The molecular and cellular basis of pathogenesis in melioidosis: how does Burkholderia pseudomallei cause disease?

Melioidosis, a febrile illness with disease states ranging from acute pneumonia or septicaemia to chronic abscesses, was first documented by Whitmore & Krishnaswami (1912). The causative agent, Burkholderia pseudomallei, was subsequently identified as a motile, gram-negative bacillus, which is principally an environmental saprophyte. Melioidosis has become an increasingly important disease in endemic areas such as northern Thailand and Australia (Currie et al., 2000). This health burden, plus the classification of B. pseudomallei as a category B biological agent (Rotz et al., 2002), has resulted in an escalation of research interest. This review focuses on the molecular and cellular basis of pathogenesis in melioidosis, with a comprehensive overview of the current knowledge on how B. pseudomallei can cause disease. The process of B. pseudomallei movement from the environmental reservoir to attachment and invasion of epithelial and macrophage cells and the subsequent intracellular survival and spread is outlined. Furthermore, the diverse assortment of virulence factors that allow B. pseudomallei to become an effective opportunistic pathogen, as well as to avoid or subvert the host immune response, is discussed. With the recent increase in genomic and molecular studies, the current understanding of the infection process of melioidosis has increased substantially, yet, much still remains to be elucidated.

Leptospira and leptospirosis

Leptospirosis is the most wide spread zoonosis worldwide; it is present in all continents except Antarctica and evidence for the carriage of Leptospira has been found in virtually all mammalian species examined. Humans most commonly become infected through occupational, recreational, or domestic contact with the urine of carrier animals, either directly or via contaminated water or soil. Leptospires are thin, helical bacteria classified into at least 12 pathogenic and 4 saprophytic species, with more than 250 pathogenic serovars. Immunity following infection is generally, but not exclusively, mediated by antibody against leptospiral LPS and restricted to antigenically related serovars. Vaccines currently available consist of killed whole cell bacterins which are used widely in animals, but less so in humans. Current work with recombinant protein antigens shows promise for the development of vaccines based on defined protective antigens. The cellular and molecular basis for virulence remains poorly understood, but comparative genomics of pathogenic and saprophytic species suggests that Leptospira expresses unique virulence determinants. However, the recent development of defined mutagenesis systems for Leptospira heralds the potential for gaining a much improved understanding of pathogenesis in leptospirosis.

Crystal structure of LipL32, the most abundant surface protein of pathogenic Leptospira spp

Spirochetes of the genus Leptospira cause leptospirosis in humans and animals worldwide. Proteins exposed on the bacterial cell surface are implicated in the pathogenesis of leptospirosis. However, the biological role of the majority of these proteins is unknown; this is principally due to the lack of genetic systems for investigating Leptospira and the absence of any structural information on leptospiral antigens. To address this, we have determined the 2.0-A-resolution structure of the lipoprotein LipL32, the most abundant outer-membrane and surface protein present exclusively in pathogenic Leptospira species. The extracellular domain of LipL32 revealed a compact, globular, "jelly-roll" fold from which projected an unusual extended beta-hairpin that served as a principal mediator of the observed crystallographic dimer. Two acid-rich patches were also identified as potential binding sites for positively charged ligands, such as laminin, to which LipL32 has a propensity to bind. Although LipL32 shared no significant sequence identity to any known protein, it possessed structural homology to the adhesins that bind components of the extracellular matrix, suggesting that LipL32 functions in an analogous manner. Moreover, the structure provides a framework for understanding the immunological role of this major surface lipoprotein.

LipL32 is an extracellular matrix-interacting protein of Leptospira spp. and Pseudoalteromonas tunicata

LipL32 is the major outer membrane protein in pathogenic Leptospira. It is highly conserved throughout pathogenic species and is expressed in vivo during human infection. While these data suggest a role in pathogenesis, a function for LipL32 has not been defined. Outer membrane proteins of gram-negative bacteria are the first line of molecular interaction with the host, and many have been shown to bind host extracellular matrix (ECM). A search for leptospiral ECM-interacting proteins identified the major outer membrane protein, LipL32. To verify this finding, recombinant LipL32 was expressed in Escherichia coli and was found to bind Matrigel ECM and individual components of ECM, including laminin, collagen I, and collagen V. Likewise, an orthologous protein found in the genome of Pseudoalteromonas tunicata strain D2 was expressed and found to be functionally similar and immunologically cross-reactive. Lastly, binding activity was mapped to the C-terminal 72 amino acids. These studies show that LipL32 and an orthologous protein in P. tunicata are immunologically cross-reactive and function as ECM-interacting proteins via a conserved C-terminal region.

Leptospira interrogans requires a functional heme oxygenase to scavenge iron from hemoglobin

The transposon TnSC189 was used to construct a mutant in the putative heme oxygenase gene hemO (LB186) of Leptospira interrogans. Unlike its parent strain, the mutant grew poorly in medium in which hemoglobin was the sole iron source. The putative heme oxygenase was over expressed in a His-tagged form, purified and was demonstrated to degrade heme in vitro. Unexpectedly, it was also found that the L. interrogans growth rate was significantly increased when medium was supplemented with hemoglobin, but only if ferrous iron sources were absent. This result was mirrored in the expression of some iron-related genes and suggests the presence of regulatory mechanisms detecting Fe2+ and hemoglobin. This is the first demonstration of a functional heme oxygenase from a spirochete.

Genome sequence of the saprophyte Leptospira biflexa provides insights into the evolution of Leptospira and the pathogenesis of leptospirosis

Leptospira biflexa is a free-living saprophytic spirochete present in aquatic environments. We determined the genome sequence of L. biflexa, making it the first saprophytic Leptospira to be sequenced. The L. biflexa genome has 3,590 protein-coding genes distributed across three circular replicons: the major 3,604 chromosome, a smaller 278-kb replicon that also carries essential genes, and a third 74-kb replicon. Comparative sequence analysis provides evidence that L. biflexa is an excellent model for the study of Leptospira evolution; we conclude that 2052 genes (61%) represent a progenitor genome that existed before divergence of pathogenic and saprophytic Leptospira species. Comparisons of the L. biflexa genome with two pathogenic Leptospira species reveal several major findings. Nearly one-third of the L. biflexa genes are absent in pathogenic Leptospira. We suggest that once incorporated into the L. biflexa genome, laterally transferred DNA undergoes minimal rearrangement due to physical restrictions imposed by high gene density and limited presence of transposable elements. In contrast, the genomes of pathogenic Leptospira species undergo frequent rearrangements, often involving recombination between insertion sequences. Identification of genes common to the two pathogenic species, L. borgpetersenii and L. interrogans, but absent in L. biflexa, is consistent with a role for these genes in pathogenesis. Differences in environmental sensing capacities of L. biflexa, L. borgpetersenii, and L. interrogans suggest a model which postulates that loss of signal transduction functions in L. borgpetersenii has impaired its survival outside a mammalian host, whereas L. interrogans has retained environmental sensory functions that facilitate disease transmission through water.

Decoration of Pasteurella multocida lipopolysaccharide with phosphocholine is important for virulence

Phosphocholine (PCho) is an important substituent of surface structures expressed by a number of bacterial pathogens. Its role in virulence has been investigated in several species, in which it has been shown to play a role in bacterial adhesion to mucosal surfaces, in resistance to antimicrobial peptides, or in sensitivity to complement-mediated killing. The lipopolysaccharide (LPS) structure of Pasteurella multocida strain Pm70, whose genome sequence is known, has recently been determined and does not contain PCho. However, LPS structures from the closely related, virulent P. multocida strains VP161 and X-73 were shown to contain PCho on their terminal galactose sugar residues. To determine if PCho was involved in the virulence of P. multocida, we used subtractive hybridization of the VP161 genome against the Pm70 genome to identify a four-gene locus (designated pcgDABC) which we show is required for the addition of the PCho residues to LPS. The proteins predicted to be encoded by pcgABC showed identity to proteins involved in choline uptake, phosphorylation, and nucleotide sugar activation of PCho. We constructed a P. multocida VP161 pcgC mutant and demonstrated that this strain produces LPS that lacks PCho on the terminal galactose residues. This pcgC mutant displayed reduced in vivo growth in a chicken infection model and was more sensitive to the chicken antimicrobial peptide fowlicidin-1 than the wild-type P. multocida strain.

Pasteurella multocida expresses two lipopolysaccharide glycoforms simultaneously, but only a single form is required for virulence: identification of two acceptor-specific heptosyl I transferases

Lipopolysaccharide (LPS) is a critical virulence determinant in Pasteurella multocida and a major antigen responsible for host protective immunity. In other mucosal pathogens, variation in LPS or lipooligosaccharide structure typically occurs in the outer core oligosaccharide regions due to phase variation. P. multocida elaborates a conserved oligosaccharide extension attached to two different, simultaneously expressed inner core structures, one containing a single phosphorylated 3-deoxy-D-manno-octulosonic acid (Kdo) residue and the other containing two Kdo residues. We demonstrate that two heptosyltransferases, HptA and HptB, add the first heptose molecule to the Kdo(1) residue and that each exclusively recognizes different acceptor molecules. HptA is specific for the glycoform containing a single, phosphorylated Kdo residue (glycoform A), while HptB is specific for the glycoform containing two Kdo residues (glycoform B). In addition, KdkA was identified as a Kdo kinase, required for phosphorylation of the first Kdo molecule. Importantly, virulence data obtained from infected chickens showed that while wild-type P. multocida expresses both LPS glycoforms in vivo, bacterial mutants that produced only glycoform B were fully virulent, demonstrating for the first time that expression of a single LPS form is sufficient for P. multocida survival in vivo. We conclude that the ability of P. multocida to elaborate alternative inner core LPS structures is due to the simultaneous expression of two different heptosyltransferases that add the first heptose residue to the nascent LPS molecule and to the expression of both a bifunctional Kdo transferase and a Kdo kinase, which results in the initial assembly of two inner core structures.

Genome sequence and identification of candidate vaccine antigens from the animal pathogen Dichelobacter nodosus

Dichelobacter nodosus causes ovine footrot, a disease that leads to severe economic losses in the wool and meat industries. We sequenced its 1.4-Mb genome, the smallest known genome of an anaerobe. It differs markedly from small genomes of intracellular bacteria, retaining greater biosynthetic capabilities and lacking any evidence of extensive ongoing genome reduction. Comparative genomic microarray studies and bioinformatic analysis suggested that, despite its small size, almost 20% of the genome is derived from lateral gene transfer. Most of these regions seem to be associated with virulence. Metabolic reconstruction indicated unsuspected capabilities, including carbohydrate utilization, electron transfer and several aerobic pathways. Global transcriptional profiling and bioinformatic analysis enabled the prediction of virulence factors and cell surface proteins. Screening of these proteins against ovine antisera identified eight immunogenic proteins that are candidate antigens for a cross-protective vaccine.

Identification of a Campylobacter jejuni protein that cross-reacts with cholera toxin

The question of whether Campylobacter jejuni produces a cholera toxin-like toxin (CTLT) has been controversial. The objective of this study was to identify the factor that cross-reacts with CT from C. jejuni. Filtrates of C. jejuni grown in four different liquid media reported to promote CTLT production were tested by Chinese hamster ovary (CHO) cell elongation assay and for reactivity with CT antibody using GM1 ganglioside enzyme-linked immunosorbent assay (ELISA) and immunoblotting. Protein sequence was determined by matrix-assisted laser desorption ionization-time of flight (MALDI TOF-TOF). Filtrates from seven reference strains reported to produce CTLT and from 80 clinical strains were negative in the CHO cell assay, but those from three reference strains and 16 clinical strains were positive by GM1 ELISA. All strains tested, including C. jejuni NCTC 11168, which does not contain a CT gene homologue, possessed a 53-kDa protein which reacted with CT antibody by immunoblotting. This band was identified as the major outer membrane protein, PorA, of C. jejuni. CT antibody reacted by immunoblotting with a recombinant PorA, but antibody to the recombinant PorA did not react with CT. Our results indicate that C. jejuni does not produce a functional CTLT, but the reactivity of PorA with CT antibody would lead to the erroneous conclusion that C. jejuni produces a functional CTLT.

Response of Leptospira interrogans to physiologic osmolarity: relevance in signaling the environment-to-host transition

Transmission of pathogenic Leptospira between mammalian hosts usually involves dissemination via soil or water contaminated by the urine of carrier animals. The ability of Leptospira to adapt to the diverse conditions found inside and outside the host is reflected in its relatively large genome size and high percentage of signal transduction genes. An exception is Leptospira borgpetersenii serovar Hardjo, which is transmitted by direct contact and appears to have lost genes necessary for survival outside the mammalian host. Invasion of host tissues by Leptospira interrogans involves a transition from a low osmolar environment outside the host to a higher physiologic osmolar environment within the host. Expression of the lipoprotein LigA and LigB adhesins is strongly induced by an upshift in osmolarity to the level found in mammalian host tissues. These data suggest that Leptospira utilizes changes in osmolarity to regulate virulence characteristics. To better understand how L. interrogans serovar Copenhageni adapts to osmolar conditions that correspond with invasion of a mammalian host, we quantified alterations in transcript levels using whole-genome microarrays. Overnight exposure in leptospiral culture medium supplemented with sodium chloride to physiologic osmolarity significantly altered the transcript levels of 6% of L. interrogans genes. Repressed genes were significantly more likely to be absent or pseudogenes in L. borgpetersenii, suggesting that osmolarity is relevant in studying the adaptation of L. interrogans to host conditions. Genes induced by physiologic osmolarity encoded a higher than expected number of proteins involved in signal transduction. Further, genes predicted to encode lipoproteins and those coregulated by temperature were overrepresented among both salt-induced and salt-repressed genes. In contrast, leptospiral homologues of hyperosmotic or general stress genes were not induced at physiologic osmolarity. These findings suggest that physiologic osmolarity is an important signal for regulation of gene expression by pathogenic leptospires during transition from ambient conditions to the host tissue environment.

Identification of novel immunogens in Pasteurella multocida

P. multocida is a Gram-negative pathogen responsible for causing diseases in animals of economic significance to livestock industries throughout the world. Current vaccines include bacterins, which provide only limited protection against homologous serotypes. Therefore there is a need for more effective vaccines to control diseases caused by P. multocida. As a step towards developing vaccines against fowl cholera, a genomics based approach was applied for the identification of novel immunogens.

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.

Effects of temperature on gene expression patterns in Leptospira interrogans serovar Lai as assessed by whole-genome microarrays

Leptospirosis is an important zoonosis of worldwide distribution. Humans become infected via exposure to pathogenic Leptospira spp. from infected animals or contaminated water or soil. The availability of genome sequences for Leptospira interrogans, serovars Lai and Copenhageni, has opened up opportunities to examine global transcription profiles using microarray technology. Temperature is a key environmental factor known to affect leptospiral protein expression. Leptospira spp. can grow in artificial media at a range of temperatures reflecting conditions found in the environment and the mammalian host. Therefore, transcriptional changes were compared between cultures grown at 20 degrees C, 30 degrees C, 37 degrees C, and 39 degrees C to represent ambient temperatures in the environment, growth under laboratory conditions, and temperatures in healthy and febrile hosts. Data from direct pairwise comparisons of the four temperatures were consolidated to examine transcriptional changes at two generalized biological conditions representing mammalian physiological temperatures (37 degrees C and 39 degrees C) versus environmental temperatures (20 degrees C and 30 degrees C). Additionally, cultures grown at 30 degrees C then shifted overnight to 37 degrees C were compared with those grown long-term at 30 degrees C and 37 degrees C to identify genes potentially expressed in the early stages of infection. Comparison of data sets from physiological versus environmental experiments with upshift experiments provided novel insights into possible transcriptional changes at different stages of infection. Changes included differential expression of chemotaxis and motility genes, signal transduction systems, and genes encoding proteins involved in alteration of the outer membrane. These findings indicate that temperature is an important factor regulating expression of proteins that facilitate invasion and establishment of disease.

Genome reduction in Leptospira borgpetersenii reflects limited transmission potential

Leptospirosis is one of the most common zoonotic diseases in the world, resulting in high morbidity and mortality in humans and affecting global livestock production. Most infections are caused by either Leptospira borgpetersenii or Leptospira interrogans, bacteria that vary in their distribution in nature and rely on different modes of transmission. We report the complete genomic sequences of two strains of L. borgpetersenii serovar Hardjo that have distinct phenotypes and virulence. These two strains have nearly identical genetic content, with subtle frameshift and point mutations being a common form of genetic variation. Starkly limited regions of synteny are shared between the large chromosomes of L. borgpetersenii and L. interrogans, probably the result of frequent recombination events between insertion sequences. The L. borgpetersenii genome is approximately 700 kb smaller and has a lower coding density than L. interrogans, indicating it is decaying through a process of insertion sequence-mediated genome reduction. Loss of gene function is not random but is centered on impairment of environmental sensing and metabolite transport and utilization. These features distinguish L. borgpetersenii from L. interrogans, a species with minimal genetic decay and that survives extended passage in aquatic environments encountering a mammalian host. We conclude that L. borgpetersenii is evolving toward dependence on a strict host-to-host transmission cycle.

Differential expression of the Bhmp39 major outer membrane proteins of Brachyspira hyodysenteriae

The enteric, anaerobic spirochete Brachyspira hyodysenteriae is the causative agent of swine dysentery, a severe mucohemorrhagic diarrheal disease of pigs that has economic significance in every major pork-producing country. Recent investigation into potential vaccine candidates has focused on the outer membrane proteins of B. hyodysenteriae. Bhmp39 (formerly Vsp39) is the most abundant surface-exposed outer membrane protein of B. hyodysenteriae; its predicted gene sequence has previously been shown to share sequence similarity to eight genes divided evenly between two paralogous loci. The peptide sequence suggested that Bhmp39 is encoded by one of these genes, bhmp39h. The biological significance of maintaining eight homologous bhmp39 genes is unclear, though it has been proposed that this may play a role in antigenic variation. In this study, real-time, reverse transcription-PCR was used to demonstrate that bhmp39f and bhmp39h were the transcripts most abundantly expressed by B. hyodysenteriae strain B204 cultured under in vitro growth conditions. Mass spectrometry data of the purified 39-kDa membrane protein showed that both Bhmp39f and Bhmp39h were present. Northern blot analysis across predicted Rho-independent terminators demonstrated that the genes of the bhmp39efgh locus result in monocistronic transcripts.

A novel phase-variable autotransporter serine protease, AusI, of Neisseria meningitidis

The sequenced genomes of pathogenic Neisseria meningitidis strains contain up to eight genes putatively encoding autotransporters, which are secreted proteins implicated in virulence. Here, we have characterized one of these genes, designated ausI, which encodes an autotransporter of the serine protease family. It was found to be specific for N. meningitidis and present in 14 out of 20 isolates, although only six of them expressed the gene. We show that expression of the gene is subject to phase variation as a result of a variable number of cytosines in a poly-C tract in the coding region. The open reading frame went out-of-phase at the poly-C tract in seven strains that did not express AusI. In the eighth strain, the open reading frame remained in frame at the poly-C tract, but it was disrupted by a premature stop codon further downstream. In accordance with its assignment as an autotransporter, a secreted AusI passenger domain was released into the extracellular milieu. This release was influenced by another autotransporter, NalP, as different forms of AusI were produced in the presence or absence of NalP. In silico sequence analysis suggested several putative functions for AusI, which, however, could not be confirmed experimentally.

Analysis of thePasteurella multocida outer membrane sub-proteome and its response to thein vivo environment of the natural host

This study describes the identification of outer membrane proteins (OMPs) of the bacterial pathogen Pasteurella multocida and an analysis of how the expression of these proteins changes during infection of the natural host. We analysed the sarcosine-insoluble membrane fractions, which are highly enriched for OMPs, from bacteria grown under a range of conditions. Initially, the OMP-containing fractions were resolved by 2-DE and the proteins identified by MALDI-TOF MS. In addition, the OMP-containing fractions were separated by 1-D SDS-PAGE and protein identifications were made using nano LC MS/MS. Using these two methods a total of 35 proteins was identified from samples obtained from organisms grown in rich culture medium. Six of the proteins were identified only by 2-DE MALDI-TOF MS, whilst 17 proteins were identified only by 1-D LC MS/MS. We then analysed the OMPs from P. multocida which had been isolated from the bloodstream of infected chickens (a natural host) or grown in iron-depleted medium. Three proteins were found to be significantly up-regulated during growth in vivo and one of these (Pm0803) was also up-regulated during growth in iron-depleted medium. After bioinformatic analysis of the protein matches, it was predicted that over one third of the combined OMPs predicted by the bioinformatics sub-cellular localisation tools PSORTB and Proteome Analyst, had been identified during this study. This is the first comprehensive proteomic analysis of the P. multocida outer membrane and the first proteomic analysis of how a bacterial pathogen modifies its outer membrane proteome during infection.

How does Pasteurella multocida respond to the host environment?

Pasteurella multocida is a Gram-negative bacterial pathogen, which causes diseases of economic importance in a wide range of animal species. The response of P. multocida to the host environment has been analysed at the transcription level, using DNA microarrays, and at the protein-expression level, using proteomics techniques. Furthermore, a growing number of P. multocida-directed mutants have been assessed for their ability to cause disease. Although technical impediments mean that it is currently difficult to analyse bacterial responses at the earliest stages of infection, it is clear that during later stages of infection the bacteria encounter host niches that require them to modify the expression of genes involved in central energy metabolism and in the uptake of various nutrients such as iron and amino acids. Furthermore, in vitro experiments have defined the varying bacterial responses to low iron and to different iron sources, including haemoglobin and transferrin. To date, most P. multocida genes shown to be upregulated during infection are involved in nutrient acquisition and metabolic processes, indicating that true virulence genes might be constitutively expressed, upregulated only during initial stages of infection or upregulated at levels below current detection limits.

Human antibody response to outer membrane protein G1a, a lipoprotein of Moraxella catarrhalis

Moraxella catarrhalis is an important cause of respiratory infections in adults with chronic obstructive pulmonary disease (COPD) and of otitis media in children. Outer membrane protein (OMP) G1a is an approximately 29-kDa surface lipoprotein and is a potential vaccine candidate. The gene that encodes OMP G1a was expressed and purified using a novel plasmid vector. [(3)H]palmitic acid labeling demonstrated that both native and recombinant OMP G1a contain covalently bound palmitic acid. To assess the expression of OMP G1a during human infection, paired sera and sputum supernatants from adults with COPD followed prospectively were studied by enzyme-linked immunosorbent assays with recombinant lipidated OMP G1a to detect antibodies made specifically during carriage of M. catarrhalis. Overall, 23% of patients developed either a serum immunoglobulin G (IgG) response (9%) or sputum IgA response (21%) to OMP G1a, following 100 episodes of acquisition and clearance of M. catarrhalis. Patients developed antibody responses at similar rates following episodes of clinical exacerbation compared to asymptomatic colonization. Serum IgG antibodies following natural infection were directed predominantly at OMP G1a epitopes that are not exposed on the bacterial surface. These data show that OMP G1a is expressed during infection of the human respiratory tract and is a target of systemic and mucosal antibodies. These observations indicate that OMP G1a, a highly conserved surface protein, should be evaluated further as a vaccine candidate.

The Leptospira interrogans lexA gene is not autoregulated

Footprinting and mutagenesis experiments demonstrated that Leptospira interrogans LexA binds the palindrome TTTGN(5)CAAA found in the recA promoter but not in the lexA promoter. In silico analysis revealed that none of the other canonical SOS genes is under direct control of LexA, making the leptospiral lexA gene the first described which is not autoregulated.

Surfaceome of Leptospira spp

The identification of the subset of outer membrane proteins exposed on the surface of a bacterial cell (the surfaceome) is critical to understanding the interactions of bacteria with their environments and greatly narrows the search for protective antigens of extracellular pathogens. The surfaceome of Leptospira was investigated by biotin labeling of viable leptospires, affinity capture of the biotinylated proteins, two-dimensional gel electrophoresis, and mass spectrometry (MS). The leptospiral surfaceome was found to be predominantly made up of a small number of already characterized proteins, being in order of relative abundance on the cell surface: LipL32 > LipL21 > LipL41. Of these proteins, only LipL32 had not been previously identified as surface exposed. LipL32 surface exposure was subsequently verified by three independent approaches: surface immunofluorescence, whole-cell enzyme-linked immunosorbent assay (ELISA), and immunoelectron microscopy. Three other proteins, Q8F8Q0 (a putative transmembrane outer membrane protein) and two proteins of 20 kDa and 55 kDa that could not be identified by MS, one of which demonstrated a high degree of labeling potentially representing an additional, as-yet-uncharacterized, surface-exposed protein. Minor labeling of p31(LipL45), GroEL, and FlaB1 was also observed. Expression of the surfaceome constituents remained unchanged under a range of conditions investigated, including temperature and the presence of serum or urine. Immunization of mice with affinity-captured surface components stimulated the production of antibodies that bound surface proteins from heterologous leptospiral strains. The surfaceomics approach is particularly amenable to protein expression profiling using small amounts of sample (<10(7) cells) offering the potential to analyze bacterial surface expression during infection.

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.