harvests energy from the proton motive force (Δp) and the TonB C-terminal domain interacts with and transduces energy to outer membrane TonB-dependent transporters (TBDTs). However, TonB's role in activating nearly one hundred TBDTs for nutrient acquisition in B. fragilis during intestinal colonization and extraintestinal infection has not been established. In this study, we show that growth was abolished in the ΔtonB3 mutant when heme, vitamin B12, Fe(III)-ferrichrome, starch, mucin-glycans, or N-linked glycans were used as a substrate for growth in vitro. Genetic complementation of the ΔtonB3 mutant with the tonB3 gene restored growth on these substrates. The ΔtonB1, ΔtonB2, ΔtonB4, ΔtonB5, and ΔtonB6 single mutants did not show a growth defect. This indicates that there was no functional compensation for the lack of TonB3, and it demonstrates that TonB3, alone, drives the TBDTs involved in the transport of essential nutrients. The ΔtonB3 mutant had a severe growth defect in a mouse model of intestinal colonization compared to the parent strain. This intestinal growth defect was enhanced in the ΔtonB3 ΔtonB6 double mutant strain which completely lost its ability to colonize the mouse intestinal tract compared to the parent strain. The ΔtonB1, ΔtonB2, ΔtonB4, and ΔtonB5 mutants did not significantly affect intestinal colonization. Moreover, the survival of the ΔtonB3 mutant strain was completely eradicated in a rat model of intra-abdominal infection. Taken together, these findings show that TonB3 was essential for survival in vivo. The genetic organization of tonB1, tonB2, tonB4, tonB5, and tonB6 gene orthologs indicates that they may interact with periplasmic and nonreceptor outer membrane proteins, but the physiological relevance of this has not been defined. Because anaerobic fermentation metabolism yields a lower Δp than aerobic respiration and B. fragilis has a reduced redox state in its periplasmic space - in contrast to an oxidative environment in aerobes - it remains to be determined if the diverse system of TonB/ExbB/ExbD orthologs encoded by B. fragilis have an increased sensitivity to PMF (relative to aerobic bacteria) to allow for the harvesting of energy under anaerobic conditions.

Development of novel iron-regulated Pasteurella multocida B: 2 bacterin and refinement of vaccine quality in terms of minimum variation in particle size and distribution vis-a-vis critical level of iron in media

To enhance the qualitative bacterial biomass per unit of media and to overcome the limitations of the existing haemorrhagic septicaemia (HS) vaccines, a comprehensive study was undertaken encompassing the role of iron on the bacterial biomass of Pasteurella multocida B: 2 to vaccine development. Trypsin digested hydrochloric acid-treated sheep blood (THSB) as a novel iron rich supplement had been devised for the first time for augmenting the qualitative bacterial biomass per unit of media which was evident with growth kinetic study. The higher recovery of iron from THSB became evident via atomic absorbance spectrophotometry. The critical level of iron in the media as well as mode of iron supplementation showed a major impact on the outer membrane protein profile of P. multocida B:2 and variation in droplet size and particle-size distribution of formulated vaccine. Immune response study against iron-regulated bacterin adjuvanted with aluminum hydroxide gel in mouse model showed that 3% THSB supplementation of casein sucrose yeast (CSY) not only augmented the growth of P. multocida B:2 significantly but conferred highest pre-challenged ELISA IgG titer and protection against pasteurellosis. Thus, THSB supplementation of CSY can resolve existing up-scaling and immunogenic potential problems of HS vaccine production.

Differences in pathogenicity and virulence-associated gene expression among Pasteurella multocida strains with high and low virulence in a lung tissue model

Pasteurella multocida capsular type A can cause a pulmonary infection, leading to serious pecuniary losses in cattle. The heterogeneity of infection outcome of P. multocida strains showing different virulence may be related to divergent expression of virulence genes. In this study, we compared the transcriptional response of virulence-associated genes in high (PMPAN001) and low (PMPAN007) virulence P. multocida capsular type A strains in lung tissues and in vitro. These clinical isolates differ in their organ bacterial loads, mRNA abundance of the same virulence genes between lung and culture medium, and extent of lung damage. Among the eight virulence-associated genes (fimA, tbpA, exbD, fur, oma87, pmHAS, nanH, and tonB), seven genes showed higher expression in lung compared with in vitro at 16 h (P ≤ 0.05) in PMPAN001, but not in PMPAN007. FimA, exbD, fur, oma87, pmHAS, and tonB gene transcripts showed significantly higher expression in PMPAN001 than in PMPAN007 in the lung tissues at 16 h post-infection (P ≤ 0.05). Specially, the virulence gene, nanH, in both strains was associated with poor expression in vitro and lung tissue (mean relative mRNA abundance values < 0.6). Strain PMPAN001 had a higher proliferation rate in vivo than strain PMPAN007. The bacterial loads of PMPAN001 in the organs increased from 12 h post-infection, with maximum bacteria count ranging from 1 million to 20 million/mg. In addition, lungs treated with PMPAN001 produced serious and extensive lesions marked with inflammation at 20 h. Overall, our results reveal that the highly expressed virulence-associated genes, fimA, exbD, fur, oma87, pmHAS, and tonB can be used as markers for assessing the virulence of P. multocida capsular type A strains.

Determination of the small RNA GcvB regulon in the Gram-negative bacterial pathogen Pasteurella multocida and identification of the GcvB seed binding region

Pasteurella multocida is a Gram-negative bacterium responsible for many important animal diseases. While a number of P. multocida virulence factors have been identified, very little is known about how gene expression and protein production is regulated in this organism. Small RNA (sRNA) molecules are critical regulators that act by binding to specific mRNA targets, often in association with the RNA chaperone protein Hfq. In this study, transcriptomic analysis of the P. multocida strain VP161 revealed a putative sRNA with high identity to GcvB from Escherichia coli and Salmonella enterica serovar Typhimurium. High-throughput quantitative liquid proteomics was used to compare the proteomes of the P. multocida VP161 wild-type strain, a gcvB mutant, and a GcvB overexpression strain. These analyses identified 46 proteins that displayed significant differential production after inactivation of gcvB, 36 of which showed increased production. Of the 36 proteins that were repressed by GcvB, 27 were predicted to be involved in amino acid biosynthesis or transport. Bioinformatic analyses of putative P. multocida GcvB target mRNAs identified a strongly conserved 10 nucleotide consensus sequence, 5'-AACACAACAT-3', with the central eight nucleotides identical to the seed binding region present within GcvB mRNA targets in E. coli and S. Typhimurium. Using a defined set of seed region mutants, together with a two-plasmid reporter system that allowed for quantification of sRNA-mRNA interactions, this sequence was confirmed to be critical for the binding of the P. multocida GcvB to the target mRNA, gltA.

The Role of TonB Gene in Edwardsiella ictaluri Virulence

Edwardsiella ictaluri is a Gram-negative facultative intracellular pathogen that causes enteric septicemia in catfish (ESC). Stress factors including poor water quality, poor diet, rough handling, overcrowding, and water temperature fluctuations increase fish susceptibility to ESC. The TonB energy transducing system (TonB-ExbB-ExbD) and TonB-dependent transporters of Gram-negative bacteria support active transport of scarce resources including iron, an essential micronutrient for bacterial virulence. Deletion of the tonB gene attenuates virulence in several pathogenic bacteria. In the current study, the role of TonB (NT01EI_RS07425) in iron acquisition and E. ictaluri virulence were investigated. To accomplish this, the E. ictaluri tonB gene was in-frame deleted. Growth kinetics, iron utilization, and virulence of the EiΔtonB mutant were determined. Loss of TonB caused a significant reduction in bacterial growth in iron-depleted medium (p > 0.05). The EiΔtonB mutant grew similarly to wild-type E. ictaluri when ferric iron was added to the iron-depleted medium. The EiΔtonB mutant was significantly attenuated in catfish compared with the parent strain (21.69 vs. 46.91% mortality). Catfish surviving infection with EiΔtonB had significant protection against ESC compared with naïve fish (100 vs. 40.47% survival). These findings indicate that TonB participates in pathogenesis of ESC and is an important E. ictaluri virulence factor.

OmpA protein sequence-based typing and virulence-associated gene profiles of Pasteurella multocida isolates associated with bovine haemorrhagic septicaemia and porcine pneumonic pasteurellosis in Thailand

Background

Pasteurella multocida is a Gram-negative bacterium that causes economically significant infections of a broad range of animal species. Pneumonic and septicaemic pasteurellosis caused by this bacterium remain important problems in pigs, cattle, and water buffaloes in Thailand. The aim of this study was to characterise the virulence-associated gene profiles and to develop an OmpA molecular typing scheme for classifying 191 bovine and porcine isolates of P. multocida collected between 1989 and 2012 in Thailand using polymerase chain reactions (PCRs), nucleotide sequencing, and sequence and structural bioinformatics analyses.

Results

PCR screening successfully characterised the profiles of 25 virulence-associated genes in all isolates. The gene profiles separated these isolates into bovine and porcine clusters based on eight genes (hgbB, hsf1, tadD, nanH, pfhA, plpE, pmHAS, and tbpA). Phylogenetic analyses of the nucleotide and protein sequences corresponding to the ompA gene, which encodes a major outer membrane surface protein, showed two major bovine and porcine clusters. Structural prediction and analysis of the dN/dS ratio revealed four hypervariable extracellular loops of the OmpA transmembrane domains. These four loops were used to develop an OmpA typing scheme. This scheme classified 186 isolates into five major loop sequence types (LST8, LST12, LST15, LST18, and LST19), consistent with the phylogenetic results. The loop regions of the bovine isolates were predicted to be more antigenic than those of the porcine isolates. Thus, molecular evolution of the OmpA proteins could be used to classify P. multocida isolates into different capsular types, host types, and, possibly, pathogenicity levels.

Conclusions

Together with the virulence-associated gene profiles, the typing reported in this work provides a better understanding of P. multocida virulence. Effective monitoring and potential strain-specific subunit vaccines could be developed based on these loop oligopeptides.

Electronic supplementary material

The online version of this article (doi:10.1186/s12917-017-1157-6) contains supplementary material, which is available to authorized users.

The RNA-Binding Chaperone Hfq Is an Important Global Regulator of Gene Expression in Pasteurella multocida and Plays a Crucial Role in Production of a Number of Virulence Factors, Including Hyaluronic Acid Capsule

The Gram-negative bacterium Pasteurella multocida is the causative agent of a number of economically important animal diseases, including avian fowl cholera. Numerous P. multocida virulence factors have been identified, including capsule, lipopolysaccharide (LPS), and filamentous hemagglutinin, but little is known about how the expression of these virulence factors is regulated. Hfq is an RNA-binding protein that facilitates riboregulation via interaction with small noncoding RNA (sRNA) molecules and their mRNA targets. Here, we show that a P. multocida hfq mutant produces significantly less hyaluronic acid capsule during all growth phases and displays reduced in vivo fitness. Transcriptional and proteomic analyses of the hfq mutant during mid-exponential-phase growth revealed altered transcript levels for 128 genes and altered protein levels for 78 proteins. Further proteomic analyses of the hfq mutant during the early exponential growth phase identified 106 proteins that were produced at altered levels. Both the transcript and protein levels for genes/proteins involved in capsule biosynthesis were reduced in the hfq mutant, as were the levels of the filamentous hemagglutinin protein PfhB2 and its secretion partner LspB2. In contrast, there were increased expression levels of three LPS biosynthesis genes, encoding proteins involved in phosphocholine and phosphoethanolamine addition to LPS, suggesting that these genes are negatively regulated by Hfq-dependent mechanisms. Taken together, these data provide the first evidence that Hfq plays a crucial role in regulating the global expression of P. multocida genes, including the regulation of key P. multocida virulence factors, capsule, LPS, and filamentous hemagglutinin.

Complete genome sequence of Pasteurella multocida HN06, a toxigenic strain of serogroup D

Pasteurella multocida is an important etiological agent that can cause many economically important diseases in a wide range of mammals and birds. Here, we report the complete genome sequence of P. multocida HN06, a toxigenic serogroup D strain isolated from a diseased pig in China.

Novel iron-regulated and Fur-regulated small regulatory RNAs in Aggregatibacter actinomycetemcomitans

Iron can regulate biofilm formation via non-coding small RNA (sRNA). To determine if iron-regulated sRNAs are involved in biofilm formation by the periodontopathogen Aggregatibacter actinomycetemcomitans, total RNA was isolated from bacteria cultured with iron supplementation or chelation. Transcriptional analysis demonstrated that the expression of four sRNA molecules (JA01-JA04) identified by bioinformatics was significantly upregulated in iron-limited medium compared with iron-rich medium. A DNA fragment encoding each sRNA promoter was able to titrate Escherichia coli ferric uptake regulator (Fur) from a Fur-repressible reporter fusion in an iron uptake regulator titration assay. Cell lysates containing recombinant AaFur shifted the mobility of sRNA-specific DNAs in a gel shift assay. Potential targets of these sRNAs, determined in silico, included genes involved in biofilm formation. The A. actinomycetemcomitans overexpressing JA03 sRNA maintained a rough phenotype on agar, but no longer adhered to uncoated polystyrene or glass, although biofilm determinant gene expression was only modestly decreased. In summary, these sRNAs have the ability to modulate biofilm formation, but their functional target genes remain to be confirmed.

Pasteurella multocida: diseases and pathogenesis

Pasteurella multocida is an enigmatic pathogen. It is remarkable both for the number and range of specific disease syndromes with which it is associated, and the wide range of host species affected. The pathogenic mechanisms involved in causing the different syndromes are, for the most part, poorly understood or completely unknown. The biochemical and serological properties of some organisms responsible for quite different syndromes appear to be similar. Thus, the molecular basis for host predilection remains unknown. The recent development of genetic manipulation systems together with the availability of multiple genome sequences should help to explain the association of particular pathological conditions with particular hosts as well as helping to elucidate pathogenic mechanisms.

Fis is essential for capsule production in Pasteurella multocida and regulates expression of other important virulence factors

P. multocida is the causative agent of a wide range of diseases of animals, including fowl cholera in poultry and wild birds. Fowl cholera isolates of P. multocida generally express a capsular polysaccharide composed of hyaluronic acid. There have been reports of spontaneous capsule loss in P. multocida, but the mechanism by which this occurs has not been determined. In this study, we identified three independent strains that had spontaneously lost the ability to produce capsular polysaccharide. Quantitative RT-PCR showed that these strains had significantly reduced transcription of the capsule biosynthetic genes, but DNA sequence analysis identified no mutations within the capsule biosynthetic locus. However, whole-genome sequencing of paired capsulated and acapsular strains identified a single point mutation within the fis gene in the acapsular strain. Sequencing of fis from two independently derived spontaneous acapsular strains showed that each contained a mutation within fis. Complementation of these strains with an intact copy of fis, predicted to encode a transcriptional regulator, returned capsule expression to all strains. Therefore, expression of a functional Fis protein is essential for capsule expression in P. multocida. DNA microarray analysis of one of the spontaneous fis mutants identified approximately 30 genes as down-regulated in the mutant, including pfhB_2, which encodes a filamentous hemagglutinin, a known P. multocida virulence factor, and plpE, which encodes the cross protective surface antigen PlpE. Therefore these experiments define for the first time a mechanism for spontaneous capsule loss in P. multocida and identify Fis as a critical regulator of capsule expression. Furthermore, Fis is involved in the regulation of a range of other P. multocida genes including important virulence factors.

Pasteurella multocida is an animal pathogen of worldwide economic significance. It causes fowl cholera in wild birds and poultry, hemorrhagic septicemia in ungulates, and atrophic rhinitis in swine. The major virulence factor in fowl cholera-causing isolates is the polysaccharide capsule, which is composed of hyaluronic acid. Although there have been reports of spontaneous capsule loss in some strains, to date there has been no systematic investigation into the molecular mechanisms of this phenomenon. In this study, we describe for the first time the underlying transcriptional mechanisms required for the expression of capsule in P. multocida, and identify a transcriptional regulator required for capsule production.

Pasteurella multocidaand bovine respiratory disease

Pasteurella multocidais a pathogenic Gram-negative bacterium that has been classified into three subspecies, five capsular serogroups and 16 serotypes.P. multocidaserogroup A isolates are bovine nasopharyngeal commensals, bovine pathogens and common isolates from bovine respiratory disease (BRD), both enzootic calf pneumonia of young dairy calves and shipping fever of weaned, stressed beef cattle.P. multocidaA:3 is the most common serotype isolated from BRD, and these isolates have limited heterogeneity based on outer membrane protein (OMP) profiles and ribotyping. Development ofP. multocida-induced pneumonia is associated with environmental and stress factors such as shipping, co-mingling, and overcrowding as well as concurrent or predisposing viral or bacterial infections. Lung lesions consist of an acute to subacute bronchopneumonia that may or may not have an associated pleuritis. Numerous virulence or potential virulence factors have been described for bovine respiratory isolates including adherence and colonization factors, iron-regulated and acquisition proteins, extracellular enzymes such as neuraminidase, lipopolysaccharide, polysaccharide capsule and a variety of OMPs. Immunity of cattle against respiratory pasteurellosis is poorly understood; however, high serum antibodies to OMPs appear to be important for enhancing resistance to the bacterium. Currently availableP. multocidavaccines for use in cattle are predominately traditional bacterins and a live streptomycin-dependent mutant. The field efficacy of these vaccines is not well documented in the literature.

Bioinformatic prediction and experimental verification of Fur-regulated genes in the extreme acidophile Acidithiobacillus ferrooxidans

The gamma-proteobacterium Acidithiobacillus ferrooxidans lives in extremely acidic conditions (pH 2) and, unlike most organisms, is confronted with an abundant supply of soluble iron. It is also unusual in that it oxidizes iron as an energy source. Consequently, it faces the challenging dual problems of (i) maintaining intracellular iron homeostasis when confronted with extremely high environmental loads of iron and (ii) of regulating the use of iron both as an energy source and as a metabolic micronutrient. A combined bioinformatic and experimental approach was undertaken to identify Fur regulatory sites in the genome of A. ferrooxidans and to gain insight into the constitution of its Fur regulon. Fur regulatory targets associated with a variety of cellular functions including metal trafficking (e.g. feoPABC, tdr, tonBexbBD, copB, cdf), utilization (e.g. fdx, nif), transcriptional regulation (e.g. phoB, irr, iscR) and redox balance (grx, trx, gst) were identified. Selected predicted Fur regulatory sites were confirmed by FURTA, EMSA and in vitro transcription analyses. This study provides the first model for a Fur-binding site consensus sequence in an acidophilic iron-oxidizing microorganism and lays the foundation for future studies aimed at deepening our understanding of the regulatory networks that control iron uptake, homeostasis and oxidation in extreme acidophiles.

A mutation in an exbD gene reduces tagetitoxin production by Pseudomonas syringae pv. tagetis

A mutant of Pseudomonas syringae pv. tagetis EB037 with limited ability to produce tagetitoxin was isolated after transposon mutagenesis and the mutation was characterized. The mutation occurred in a gene with a high degree of sequence identity to exbD. exbD is contiguous with tonB and exbB upstream and with a gene for a TonB-dependent receptor downstream. Using reverse transcription - polymerase chain reaction with RNA from the wild-type and exbD mutant strains, we demonstrated that the mutation in exbD did not have a polar affect on the expression of downstream genes. The exbD mutant was able to grow well in conditions where iron is not freely available. Siderophore production by the exbD mutant was similar to that of the wild-type strain. We conclude that the mutation in exbD disrupts tagetitoxin production without compromising iron metabolism. The results indicate that tagetitoxin export by P. syringae pv. tagetis involves an efflux pump that requires a functional TonB system that is not essential for normal iron metabolism.

The response of Mannheimia haemolytica to iron limitation: implications for the acquisition of iron in the bovine lung

Mannheimia haemolytica is the major causative agent of shipping fever, a severe pneumonia in cattle causing high morbidity and mortality. A prerequisite of successful lung colonization by M. haemolytica is the necessity to adapt to the paucity of iron. The lack of genome information has precluded an assessment of the genetic repertoire available to M. haemolytica to adapt to low iron environments. To close this knowledge-gap, we have determined 90% of a virulent M. haemolytica serotype A1 genome sequence and produced a microarray in order to study gene expression under iron-limiting growth for 15, 30 and 60 min. M. haemolytica responded to iron limitation by the up-regulation of transcripts coding for receptors and ABC-type transporters of transferrin, haemoglobin, haem and siderophores. Real time PCR analysis of lung tissue from Mannheimia-infected calves demonstrated the in vivo transcription of two potential haemoglobin receptors, hmbR1 and hmbR2. The relative hmbR1 and hmbR2 transcript levels in the infected lung tissue were comparable to the induced levels observed under iron-limiting growth, demonstrating in vivo induction of receptor transcription in the context of an infection. When the iron response of M. haemolytica was compared to the iron response of Pasteurella multocida, another pathogen colonizing the bovine lung, only few homologous genes were induced in both organisms. These included the haemoglobin receptor hmbR2 and the periplasmic transport systems yfeABCD and fbpABC. The comparative analysis suggests that the two pathogens use different strategies to adapt to the iron-limiting environment in the bovine host.

Pasteurella multocida pathogenesis: 125 years after Pasteur

Pasteurella multocida was first shown to be the causative agent of fowl cholera by Louis Pasteur in 1881. Since then, this Gram-negative bacterium has been identified as the causative agent of many other economically important diseases in a wide range of hosts. The mechanisms by which these bacteria can invade the mucosa, evade innate immunity and cause systemic disease are slowly being elucidated. Key virulence factors identified to date include capsule and lipopolysaccharide. The capsule is clearly involved in bacterial avoidance of phagocytosis and resistance to complement, while complete lipopolysaccharide is critical for bacterial survival in the host. A number of other virulence factors have been identified by both directed and random mutagenesis, including Pasteurella multocida toxin (PMT), putative surface adhesins and iron acquisition proteins. However, it is likely that many key virulence factors are yet to be identified, including those required for initial attachment and invasion of host cells and for persistence in a relatively nutrient poor and hostile environment.

Interaction of Pasteurella multocida with free-living amoebae

Pasteurella multocida is a highly infectious, facultative intracellular bacterium which causes fowl cholera in birds. This study reports, for the first time, the observed interaction between P. multocida and free-living amoebae. Amoebal trophozoites were coinfected with fowl-cholera-causing P. multocida strain X-73 that expressed the green fluorescent protein (GFP). Using confocal fluorescence microscopy, GFP expressing X-73 was located within the trophozoite. Transmission electron microscopy of coinfection preparations revealed clusters of intact X-73 cells in membrane-bound vacuoles within the trophozoite cytoplasm. A coinfection assay employing gentamicin to kill extracellular bacteria was used to assess the survival and replication of P. multocida within amoebae. In the presence of amoebae, the number of recoverable intracellular X-73 cells increased over a 24-h period; in contrast, X-73 cultured alone in assay medium showed a consistent decline in growth. Cytotoxicity assays and microscopy showed that X-73 was able to lyse and exit the amoebal cells approximately 18 h after coinfection. The observed interaction between P. multocida and amoebae can be considered as an infective process as the bacterium was able to invade, survive, replicate, and lyse the amoebal host. This raises the possibility that similar interactions occur in vivo between P. multocida and host cells. Free-living amoebae are ubiquitous within water and soil environments, and P. multocida has been observed to survive within these same ecosystems. Thus, our findings suggest that the interaction between P. multocida and amoebae may occur within the natural environment.

The pnhA gene of Pasteurella multocida encodes a dinucleoside oligophosphate pyrophosphatase member of the Nudix hydrolase superfamily

The pnhA gene of Pasteurella multocida encodes PnhA, which is a member of the Nudix hydrolase subfamily of dinucleoside oligophosphate pyrophosphatases. PnhA hydrolyzes diadenosine tetra-, penta-, and hexaphosphates with a preference for diadenosine pentaphosphate, from which it forms ATP and ADP. PnhA requires a divalent metal cation, Mg(2+) or Mn(2+), and prefers an alkaline pH of 8 for optimal activity. A P. multocida strain that lacked a functional pnhA gene, ACP13, was constructed to further characterize the function of PnhA. The cellular size of ACP13 was found to be 60% less than that of wild-type P. multocida, but the growth rate of ACP13 and its sensitivity to heat shock conditions were similar to those of the wild type, and the wild-type cell size was restored in the presence of a functional pnhA gene. Wild-type and ACP13 strains were tested for virulence by using the chicken embryo lethality model, and ACP13 was found to be up to 1,000-fold less virulent than the wild-type strain. This is the first study to use an animal model in assessing the virulence of a bacterial strain that lacked a dinucleoside oligophosphate pyrophosphatase and suggests that the pyrophosphatase PnhA, catalyzing the hydrolysis of diadenosine pentaphosphates, may also play a role in facilitating P. multocida pathogenicity in the host.

The Pasteurella multocida nrfE gene is upregulated during infection and is essential for nitrite reduction but not for virulence

Pasteurella multocida is the causative agent of a range of diseases with economic importance in production animals. Many systems have been employed to identify virulence factors of P. multocida, including in vivo expression technology (IVET), signature-tagged mutagenesis, and whole-genome expression profiling. In a previous study in which IVET was used with P. multocida, nrfE was identified as a gene that is preferentially expressed in vivo. In Escherichia coli, nrfE is part of the formate-dependent nitrite reductase system involved in utilizing available nitrite as an electron accepter during growth under anaerobic conditions. In this study, we constructed an isogenic P. multocida strain that was unable to reduce nitrite under either aerobic or anaerobic conditions, thereby demonstrating that P. multocida nrfE is essential for nitrite reduction. However, the nrfE mutant was still virulent in mice. Real-time reverse transcription-PCR analysis indicated that nrfE was regulated independently of nrfABCD by an independent promoter that is likely to be upregulated in vivo.

Identification and characterization of the TonB region and its role in transferrin-mediated iron acquisition in Haemophilus parasuis

Haemophilus parasuis is the causative agent of Glässer's disease, which is responsible for considerable economic losses in the pig-rearing industry. The aim of the study reported here was the identification, sequencing and molecular characterization of the TonB region that includes tonB, exbBD, and tbpBA genes in H. parasuis. In addition, two fusion proteins were generated. One of them (pGEX-6P-1-GST-TbpB) contained the first 501 amino acids of H. parasuis TbpB protein, while the second (pBAD-Thio-TbpB-V5-His) included the first 102 amino acids of H. parasuis TbpB N-terminus domain. A panel of 14 hybridomas secreting monoclonal antibodies was raised against the two recombinant TbpB fusion proteins. Furthermore, to assess whether the expression of the H. parasuis ExbB, TbpB, and TbpA proteins was upregulated under conditions of restricted availability of iron, a rabbit polyclonal antibody against H. parasuis TbpB-His fusion protein was produced. A rabbit polyclonal antibody against serotype 7 of Actinobacillus pleuropneumoniae ExbB and TbpA proteins was also used for the detection of the homologous proteins in H. parasuis. Overall, the data indicate that H. parasuis, like other members of the Pasteurellaceae family, possesses the genetic elements of the TonB region for iron acquisition and the transferrin-binding proteins encoded under this region are upregulated under restricted iron availability.

TheexbDgene ofPhotorhabdus temperatais required for full virulence in insects and symbiosis with the nematodeHeterorhabditis

Photorhabdus are bacteria found colonizing the gut of a specialized stage of the nematode Heterorhabditis, called the infective juvenile (IJ). The IJ is a free-living stage of the nematode that seeks out and infects insect larvae. Once inside the insect the IJ release Photorhabdus into the haemolymph where the bacteria rapidly proliferate, killing the insect within 48-72 h. The nematodes grow and reproduce in the insect cadaver by feeding on the Photorhabdus biomass. In this study we use Photorhabdus temperata K122 to show that genes involved in iron acquisition play a key role during the course of the tripartite bacteria-nematode-insect interaction. We show that a strain carrying a mutation in a gene with homology to exbD, encoding a component of the TonB complex, is unable to grow well in conditions where iron is not freely available. In addition, this mutant, BMM417, requires a longer time to kill the insect larvae than the wild-type bacteria and this defect in pathogenicity is complemented by the co-injection of iron. Moreover, the increase in LT(50) observed with BMM417 is correlated with a significantly slower in vivo growth rate suggesting that iron is limiting in the insect. We also show that BMM417 is unable to support the growth and development of the Heterorhabditis nematode. Addition of exogenous iron to the growth media restores nematode growth and development on BMM417, suggesting that aspects of iron metaboism in Photorhabdus are important during the symbiosis with the nematode.

A heptosyltransferase mutant of Pasteurella multocida produces a truncated lipopolysaccharide structure and is attenuated in virulence

Pasteurella multocida is the causative agent of fowl cholera in birds. In a previous study using signature-tagged mutagenesis, we identified a mutant, AL251, which was attenuated for virulence in mice and in the natural chicken host. Sequence analysis indicated that AL251 had an insertional inactivation of the gene waaQ(PM), encoding a putative heptosyl transferase, required for the addition of heptose to lipopolysaccharide (LPS) (M. Harper, J. D. Boyce, I. W. Wilkie, and B. Adler, Infect. Immun. 71:5440-5446, 2003). In the present study, using mass spectrometry and nuclear magnetic resonance, we have confirmed the identity of the enzyme encoded by waaQ(PM) as a heptosyl transferase III and demonstrated that the predominant LPS glycoforms isolated from this mutant are severely truncated. Complementation experiments demonstrated that providing a functional waaQ(PM) gene in trans can restore both the LPS to its full length and growth in mice to wild-type levels. Furthermore, we have shown that mutant AL251 is unable to cause fowl cholera in chickens and that the attenuation observed is not due to increased serum sensitivity.

Signature-tagged mutagenesis of Pasteurella multocida identifies mutants displaying differential virulence characteristics in mice and chickens

Pasteurella multocida is the causative agent of fowl cholera in birds. Signature-tagged mutagenesis (STM) was used to identify potential virulence factors in a mouse septicemia disease model and a chicken fowl cholera model. A library of P. multocida mutants was constructed with a modified Tn916 and screened for attenuation in both animal models. Mutants identified by the STM screening were confirmed as attenuated by competitive growth assays in both chickens and mice. Of the 15 mutants identified in the chicken model, only 5 were also attenuated in mice, showing for the first time the presence of host-specific virulence factors and indicating the importance of screening for attenuation in the natural host.

fur-independent regulation of the Pasteurella multocida hbpA gene encoding a haemin-binding protein

Treatment of bacterial cultures with chelating agents such as 2,2'-dipyridyl (DPD) induces expression of iron-regulated genes. It is known that in the gamma-Proteobacteria, the Fur protein is the major regulator of genes encoding haem- or haemoglobin-binding proteins. Electrophoretic analysis of outer-membrane proteins of the gamma-proteobacterium Pasteurella multocida has revealed the induction of two proteins of 60 and 40 kDa in DPD-treated cultures in both wild-type and fur-defective strains. These two proteins have the same N-terminal amino acid sequence, which identifies this protein as the product of the PM0592 ORF. Analysis of the sequence of this ORF, which encodes a protein of 60 kDa, revealed the presence of a hexanucleotide (AAAAAA) at which a programmed translational frameshift can occur giving rise to a 40 kDa protein. Analyses conducted in Escherichia coli, using the complete PM0592 ORF and a derivative truncated at the hexanucleotide position, have shown that both polypeptides bind haemin. For this reason, the PM0592 ORF product has been designated HbpA (for haemin-binding protein). Expression studies using both RT-PCR and lacZ fusions, as well as electrophoretic profiles of outer-membrane protein composition, have demonstrated that the hbpA gene is negatively regulated by iron, manganese and haemin through a fur-independent pathway. Despite the fact that serum of mice infected with P. multocida contained antibodies that reacted with both the 60 and 40 kDa products of the hbpA gene, these proteins did not offer protection when used in immunization assays against this micro-organism.

Functional characterization of HgbB, a new hemoglobin binding protein of Pasteurella multocida

The biological function and role in pathogenesis of a Pasteurella multocida A:1 strain hemoglobin binding protein was investigated. The hgbB gene from the P. multocida A:1 strain, VP161, was cloned and characterized. hgbB was 2991 bp in length and encoded a mature length protein of 111 kDa. HgbB was predicted to be an outer membrane protein and shared 68 and 69% similarity to the hemoglobin/hemoglobin-haptoglobin binding protein, HI0712 from Haemophilus influenzae Rd and HgpC, from H. influenzae b, respectively. HgbB exhibited features typical of TonB dependent receptors, including seven conserved regions typical of these proteins, and conserved invariant residues. Escherichia coli expressing recombinant HgbB was found to bind hemoglobin in a solid phase dot blot binding assay. However, when a truncated form of the protein was expressed in E. coli, cells could no longer bind hemoglobin. Insertional inactivation of hgbB did not affect the ability of P. multocida to bind hemoglobin, nor its ability to produce disease in a mouse model. In addition, recombinant HgbB did not confer any protection against homologous or heterologous challenge.

The high-affinity zinc-uptake system znuACB is under control of the iron-uptake regulator (fur) gene in the animal pathogen Pasteurella multocida

The Pasteurella multocida znuACB genes encoding a high-affinity zinc-uptake system have been identified and cloned. In contrast to what happens in Escherichia coli, znuA is not physically linked to znuCB. Through lacZ transcriptional fusions it has been demonstrated that zinc negatively regulates both znuA and znuCB operons. Nevertheless, and contrary to that determined so far for all other znuACB bacterial systems known, P. multocida znuACB genes are not under control of the zur gene, which is absent in this bacterial species, but rather are under its iron-uptake regulator (fur) gene. Furthermore, construction of defective mutants has demonstrated that P. multocida znuA and znuCB transcriptional units are required for virulence of this organism in a mouse model.