Aromatic compound-dependent Brucella suis is attenuated in both cultured cells and mouse models

An Evolutionary Conservation and Druggability Analysis of Enzymes Belonging to the Bacterial Shikimate Pathway

Enzymes belonging to the shikimate pathway have long been considered promising targets for antibacterial drugs because they have no counterpart in mammals and are essential for bacterial growth and virulence. However, despite decades of research, there are currently no clinically relevant antibacterial drugs targeting any of these enzymes, and there are legitimate concerns about whether they are sufficiently druggable, i.e., whether they can be adequately modulated by small and potent drug-like molecules. In the present work, in silico analyses combining evolutionary conservation and druggability are performed to determine whether these enzymes are candidates for broad-spectrum antibacterial therapy. The results presented here indicate that the substrate-binding sites of most enzymes in this pathway are suitable drug targets because of their reasonable conservation and druggability scores. An exception was the substrate-binding site of 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase, which was found to be undruggable because of its high content of charged residues and extremely high overall polarity. Although the presented study was designed from the perspective of broad-spectrum antibacterial drug development, this workflow can be readily applied to any antimicrobial target analysis, whether narrow- or broad-spectrum. Moreover, this research also contributes to a deeper understanding of these enzymes and provides valuable insights into their properties.

Mining the Flavoproteome of Brucella ovis, the Brucellosis Causing Agent in Ovis aries

Flavoproteins are a diverse class of proteins that are mostly enzymes and contain as cofactors flavin mononucleotide (FMN) and/or flavin adenine dinucleotide (FAD), which enable them to participate in a wide range of physiological reactions. We have compiled 78 potential proteins building the flavoproteome of Brucella ovis (B. ovis), the causative agent of ovine brucellosis. The curated list of flavoproteins here reported is based on (i) the analysis of sequence, structure and function of homologous proteins, and their classification according to their structural domains, clans, and expected enzymatic functions; (ii) the constructed phylogenetic trees of enzyme functional classes using 19 Brucella strains and 26 pathogenic and/or biotechnological relevant alphaproteobacteria together with B. ovis; and (iii) the evaluation of the genetic context for each entry. Candidates account for ∼2.7% of the B. ovis proteome, and 75% of them use FAD as cofactor. Only 55% of these flavoproteins belong to the core proteome of Brucella and contribute to B. ovis processes involved in maintenance activities, survival and response to stress, virulence, and/or infectivity. Several of the predicted flavoproteins are highly divergent in Brucella genus from revised proteins and for them it is difficult to envisage a clear function. This might indicate modified catalytic activities or even divergent processes and mechanisms still not identified. We have also detected the lack of some functional flavoenzymes in B. ovis, which might contribute to it being nonzoonotic. Finally, potentiality of B. ovis flavoproteome as the source of antimicrobial targets or biocatalyst is discussed. IMPORTANCE Some microorganisms depend heavily on flavin-dependent activities, but others maintain them at a minimum. Knowledge about flavoprotein content and functions in different microorganisms will help to identify their metabolic requirements, as well as to benefit either industry or health. Currently, most flavoproteins from the sheep pathogen Brucella ovis are only automatically annotated in databases, and only two have been experimentally studied. Indeed, certain homologues with unknown function are not characterized, and they might relate to still not identified mechanisms or processes. Our research has identified 78 members that comprise its flavoproteome, 76 of them flavoenzymes, which mainly relate to bacteria survival, virulence, and/or infectivity. The list of flavoproteins here presented allows us to better understand the peculiarities of Brucella ovis and can be applied as a tool to search for candidates as new biocatalyst or antimicrobial targets.

Immunometabolism in human brucellosis: An emerging field of investigation

In recent years, extreme attention has been focused on the role of immunometabolism in the regulation of immune cell responses in healthy individuals during infection, autoimmunity, and cancer. In the infection biology area, it has been shown that there is a close relationship between the immune system and the host metabolic changes. Brucella species is an intracellular coccobacillus that infects humans and mammals, which led to brucellosis. Brucella species with host-specific evolutionary mechanisms allow it to hide from or manipulate cellular immunity and achieve intracellular persistence. Intracellular bacterial pathogens such as Brucella species also employ host cell resources to replicate and persist inside the host. Targeting these host systems is one promising strategy for developing novel antimicrobials to tackle intracellular infections. This study will summarize the role of metabolic reprogramming in immune cells and their relationship to brucellosis.

Sub-Inhibitory Doses of Individual Constituents of Essential Oils Can Select for Staphylococcus aureus Resistant Mutants

Increased bacterial resistance to food preservation technologies represents a risk for food safety and shelf-life. The use of natural antimicrobials, such as essential oils (EOs) and their individual constituents (ICs), has been proposed to avoid the generation of antimicrobial resistance. However, prolonged application of ICs might conceivably lead to the emergence of resistant strains. Hence, this study was aimed toward applying sub-inhibitory doses of the ICs carvacrol, citral, and (+)-limonene oxide to Staphylococcus aureus USA300, in order to evaluate the emergence of resistant strains and to identify the genetic modifications responsible for their increased resistance. Three stable-resistant strains, CAR (from cultures with carvacrol), CIT (from cultures with citral), and OXLIM (from cultures with (+)-limonene oxide) were isolated, showing an increased resistance against the ICs and a higher tolerance to lethal treatments by ICs or heat. Whole-genome sequencing revealed in CAR a large deletion in a region that contained genes encoding transcriptional regulators and metabolic enzymes. CIT showed a single missense mutation in aroC (N187K), which encodes for chorismate synthase; and in OXLIM a missense mutation was detected in rpoB (A862V), which encodes for RNA polymerase subunit beta. This study provides a first detailed insight into the mechanisms of action and S. aureus resistance arising from exposure to carvacrol, citral, and (+)-limonene oxide.

Brucella central carbon metabolism: an update

The brucellae are facultative intracellular pathogens causing brucellosis, an important zoonosis. Here, we review the nutritional, genetic, proteomic and transcriptomic studies on Brucella carbon uptake and central metabolism, information that is needed for a better understanding of Brucella virulence. There is no uniform picture across species but the studies suggest primary and/or secondary transporters for unknown carbohydrates, lactate, glycerol phosphate, erythritol, xylose, ribose, glucose and glucose/galactose, and routes for their incorporation to central metabolism, including an erythritol pathway feeding the pentose phosphate cycle. Significantly, all brucellae lack phosphoenolpyruvate synthase and phosphofructokinase genes, which confirms previous evidence on glycolysis absence, but carry all Entner-Doudoroff (ED) pathway and Krebs cycle (and glyoxylate pathway) genes. However, glucose catabolism proceeds through the pentose phosphate cycle in the classical species, and the ED pathway operates in some rodent-associated brucellae, suggesting an ancestral character for this pathway in this group. Gluconeogenesis is functional but does not rely exclusively on classical fructose bisphosphatases. Evidence obtained using infection models is fragmentary but suggests the combined or sequential use of hexoses/pentoses, amino acids and gluconeogenic substrates. We also discuss the role of the phosphotransferase system, stringent reponse, quorum sensing, BvrR/S and sRNAs in metabolism control, an essential aspect of the life style of facultative intracellular parasites.

Attenuating gene expression (AGE) for vaccine development

Live attenuated vaccines are adept in stimulating protective immunity. Methods for generating such vaccines have largely adopted strategies used with Salmonella enterica. Yet, when similar strategies were tested in other gram-negative bacteria, the virulence factors or genes responsible to incapacitate Salmonella often failed in providing the desired outcome. Consequently, conventional live vaccines rely on prior knowledge of the pathogen's virulence factors to successfully attenuate them. This can be problematic since such bacterial pathogens normally harbor thousands of genes. To circumvent this problem, we found that overexpression of bacterial appendages, e.g., fimbriae, capsule, and flagella, could successfully attenuate wild-type (wt) Salmonella enterica serovar Typhimurium. Further analysis revealed these attenuated Salmonella strains conferred protection against wt S. Typhimurium challenge as effectively as genetically defined Salmonella vaccines. We refer to this strategy as attenuating gene expression (AGE), a simple efficient approach in attenuating bacterial pathogens, greatly facilitating the construction of live vaccines.

Development of Burkholderia mallei and pseudomallei vaccines

Burkholderia mallei and Burkholderia pseudomallei are Gram-negative bacteria that cause glanders and melioidosis, respectively. Inhalational infection with either organism can result in severe and rapidly fatal pneumonia. Inoculation by the oral and cutaneous routes can also produce infection. Chronic infection may develop after recovery from acute infection with both agents, and control of infection with antibiotics requires prolonged treatment. Symptoms for both meliodosis and glanders are non-specific, making diagnosis difficult. B. pseudomallei can be located in the environment, but in the host, B. mallei and B. psedomallei are intracellular organisms, and infection results in similar immune responses to both agents. Effective early innate immune responses are critical to controlling the early phase of the infection. Innate immune signaling molecules such as TLR, NOD, MyD88, and pro-inflammatory cytokines such as IFN-γ and TNF-α play key roles in regulating control of infection. Neutrophils and monocytes are critical cells in the early infection for both microorganisms. Both monocytes and macrophages are necessary for limiting dissemination of B. pseudomallei. In contrast, the role of adaptive immune responses in controlling Burkholderia infection is less well understood. However, T cell responses are critical for vaccine protection from Burkholderia infection. At present, effective vaccines for prevention of glanders or meliodosis have not been developed, although recently development of Burkholderia vaccines has received renewed attention. This review will summarize current and past approaches to develop B. mallei and B. pseudomalllei vaccines, with emphasis on immune mechanisms of protection and the challenges facing the field. At present, immunization with live attenuated bacteria provides the most effective and durable immunity, and it is important therefore to understand the immune correlates of protection induced by live attenuated vaccines. Subunit vaccines have typically provided less robust immunity, but are safer to administer to a wider variety of people, including immune compromised individuals because they do not reactivate or cause disease. The challenges facing B. mallei and B. pseudomalllei vaccine development include identification of broadly protective antigens, design of efficient vaccine delivery and adjuvant systems, and a better understanding of the correlates of protection from both acute and chronic infection.

Progress in Brucella vaccine development

Brucella spp. are zoonotic, facultative intracellular pathogens, which cause animal and human disease. Animal disease results in abortion of fetuses; in humans, it manifests flu-like symptoms with an undulant fever, with osteoarthritis as a common complication of infection. Antibiotic regimens for human brucellosis patients may last several months and are not always completely effective. While there are no vaccines for humans, several licensed live Brucella vaccines are available for use in livestock. The performance of these animal vaccines is dependent upon the host species, dose, and route of immunization. Newly engineered live vaccines, lacking well-defined virulence factors, retain low residual virulence, are highly protective, and may someday replace currently used animal vaccines. These also have possible human applications. Moreover, due to their enhanced safety and efficacy in animal models, subunit vaccines for brucellosis show great promise for their application in livestock and humans. This review summarizes the progress of brucellosis vaccine development and presents an overview of candidate vaccines.

Impact of homologous and non-homologous recombination in the genomic evolution of Escherichia coli

Background

Escherichia coli is an important species of bacteria that can live as a harmless inhabitant of the guts of many animals, as a pathogen causing life-threatening conditions or freely in the non-host environment. This diversity of lifestyles has made it a particular focus of interest for studies of genetic variation, mainly with the aim to understand how a commensal can become a deadly pathogen. Many whole genomes of E. coli have been fully sequenced in the past few years, which offer helpful data to help understand how this important species evolved.

Results

We compared 27 whole genomes encompassing four phylogroups of Escherichia coli (A, B1, B2 and E). From the core-genome we established the clonal relationships between the isolates as well as the role played by homologous recombination during their evolution from a common ancestor. We found strong evidence for sexual isolation between three lineages (A+B1, B2, E), which could be explained by the ecological structuring of E. coli and may represent on-going speciation. We identified three hotspots of homologous recombination, one of which had not been previously described and contains the aroC gene, involved in the essential shikimate metabolic pathway. We also described the role played by non-homologous recombination in the pan-genome, and showed that this process was highly heterogeneous. Our analyses revealed in particular that the genomes of three enterohaemorrhagic (EHEC) strains within phylogroup B1 have converged from originally separate backgrounds as a result of both homologous and non-homologous recombination.

Conclusions

Recombination is an important force shaping the genomic evolution and diversification of E. coli, both by replacing fragments of genes with an homologous sequence and also by introducing new genes. In this study, several non-random patterns of these events were identified which correlated with important changes in the lifestyle of the bacteria, and therefore provide additional evidence to explain the relationship between genomic variation and ecological adaptation.

Internal affairs: investigating the Brucella intracellular lifestyle

Bacteria of the genus Brucella are Gram-negative pathogens of several animal species that cause a zoonotic disease in humans known as brucellosis or Malta fever. Within their hosts, brucellae reside within different cell types where they establish a replicative niche and remain protected from the immune response. The aim of this article is to discuss recent advances in the field in the specific context of the Brucella intracellular 'lifestyle'. We initially discuss the different host cell targets and their relevance during infection. As it represents the key to intracellular replication, the focus is then set on the maturation of the Brucella phagosome, with particular emphasis on the Brucella factors that are directly implicated in intracellular trafficking and modulation of host cell signalling pathways. Recent data on the role of the type IV secretion system are discussed, novel effector molecules identified and how some of them impact on trafficking events. Current knowledge on Brucella gene regulation and control of host cell death are summarized, as they directly affect intracellular persistence. Understanding how Brucella molecules interplay with their host cell targets to modulate cellular functions and establish the intracellular niche will help unravel how this pathogen causes disease.

The genome of a pathogenic rhodococcus: cooptive virulence underpinned by key gene acquisitions

We report the genome of the facultative intracellular parasite Rhodococcus equi, the only animal pathogen within the biotechnologically important actinobacterial genus Rhodococcus. The 5.0-Mb R. equi 103S genome is significantly smaller than those of environmental rhodococci. This is due to genome expansion in nonpathogenic species, via a linear gain of paralogous genes and an accelerated genetic flux, rather than reductive evolution in R. equi. The 103S genome lacks the extensive catabolic and secondary metabolic complement of environmental rhodococci, and it displays unique adaptations for host colonization and competition in the short-chain fatty acid–rich intestine and manure of herbivores—two main R. equi reservoirs. Except for a few horizontally acquired (HGT) pathogenicity loci, including a cytoadhesive pilus determinant (rpl) and the virulence plasmid vap pathogenicity island (PAI) required for intramacrophage survival, most of the potential virulence-associated genes identified in R. equi are conserved in environmental rhodococci or have homologs in nonpathogenic Actinobacteria. This suggests a mechanism of virulence evolution based on the cooption of existing core actinobacterial traits, triggered by key host niche–adaptive HGT events. We tested this hypothesis by investigating R. equi virulence plasmid-chromosome crosstalk, by global transcription profiling and expression network analysis. Two chromosomal genes conserved in environmental rhodococci, encoding putative chorismate mutase and anthranilate synthase enzymes involved in aromatic amino acid biosynthesis, were strongly coregulated with vap PAI virulence genes and required for optimal proliferation in macrophages. The regulatory integration of chromosomal metabolic genes under the control of the HGT–acquired plasmid PAI is thus an important element in the cooptive virulence of R. equi.

Rhodococcus is a prototypic genus within the Actinobacteria, one of the largest microbial groups on Earth. Many of the ubiquitous rhodococcal species are biotechnologically useful due to their metabolic versatility and biodegradative properties. We have deciphered the genome of a facultatively parasitic Rhodococcus, the animal and human pathogen R. equi. Comparative genomic analyses of related species provide a unique opportunity to increase our understanding of niche-adaptive genome evolution and specialization. The environmental rhodococci have much larger genomes, richer in metabolic and degradative pathways, due to gene duplication and acquisition, not genome contraction in R. equi. This probably reflects that the host-associated R. equi habitat is more stable and favorable than the chemically diverse but nutrient-poor environmental niches of nonpathogenic rhodococci, necessitating metabolically more complex, expanded genomes. Our work also highlights that the recruitment or cooption of core microbial traits, following the horizontal acquistion of a few critical genes that provide access to the host niche, is an important mechanism in actinobacterial virulence evolution. Gene cooption is a key evolutionary mechanism allowing rapid adaptive change and novel trait acquisition. Recognizing the contribution of cooption to virulence provides a rational framework for understanding and interpreting the emergence and evolution of microbial pathogenicity.

Health considerations regarding horizontal transfer of microbial transgenes present in genetically modified crops

The potential effects of horizontal gene transfer on human health are an important item in the safety assessment of genetically modified organisms. Horizontal gene transfer from genetically modified crops to gut microflora most likely occurs with transgenes of microbial origin. The characteristics of microbial transgenes other than antibiotic-resistance genes in market-approved genetically modified crops are reviewed. These characteristics include the microbial source, natural function, function in genetically modified crops, natural prevalence, geographical distribution, similarity to other microbial genes, known horizontal transfer activity, selective conditions and environments for horizontally transferred genes, and potential contribution to pathogenicity and virulence in humans and animals. The assessment of this set of data for each of the microbial genes reviewed does not give rise to health concerns. We recommend including the above-mentioned items into the premarket safety assessment of genetically modified crops carrying transgenes other than those reviewed in the present study.

Survival of the fittest: how Brucella strains adapt to their intracellular niche in the host

Brucella strains produce abortion and infertility in their natural hosts and a zoonotic disease in humans known as undulant fever. These bacteria do not produce classical virulence factors, and their capacity to successfully survive and replicate within a variety of host cells underlies their pathogenicity. Extensive replication of the brucellae in placental trophoblasts is associated with reproductive tract pathology in natural hosts, and prolonged persistence in macrophages leads to the chronic infections that are a hallmark of brucellosis in both natural hosts and humans. This review describes how Brucella strains have efficiently adapted to their intracellular lifestyle in the host.

Targeting of the Brucella suis virulence factor histidinol dehydrogenase by histidinol analogues results in inhibition of intramacrophagic multiplication of the pathogen

Brucella suis histidinol dehydrogenase (HDH) can be efficiently targeted by substrate analogues. The growth of this pathogen in minimal medium was inhibited and the multiplication in human macrophages was totally abolished in the presence of the drugs. These effects have been shown to be correlated with the previously described inhibition of Brucella HDH activity.

Deletion of znuA virulence factor attenuates Brucella abortus and confers protection against wild-type challenge

znuA is known to be an important factor for survival and normal growth under low Zn(2+) concentrations for Escherichia coli, Haemophilus spp., Neisseria gonorrhoeae, and Pasteurella multocida. We hypothesized that the znuA gene present in Brucella melitensis 16 M would be similar to znuA in B. abortus and questioned whether it may also be an important factor for growth and virulence of Brucella abortus. Using the B. melitensis 16 M genome sequence, primers were designed to construct a B. abortus deletion mutant. A znuA knockout mutation in B. abortus 2308 (DeltaznuA) was constructed and found to be lethal in low-Zn(2+) medium. When used to infect macrophages, DeltaznuA B. abortus showed minimal growth. Further study with DeltaznuA B. abortus showed that its virulence in BALB/c mice was attenuated, and most of the bacteria were cleared from the spleen within 8 weeks. Protection studies confirmed the DeltaznuA mutant as a potential live vaccine, since protection against wild-type B. abortus 2308 challenge was as effective as that obtained with the RB51 or S19 vaccine strain.

Identification of a new virulence factor, BvfA, in Brucella suis

We report the identification of BvfA (for Brucella virulence factor A), a small periplasmic protein unique to the genus Brucella, which is essential for the virulence of Brucella suis. A BvfA knockout mutant was highly attenuated both in in vitro macrophage infection assays and in vivo in the murine model of brucellosis. Fluorescence-activated cell sorting analysis with green fluorescent protein fusions showed that the expression of bvfA is induced within macrophages by phagosome acidification and coregulated with the B. suis virB operon, suggesting that it too may play a role in the establishment of the intracellular replication niche.

Intact purine biosynthesis pathways are required for wild-type virulence of Brucella abortus 2308 in the BALB/c mouse model

Brucella abortus 2308 derivatives with mini-Tn5 insertions in purE, purL, and purD display significant attenuation in the BALB/c mouse model, while isogenic mutants with mini-Tn5 insertions in pheA, trpB, and dagA display little or no attenuation in cultured murine macrophages or mice. These experimental findings confirm the importance of the purine biosynthesis pathways for the survival and replication of the brucellae in host macrophages. In contrast to previous reports, however, these results indicate that exogenous tryptophan and phenylalanine are available for use by the brucellae in the phagosomal compartment.

Adaptation of the Brucellae to their intracellular niche

Members of the bacterial genus Brucella are facultative intracellular pathogens that reside predominantly within membrane-bound compartments within two host cell types, macrophages and placental trophoblasts. Within macrophages, the brucellae route themselves to an intracellular compartment that is favourable for survival and replication, and they also appear to be well-adapted from a physiological standpoint to withstand the environmental conditions encountered during prolonged residence in this intracellular niche. Much less is known about the interactions of the Brucella with placental trophoblasts, but experimental evidence suggests that these bacteria use an iron acquisition system to support extensive intracellular replication within these host cells that is not required for survival and replication in host macrophages. Thus, it appears that the brucellae rely upon the products of distinct subsets of genes to adapt successfully to the environmental conditions encountered within the two cell types within which they reside in their mammalian hosts.

[Strategies for bacterial virulence genes identification]

One of the recent preoccupations of medical microbiology has been to characterise the mechanisms of virulence of bacterial pathogens at the molecular level. One hundred years after Koch, Stanley Falkow proposed a new, molecular version of Koch's postulates to define avirulence gene: (a) the gene confers a certain phenotype to the studied bacteria, (b) inactivation of the gene abolishes the phenotype, (c)reintroduction of the gene restores the wild type to the mutant. Although this strategy, based upon mutagenesis and the use of experimental models, allows the identification of many genes, it is not comprehensive. Other methods can be used to complete the identification of virulence factors such as differential expression, either at the level of transcription (transcriptome) or at the level of protein expression (proteome). All these techniques are now supported by the data from complete genome sequencing projects. The pool of information obtained from these approaches allows the definition of the 'virulome', which is the assembly of factors a pathogen requires for virulence. Understanding the virulome will open the way to the development of new strategies for vaccination or the development of new generation of antimicrobials.

Brucella stationary-phase gene expression and virulence

The capacity of the Brucella spp. to establish and maintain long-term residence in the phagosomal compartment of host macrophages is critical to their ability to produce chronic infections in their mammalian hosts. The RNA binding protein host factor I (HF-I) encoded by the hfq gene is required for the efficient translation of the stationary-phase sigma factor RpoS in many bacteria, and a Brucella abortus hfq mutant displays a phenotype in vitro, which suggests that it has a generalized defect in stationary-phase physiology. The inability of the B. abortus hfq mutant to survive and replicate in a wild-type manner in cultured murine macrophages, and the profound attenuation displayed by this strain and its B. melitensis counterpart in experimentally infected animals indicate that stationary-phase physiology plays an essential role in the capacity of the brucellae to establish and maintain long-term intracellular residence in host macrophages. The nature of the Brucella HF-I-regulated genes that have been identified to date suggests that the corresponding gene products contribute to the remarkable capacity of the brucellae to resist the harsh environmental conditions they encounter during their prolonged residence in the phagosomal compartment.

Brucellapathogenesis, genes identified from random large-scale screens

Pathogenicity islands, specialized secretion systems, virulence plasmids, fimbriae, pili, adhesins, and toxins are all classical bacterial virulence factors. However, many of these factors, though widespread among bacterial pathogens, are not necessarily found among bacteria that colonize eukaryotic cells in a pathogenic/symbiotic relationship. Bacteria that form these relationships have developed other strategies to infect and grow in their hosts. This is particularly true for Brucella and other members of the class Proteobacteria. Thus far the identification of virulence factors for Brucella has been largely dependent on large-scale screens and testing in model systems. The genomes of the facultative intracellular pathogens Brucella melitensis and Brucella suis were sequenced recently. This has identified several more potential virulence factors for Brucella that were not found in large screens. Here, we present an overall view of Brucella virulence by compiling virulence data from the study of 184 attenuated mutants.

Molecular host-pathogen interaction in brucellosis: current understanding and future approaches to vaccine development for mice and humans

Brucellosis caused by Brucella spp. is a major zoonotic disease. Control of brucellosis in agricultural animals is a prerequisite for the prevention of this disease in human beings. Recently, Brucella melitensis was declared by the Centers for Disease Control and Prevention to be one of three major bioterrorist agents due to the expense required for the treatment of human brucellosis patients. Also, the economic agricultural loss due to bovine brucellosis emphasizes the financial impact of brucellosis in society. Thus, vaccination might efficiently solve this disease. Currently, B. abortus RB51 and B. melitensis REV.1 are used to immunize cattle and to immunize goats and sheep, respectively, in many countries. However, these genetically undefined strains still induce abortion and persistent infection, raising questions of safety and efficiency. In fact, the REV.1 vaccine is quite virulent and apparently unstable, creating the need for improved vaccines for B. melitensis. In addition, Brucella spp. may or may not provide cross-protection against infection by heterologous Brucella species, hampering the acceleration of vaccine development. This review provides our current understanding of Brucella pathogenesis and host immunity for the development of genetically defined efficient vaccine strains. Additionally, conditions required for an effective Brucella vaccine strain as well as the future research direction needed to investigate Brucella pathogenesis and host immunity are postulated.

Seeking a niche: putative contributions of the hfq and bacA gene products to the successful adaptation of the brucellae to their intracellular home

Long-term residence of the brucellae in the phagosomal compartment of host macrophages is essential to their ability to produce disease in both natural and experimental hosts. Correspondingly, the Brucella spp. appear to be well adapted to resist the multiple environmental stresses they encounter in their intracellular home. This brief review will focus on the contributions of the hfq and bacA gene products to this adaptation. Studies with Brucella hfq mutants suggest that stationary phase physiology is critical for successful long-term residence in host macrophages. Analysis of Brucella bacA mutants, on the other hand, reveal very striking parallels between the strategies employed by the rhizobia to establish and maintain protracted intracellular residence in their plant host and those used by the brucellae during their long-term survival in the phagosomal compartment of host macrophages.

The analysis of the intramacrophagic virulome of Brucella suis deciphers the environment encountered by the pathogen inside the macrophage host cell

The pathogen Brucella suis resides and multiplies within a phagocytic vacuole of its host cell, the macrophage. The resulting complex relationship has been investigated by the analysis of the set of genes required for virulence, which we call intramacrophagic virulome. Ten thousand two hundred and seventy-two miniTn5 mutants of B. suis constitutively expressing gfp were screened by fluorescence microscopy for lack of intracellular multiplication in human macrophages. One hundred thirty-one such mutants affected in 59 different genes could be isolated, and a function was ascribed to 53 of them. We identified genes involved in (i) global adaptation to the intracellular environment, (ii) amino acid, and (iii) nucleotide synthesis, (iv) sugar metabolism, (v) oxidoreduction, (vi) nitrogen metabolism, (vii) regulation, (viii) disulphide bond formation, and (ix) lipopolysaccharide biosynthesis. Results led to the conclusion that the replicative compartment of B. suis is poor in nutrients and characterized by low oxygen tension, and that nitrate may be used for anaerobic respiration. Intramacrophagic virulome analysis hence allowed the description of the nature of the replicative vacuole of the pathogen in the macrophage and extended our understanding of the niche in which B. suis resides. We propose calling this specific compartment "brucellosome."

Ferrochelatase is present in Brucella abortus and is critical for its intracellular survival and virulence

Brucella spp. are pathogenic bacteria that cause brucellosis, an animal disease which can also affect humans. Although understanding the pathogenesis is important for the health of animals and humans, little is known about virulence factors associated with it. In order for chronic disease to be established, Brucella spp. have developed the ability to survive inside phagocytes by evading cell defenses. It hides inside vacuoles, where it then replicates, indicating that it has an active metabolism. The purpose of this work was to obtain better insight into the intracellular metabolism of Brucella abortus. During a B. abortus genomic sequencing project, a clone coding a putative gene homologous to hemH was identified and sequenced. The amino acid sequence revealed high homology to members of the ferrochelatase family. A knockout mutant displayed auxotrophy for hemin, defective intracellular survival inside J774 and HeLa cells, and lack of virulence in BALB/c mice. This phenotype was overcome by complementing the mutant strain with a plasmid harboring wild-type hemH. These data demonstrate that B. abortus synthesizes its own heme and also has the ability to use an external source of heme; however, inside cells, there is not enough available heme to support its intracellular metabolism. It is concluded that ferrochelatase is essential for the multiplication and intracellular survival of B. abortus and thus for the establishment of chronic disease as well.