The Brucella suis virB operon is induced intracellularly in macrophages

RegA Plays a Key Role in Oxygen-Dependent Establishment of Persistence and in Isocitrate Lyase Activity, a Critical Determinant of In vivo Brucella suis Pathogenicity

For aerobic human pathogens, adaptation to hypoxia is a critical factor for the establishment of persistent infections, as oxygen availability is low inside the host. The two-component system RegB/A of Brucella suis plays a central role in the control of respiratory systems adapted to oxygen deficiency, and in persistence in vivo. Using an original "in vitro model of persistence" consisting in gradual oxygen depletion, we compared transcriptomes and proteomes of wild-type and ΔregA strains to identify the RegA-regulon potentially involved in the set-up of persistence. Consecutive to oxygen consumption resulting in growth arrest, 12% of the genes in B. suis were potentially controlled directly or indirectly by RegA, among which numerous transcriptional regulators were up-regulated. In contrast, genes or proteins involved in envelope biogenesis and in cellular division were repressed, suggesting a possible role for RegA in the set-up of a non-proliferative persistence state. Importantly, the greatest number of the RegA-repressed genes and proteins, including aceA encoding the functional IsoCitrate Lyase (ICL), were involved in energy production. A potential consequence of this RegA impact may be the slowing-down of the central metabolism as B. suis progressively enters into persistence. Moreover, ICL is an essential determinant of pathogenesis and long-term interactions with the host, as demonstrated by the strict dependence of B. suis on ICL activity for multiplication and persistence during in vivo infection. RegA regulates gene or protein expression of all functional groups, which is why RegA is a key regulator of B. suis in adaptation to oxygen depletion. This function may contribute to the constraint of bacterial growth, typical of chronic infection. Oxygen-dependent activation of two-component systems that control persistence regulons, shared by several aerobic human pathogens, has not been studied in Brucella sp. before. This work therefore contributes significantly to the unraveling of persistence mechanisms in this important zoonotic pathogen.

Promotion and Rescue of Intracellular Brucella neotomae Replication during Coinfection with Legionella pneumophila

We established a new Brucella neotomaein vitro model system for study of type IV secretion system-dependent (T4SS) pathogenesis in the Brucella genus. Importantly, B. neotomae is a rodent pathogen, and unlike B. abortus, B. melitensis, and B. suis, B. neotomae has not been observed to infect humans. It therefore can be handled more facilely using biosafety level 2 practices. More particularly, using a series of novel fluorescent protein and lux operon reporter systems to differentially label pathogens and track intracellular replication, we confirmed T4SS-dependent intracellular growth of B. neotomae in macrophage cell lines. Furthermore, B. neotomae exhibited early endosomal (LAMP-1) and late endoplasmic reticulum (calreticulin)-associated phagosome maturation. These findings recapitulate prior observations for human-pathogenic Brucella spp. In addition, during coinfection experiments with Legionella pneumophila, we found that defective intracellular replication of a B. neotomae T4SS virB4 mutant was rescued and baseline levels of intracellular replication of wild-type B. neotomae were significantly stimulated by coinfection with wild-type but not T4SS mutant L. pneumophila Using confocal microscopy, it was determined that intracellular colocalization of B. neotomae and L. pneumophila was required for rescue and that colocalization came at a cost to L. pneumophila fitness. These findings were not completely expected based on known temporal and qualitative differences in the intracellular life cycles of these two pathogens. Taken together, we have developed a new system for studying in vitroBrucella pathogenesis and found a remarkable T4SS-dependent interplay between Brucella and Legionella during macrophage coinfection.

Postreplication Roles of the Brucella VirB Type IV Secretion System Uncovered via Conditional Expression of the VirB11 ATPase

Brucella abortus, the bacterial agent of the worldwide zoonosis brucellosis, primarily infects host phagocytes, where it undergoes an intracellular cycle within a dedicated membrane-bound vacuole, the Brucella-containing vacuole (BCV). Initially of endosomal origin (eBCV), BCVs are remodeled into replication-permissive organelles (rBCV) derived from the host endoplasmic reticulum, a process that requires modulation of host secretory functions via delivery of effector proteins by the Brucella VirB type IV secretion system (T4SS). Following replication, rBCVs are converted into autophagic vacuoles (aBCVs) that facilitate bacterial egress and subsequent infections, arguing that the bacterium sequentially manipulates multiple cellular pathways to complete its cycle. The VirB T4SS is essential for rBCV biogenesis, as VirB-deficient mutants are stalled in eBCVs and cannot mediate rBCV biogenesis. This has precluded analysis of whether the VirB apparatus also drives subsequent stages of the Brucella intracellular cycle. To address this issue, we have generated a B. abortus strain in which VirB T4SS function is conditionally controlled via anhydrotetracycline (ATc)-dependent complementation of a deletion of the virB11 gene encoding the VirB11 ATPase. We show in murine bone marrow-derived macrophages (BMMs) that early VirB production is essential for optimal rBCV biogenesis and bacterial replication. Transient expression of virB11 prior to infection was sufficient to mediate normal rBCV biogenesis and bacterial replication but led to T4SS inactivation and decreased aBCV formation and bacterial release, indicating that these postreplication stages are also T4SS dependent. Hence, our findings support the hypothesis of additional, postreplication roles of type IV secretion in the Brucella intracellular cycle.

Many intracellular bacterial pathogens encode specialized secretion systems that deliver effector proteins into host cells to mediate the multiple stages of their intracellular cycles. Because these intracellular events occur sequentially, classical genetic approaches cannot address the late roles that these apparatuses play, as secretion-deficient mutants cannot proceed past their initial defect. Here we have designed a functionally controllable VirB type IV secretion system (T4SS) in the bacterial pathogen Brucella abortus to decipher its temporal requirements during the bacterium’s intracellular cycle in macrophages. By controlling production of the VirB11 ATPase, which energizes the T4SS, we show not only that this apparatus is required early to generate the Brucella replicative organelle but also that it contributes to completion of the bacterium’s cycle and bacterial egress. Our findings expand upon the pathogenic functions of the Brucella VirB T4SS and illustrate targeting of secretion ATPases as a useful strategy to manipulate the activity of bacterial secretion systems.

Identification of OtpR regulated sRNAs in Brucella melitensis expressed under acidic stress and their roles in pathogenesis and metabolism

Small RNAs (sRNAs) are the small regulatory molecules that regulate various biological processes in bacteria. Though the functions of sRNAs are well documented, very little information is available on the sRNAs of Brucella spp. The otpR is the response regulator of a two-component regulatory system, which plays a significant role in Brucella virulence. In this study, we identified the sRNAs expressed in B. melitensis 16M and its otpR mutant under acidic stress from the RNAseq dataset. We identified 94 trans-encoded and 948 cis-encoded sRNAs in B. melitensis 16M. In B. melitensis 16M ΔotpR under acidic stress 99 trans-encoded and 877 cis-encoded sRNAs were identified. Among these, 12 trans-encoded and 43 cis-encoded sRNAs were upregulated in B. melitensis 16M ΔotpR, with an adjusted P-value≤0.05. The mRNA targets of these sRNAs were predicted. Further, the levels of mRNA targets were examined, and the sRNA-mediated regulatory network was predicted. Functional classification and pathway analysis of mRNA targets provided evidence that sRNAs are involved in different metabolic pathways including carbohydrates, amino acids, lipids, nucleotides transport and metabolism, cell membrane biogenesis and intracellular trafficking of Brucella. We also found that eight transcriptional regulators including a quorum sensing regulator, vjbR are down-regulated by sRNAs. These transcriptional regulators might be responsible for the regulation of several other genes in the otpR mutant. The trans-encoded BsnR88 and cis-encoded BsnR980, BsnR998, BsnR881, BsnR1001, BsnR891, BsnR883, BsnR905 are regulating virB operon genes coding for type IV secretion system (T4SS), which is the major virulence factor of Brucella.

Microscopy-based Assays for High-throughput Screening of Host Factors Involved in Brucella Infection of Hela Cells

Brucella species are facultative intracellular pathogens that infect animals as their natural hosts. Transmission to humans is most commonly caused by direct contact with infected animals or by ingestion of contaminated food and can lead to severe chronic infections. Brucella can invade professional and non-professional phagocytic cells and replicates within endoplasmic reticulum (ER)-derived vacuoles. The host factors required for Brucella entry into host cells, avoidance of lysosomal degradation, and replication in the ER-like compartment remain largely unknown. Here we describe two assays to identify host factors involved in Brucella entry and replication in HeLa cells. The protocols describe the use of RNA interference, while alternative screening methods could be applied. The assays are based on the detection of fluorescently labeled bacteria in fluorescently labeled host cells using automated wide-field microscopy. The fluorescent images are analyzed using a standardized image analysis pipeline in CellProfiler which allows single cell-based infection scoring. In the endpoint assay, intracellular replication is measured two days after infection. This allows bacteria to traffic to their replicative niche where proliferation is initiated around 12 hr after bacterial entry. Brucella which have successfully established an intracellular niche will thus have strongly proliferated inside host cells. Since intracellular bacteria will greatly outnumber individual extracellular or intracellular non-replicative bacteria, a strain constitutively expressing GFP can be used. The strong GFP signal is then used to identify infected cells. In contrast, for the entry assay it is essential to differentiate between intracellular and extracellular bacteria. Here, a strain encoding for a tetracycline-inducible GFP is used. Induction of GFP with simultaneous inactivation of extracellular bacteria by gentamicin enables the differentiation between intracellular and extracellular bacteria based on the GFP signal, with only intracellular bacteria being able to express GFP. This allows the robust detection of single intracellular bacteria before intracellular proliferation is initiated.

Mechanism and Function of Type IV Secretion During Infection of the Human Host

Bacterial pathogens employ type IV secretion systems (T4SSs) for various purposes to aid in survival and proliferation in eukaryotic hosts. One large T4SS subfamily, the conjugation systems, confers a selective advantage to the invading pathogen in clinical settings through dissemination of antibiotic resistance genes and virulence traits. Besides their intrinsic importance as principle contributors to the emergence of multiply drug-resistant "superbugs," detailed studies of these highly tractable systems have generated important new insights into the mode of action and architectures of paradigmatic T4SSs as a foundation for future efforts aimed at suppressing T4SS machine function. Over the past decade, extensive work on the second large T4SS subfamily, the effector translocators, has identified a myriad of mechanisms employed by pathogens to subvert, subdue, or bypass cellular processes and signaling pathways of the host cell. An overarching theme in the evolution of many effectors is that of molecular mimicry. These effectors carry domains similar to those of eukaryotic proteins and exert their effects through stealthy interdigitation of cellular pathways, often with the outcome not of inducing irreversible cell damage but rather of reversibly modulating cellular functions. This article summarizes the major developments for the actively studied pathogens with an emphasis on the structural and functional diversity of the T4SSs and the emerging common themes surrounding effector function in the human host.

Establishment of Chronic Infection: Brucella's Stealth Strategy

Brucella is a facultative intracellular pathogen that causes zoonotic infection known as brucellosis which results in abortion and infertility in natural host. Humans, especially in low income countries, can acquire infection by direct contact with infected animal or by consumption of animal products and show high morbidity, severe economic losses and public health problems. However for survival, host cells develop complex immune mechanisms to defeat and battle against attacking pathogens and maintain a balance between host resistance and Brucella virulence. On the other hand as a successful intracellular pathogen, Brucella has evolved multiple strategies to evade immune response mechanisms to establish persistent infection and replication within host. In this review, we mainly summarize the "Stealth" strategies employed by Brucella to modulate innate and the adaptive immune systems, autophagy, apoptosis and possible role of small noncoding RNA in the establishment of chronic infection. The purpose of this review is to give an overview for recent understanding how this pathogen evades immune response mechanisms of host, which will facilitate to understanding the pathogenesis of brucellosis and the development of novel, more effective therapeutic approaches to treat brucellosis.

Staphylococcus aureus Strain USA300 Perturbs Acquisition of Lysosomal Enzymes and Requires Phagosomal Acidification for Survival inside Macrophages

Methicillin-resistant Staphylococcus aureus (MRSA) causes invasive, drug-resistant skin and soft tissue infections. Reports that S. aureus bacteria survive inside macrophages suggest that the intramacrophage environment may be a niche for persistent infection; however, mechanisms by which the bacteria might evade macrophage phagosomal defenses are unclear. We examined the fate of the S. aureus-containing phagosome in THP-1 macrophages by evaluating bacterial intracellular survival and phagosomal acidification and maturation and by testing the impact of phagosomal conditions on bacterial viability. Multiple strains of S. aureus survived inside macrophages, and in studies using the MRSA USA300 clone, the USA300-containing phagosome acidified rapidly and acquired the late endosome and lysosome protein LAMP1. However, fewer phagosomes containing live USA300 bacteria than those containing dead bacteria associated with the lysosomal hydrolases cathepsin D and β-glucuronidase. Inhibiting lysosomal hydrolase activity had no impact on intracellular survival of USA300 or other S. aureus strains, suggesting that S. aureus perturbs acquisition of lysosomal enzymes. We examined the impact of acidification on S. aureus intramacrophage viability and found that inhibitors of phagosomal acidification significantly impaired USA300 intracellular survival. Inhibition of macrophage phagosomal acidification resulted in a 30-fold reduction in USA300 expression of the staphylococcal virulence regulator agr but had little effect on expression of sarA, saeR, or sigB. Bacterial exposure to acidic pH in vitro increased agr expression. Together, these results suggest that S. aureus survives inside macrophages by perturbing normal phagolysosome formation and that USA300 may sense phagosomal conditions and upregulate expression of a key virulence regulator that enables its intracellular survival.

Brucella abortus Cell Cycle and Infection Are Coordinated

Brucellae are facultative intracellular pathogens. The recent development of methods and genetically engineered strains allowed the description of cell-cycle progression of Brucella abortus, including unipolar growth and the ordered initiation of chromosomal replication. B. abortus cell-cycle progression is coordinated with intracellular trafficking in the endosomal compartments. Bacteria are first blocked at the G1 stage, growth and chromosome replication being resumed shortly before reaching the intracellular proliferation compartment. The control mechanisms of cell cycle are similar to those reported for the bacterium Caulobacter crescentus, and they are crucial for survival in the host cell. The development of single-cell analyses could also be applied to other bacterial pathogens to investigate their cell-cycle progression during infection.

Type IV secretion system of Brucella spp. and its effectors

Brucella spp. are intracellular bacterial pathogens that cause infection in domestic and wild animals. They are often used as model organisms to study intracellular bacterial infections. Brucella VirB T4SS is a key virulence factor that plays important roles in mediating intracellular survival and manipulating host immune response to infection. In this review, we discuss the roles of Brucella VirB T4SS and 15 effectors that are proposed to be crucial for Brucella pathogenesis. VirB T4SS regulates the inflammation response and manipulates vesicle trafficking inside host cells. VirB T4SS also plays crucial roles in the inhibition of the host immune response and intracellular survival during infection. Here, we list the key molecular events in the intracellular life cycle of Brucella that are potentially targeted by the VirB T4SS effectors. Elucidating the functions of these effectors will help clarify the molecular role of T4SS during infection. Furthermore, studying the effectors secreted by Brucella spp. might provide insights into the mechanisms used by the bacteria to hijack the host signaling pathways and aid in the development of better vaccines and therapies against brucellosis.

RNA-seq reveals the critical role of OtpR in regulating Brucella melitensis metabolism and virulence under acidic stress

The response regulator OtpR is critical for the growth, morphology and virulence of Brucella melitensis. Compared to its wild type strain 16 M, B. melitensis 16 MΔotpR mutant has decreased tolerance to acid stress. To analyze the genes regulated by OtpR under acid stress, we performed RNA-seq whole transcriptome analysis of 16 MΔotpR and 16 M. In total, 501 differentially expressed genes were identified, including 390 down-regulated and 111 up-regulated genes. Among these genes, 209 were associated with bacterial metabolism, including 54 genes involving carbohydrate metabolism, 13 genes associated with nitrogen metabolism, and seven genes associated with iron metabolism. The 16 MΔotpR also decreased capacity to utilize different carbon sources and to tolerate iron limitation in culture experiments. Notably, OtpR regulated many Brucella virulence factors essential for B. melitensis intracellular survival. For instance, the virB operon encoding type IV secretion system was significantly down-regulated, and 36 known transcriptional regulators (e.g., vjbR and blxR) were differentially expressed in 16 MΔotpR. Selected RNA-seq results were experimentally confirmed by RT-PCR and RT-qPCR. Overall, these results deciphered differential phenomena associated with virulence, environmental stresses and cell morphology in 16 MΔotpR and 16 M, which provided important information for understanding the detailed OtpR-regulated interaction networks and Brucella pathogenesis.

The changing nature of the Brucella-containing vacuole

Bacteria of the genus Brucella are intracellular vacuolar pathogens of mammals that cause the worldwide zoonosis brucellosis, and reside within phagocytes of infected hosts to promote their survival, persistence and proliferation. These traits are essential to the bacterium's ability to cause disease and have been the subject of much investigation to gain an understanding of Brucella pathogenic mechanisms. Although the endoplasmic reticulum-derived nature of the Brucella replicative niche has been long known, major strides have recently been made in deciphering the molecular mechanisms of its biogenesis, including the identification of bacterial determinants and host cellular pathways involved in this process. Here I will review and discuss the most recent advances in our knowledge of Brucella intracellular pathogenesis, with an emphasis on bacterial exploitation of the host endoplasmic reticulum-associated functions, and how autophagy-related processes contribute to the bacterium's intracellular cycle.

Immunization with Brucella VirB proteins reduces organ colonization in mice through a Th1-type immune response and elicits a similar immune response in dogs

VirB proteins from Brucella spp. constitute the type IV secretion system, a key virulence factor mediating the intracellular survival of these bacteria. Here, we assessed whether a Th1-type immune response against VirB proteins may protect mice from Brucella infection and whether this response can be induced in the dog, a natural host for Brucella. Splenocytes from mice immunized with VirB7 or VirB9 responded to their respective antigens with significant and specific production of gamma interferon (IFN-γ), whereas interleukin-4 (IL-4) was not detected. Thirty days after an intraperitoneal challenge with live Brucella abortus, the spleen load of bacteria was almost 1 log lower in mice immunized with VirB proteins than in unvaccinated animals. As colonization reduction seemed to correlate with a Th1-type immune response against VirB proteins, we decided to assess whether such a response could be elicited in the dog. Peripheral blood mononuclear cells (PBMCs) from dogs immunized with VirB proteins (three subcutaneous doses in QuilA adjuvant) produced significantly higher levels of IFN-γ than cells from control animals upon in vitro stimulation with VirB proteins. A skin test to assess specific delayed-type hypersensitivity was positive in 4 out of 5 dogs immunized with either VirB7 or VirB9. As both proteins are predicted to locate in the outer membrane of Brucella organisms, the ability of anti-VirB antibodies to mediate complement-dependent bacteriolysis of B. canis was assessed in vitro. Sera from dogs immunized with either VirB7 or VirB9, but not from those receiving phosphate-buffered saline (PBS), produced significant bacteriolysis. These results suggest that VirB-specific responses that reduce organ colonization by Brucella in mice can be also elicited in dogs.

Bacteria, the endoplasmic reticulum and the unfolded protein response: friends or foes?

The unfolded protein response (UPR) is a cytoprotective response that is aimed at restoring cellular homeostasis following physiological stress exerted on the endoplasmic reticulum (ER), which also invokes innate immune signalling in response to invading microorganisms. Although it has been known for some time that the UPR is modulated by various viruses, recent evidence indicates that it also has multiple roles during bacterial infections. In this Review, we describe how bacteria interact with the ER, including how bacteria induce the UPR, how subversion of the UPR promotes bacterial proliferation and how the UPR contributes to innate immune responses against invading bacteria.

The predicted ABC transporter AbcEDCBA is required for type IV secretion system expression and lysosomal evasion by Brucella ovis

Brucella ovis is a major cause of reproductive failure in rams and it is one of the few well-described Brucella species that is not zoonotic. Previous work showed that a B. ovis mutant lacking a species-specific ABC transporter (ΔabcBA) was attenuated in mice and was unable to survive in macrophages. The aim of this study was to evaluate the role of this ABC transporter during intracellular survival of B. ovis. In HeLa cells, B. ovis WT was able to survive and replicate at later time point (48 hpi), whereas an ΔabcBA mutant was attenuated at 24 hpi. The reduced survival of the ΔabcBA mutant was associated with a decreased ability to exclude the lysosomal marker LAMP1 from its vacuolar membrane, suggesting a failure to establish a replicative niche. The ΔabcBA mutant showed a reduced abundance of the Type IV secretion system (T4SS) proteins VirB8 and VirB11 in both rich and acid media, when compared to WT B. ovis. However, mRNA levels of virB1, virB8, hutC, and vjbR were similar in both strains. These results support the notion that the ABC transporter encoded by abcEDCBA or its transported substrate acts at a post-transcriptional level to promote the optimal expression of the B. ovis T4SS within infected host cells.

Secretome of obligate intracellular Rickettsia

The genus Rickettsia (Alphaproteobacteria, Rickettsiales, Rickettsiaceae) is comprised of obligate intracellular parasites, with virulent species of interest both as causes of emerging infectious diseases and for their potential deployment as bioterrorism agents. Currently, there are no effective commercially available vaccines, with treatment limited primarily to tetracycline antibiotics, although others (e.g. josamycin, ciprofloxacin, chloramphenicol, and azithromycin) are also effective. Much of the recent research geared toward understanding mechanisms underlying rickettsial pathogenicity has centered on characterization of secreted proteins that directly engage eukaryotic cells. Herein, we review all aspects of the Rickettsia secretome, including six secretion systems, 19 characterized secretory proteins, and potential moonlighting proteins identified on surfaces of multiple Rickettsia species. Employing bioinformatics and phylogenomics, we present novel structural and functional insight on each secretion system. Unexpectedly, our investigation revealed that the majority of characterized secretory proteins have not been assigned to their cognate secretion pathways. Furthermore, for most secretion pathways, the requisite signal sequences mediating translocation are poorly understood. As a blueprint for all known routes of protein translocation into host cells, this resource will assist research aimed at uniting characterized secreted proteins with their apposite secretion pathways. Furthermore, our work will help in the identification of novel secreted proteins involved in rickettsial 'life on the inside'.

Early transcriptional responses of bovine chorioallantoic membrane explants to wild type, ΔvirB2 or ΔbtpB Brucella abortus infection

The pathogenesis of the Brucella-induced inflammatory response in the bovine placenta is not completely understood. In this study we evaluated the role of the B. abortus Type IV secretion system and the anti-inflammatory factor BtpB in early interactions with bovine placental tissues. Transcription profiles of chorioallantoic membrane (CAM) explants inoculated with wild type (strain 2308), ΔvirB2 or ΔbtpB Brucella abortus were compared by microarray analysis at 4 hours post infection. Transcripts with significant variation (>2 fold change; P<0.05) were functionally classified, and transcripts related to defense and inflammation were assessed by quantitative real time RT-PCR. Infection with wild type B. abortus resulted in slightly more genes with decreased than increased transcription levels. Conversely, infection of trophoblastic cells with the ΔvirB2 or the ΔbtpB mutant strains, that lack a functional T4SS or that has impaired inhibition of TLR signaling, respectively, induced more upregulated than downregulated genes. Wild type Brucella abortus impaired transcription of host genes related to immune response when compared to ΔvirB and ΔbtpB mutants. Our findings suggest that proinflammatory genes are negatively modulated in bovine trophoblastic cells at early stages of infection. The virB operon and btpB are directly or indirectly related to modulation of these host genes. These results shed light on the early interactions between B. abortus and placental tissue that ultimately culminate in inflammatory pathology and abortion.

A new method for simultaneous gene deletion and down-regulation in Brucella melitensis Rev.1

In this study, our aim was to integrate an antisense expression cassette in bacterial chromosome for providing a long-term expression down-regulation in a bid to develop a new approach for simultaneous deletion and down-regulation of target genes in bacterial system. Therefore, we were used this approach for simultaneous deletion of the perosamine synthetase (per) gene and down-regulation of the virB1 expression in Brucella melitensis Rev.1. The per gene, which is one of the LPS O-chain coding genes, was replaced by homologous recombination with an antisense virB1 expression cassette together with kanamycin resistance cassette (kan(R)). Deletion of the per gene was characterized by PCR analysis and DNA sequencing. The expression of antisense virB1 cassette was confirmed by RT-PCR. Down-regulation of the virB1 mRNA expression was quantified by real-time RT-PCR using virB1 specific primers relative to the groEL reference gene. The survival rate of mutant strain was evaluated by CFU count in the BALB/c mice. The virB1 mRNA expression was down-regulated on average 10-fold in mutant strain as compared to parental strain. The loss of per gene function and decrease of the virB1 mRNA expression resulted in reduced entry and survival of the mutant Rev.1 strain in BALB/c mice splenocytes. We propose that this method can be used for simultaneous regulation of multiple genes expression.

TraK and TraB are conserved outer membrane proteins of the Neisseria gonorrhoeae Type IV secretion system and are expressed at low levels in wild-type cells

Neisseria gonorrhoeae uses a type IV secretion system (T4SS) to secrete chromosomal DNA into the medium, and this DNA is effective in transforming other gonococci via natural transformation. In addition, the T4SS is important in the initial stages of biofilm development and mediates intracellular iron uptake in the absence of TonB. To better understand the mechanism of type IV secretion in N. gonorrhoeae, we examined the expression levels and localization of two predicted T4SS outer membrane proteins, TraK and TraB, in the wild-type strain as well as in overexpression strains and in a strain lacking all of the T4SS proteins. Despite very low sequence similarity to known homologues, TraB (VirB10 homolog) and TraK (VirB9 homolog) localized similarly to related proteins in other systems. Additionally, we found that TraV (a VirB7 homolog) interacts with TraK, as in other T4SSs. However, unlike in other systems, neither TraK nor TraB required the presence of other T4SS components for proper localization. Unlike other gonococcal T4SS proteins we have investigated, protein levels of the outer membrane proteins TraK and TraB were extremely low in wild-type cells and were undetectable by Western blotting unless overexpressed or tagged with a FLAG3 triple-epitope tag. Localization of TraK-FLAG3 in otherwise wild-type cells using immunogold electron microscopy of thin sections revealed a single gold particle on some cells. These results suggest that the gonococcal T4SS may be present in single copy per cell and that small amounts of T4SS proteins TraK and TraB are sufficient for DNA secretion.

A Rab-centric perspective of bacterial pathogen-occupied vacuoles

The ability to create and maintain a specialized organelle that supports bacterial replication is an important virulence property for many intracellular pathogens. Living in a membrane-bound vacuole presents inherent challenges, including the need to remodel a plasma membrane-derived organelle into a novel structure that will expand and provide essential nutrients to support replication, while also having the vacuole avoid membrane transport pathways that target bacteria for destruction in lysosomes. It is clear that pathogenic bacteria use different strategies to accomplish these tasks. The dynamics by which host Rab GTPases associate with pathogen-occupied vacuoles provide insight into the mechanisms used by different bacteria to manipulate host membrane transport. In this review we highlight some of the strategies bacteria use to maintain a pathogen-occupied vacuole by focusing on the Rab proteins involved in biogenesis and maintenance of these novel organelles.

Proteomic analysis of detergent resistant membrane domains during early interaction of macrophages with rough and smooth Brucella melitensis

The plasma membrane contains discrete nanometer-sized domains that are resistant to non-ionic detergents, and which are called detergent resistant membrane domains (DRMDs) or lipid rafts. Exposure of host cells to pathogenic bacteria has been shown to induce the re-distribution of specific host proteins between DRMDs and detergent soluble membranes, which leads to the initiation of cell signaling that enable pathogens to access host cells. DRMDs have been shown to play a role in the invasion of Brucella into host macrophages and the formation of replicative phagosomes called Brucella-containing vacuoles (BCVs). In this study we sought to characterize changes to the protein expression profiles in DRMDs and to respective cellular pathways and networks of Mono Mac 6 cells in response to the adherence of rough VTRM1 and smooth 16 M B. melitensis strains. DRMDs were extracted from Mono Mac 6 cells exposed for 2 minutes at 4°C to Brucella (no infection occurs) and from unexposed control cells. Protein expression was determined using the non-gel based quantitative iTRAQ (Isobaric Tags for Relative and Absolute Quantitation) mass spectrometry technique. Using the identified iTRAQ proteins we performed enrichment analyses and probed constructed human biochemical networks for interactions and metabolic reactions. We identified 149 proteins, which either became enriched, depleted or whose amounts did not change in DRMDs upon Brucella exposure. Several of these proteins were distinctly enriched or depleted in DRMDs upon exposure to rough and smooth B. melitensis strains which results in the differential engagement of cellular pathways and networks immediately upon Brucella encounter. For some of the proteins such as myosin 9, small G protein signaling modulator 3, lysine-specific demethylase 5D, erlin-2, and voltage-dependent anion-selective channel protein 2, we observed extreme differential depletion or enrichment in DRMDs. The identified proteins and pathways could provide the basis for novel ways of treating or diagnosing Brucellosis.

The Brucella suis IbpA heat-shock chaperone is not required for virulence or for expression of the VirB type IV secretion system VirB8 protein

Brucella suis, facultative intracellular bacterial pathogen of mammals, and Agrobacterium tumefaciens, a plant pathogen, both use a VirB type IV secretion system (T4SS) to translocate effector molecules into host cells. HspL, an α-crystalline-type small heat-shock protein, acts as a chaperone for the Agrobacterium VirB8 protein, an essential component of the VirB system. An Agrobacterium mutant lacking hspL is attenuated due to a misfunctional T4SS. We have investigated whether IbpA (BRA0051), the Brucella HspL homologue, plays a similar role. Unlike HspL, IbpA does not interact with VirB8, and an IbpA mutant shows full virulence and no defect in VirB expression. These data show that the Brucella α-crystalline-type small heat-shock protein IbpA is not required for Brucella virulence.

SIGNIFICANCE AND IMPACT OF STUDY

Many bacteria use type IV secretion systems (T4SS), multi-protein machines, to translocate DNA and protein substrates across their envelope. Understanding how T4SS function is important as they play major roles in the spread of plasmids carrying antibiotic resistance and in pathogenicity. In the plant pathogen Agrobacterium tumefaciens, HspL, an α-crystalline-type small heat-shock protein, acts as a chaperone for the essential type IV secretion system component VirB8. Here, we show that this is not the case for all T4SS; in the zoonotic pathogen Brucella suis, IbpA, the protein most related to HspL, does not play this role.

Brucella dissociation is essential for macrophage egress and bacterial dissemination

It has long been observed that smooth Brucella can dissociate into rough mutants that are cytotoxic to macrophages. However, the in vivo biological significance and/or mechanistic details of Brucella dissociation and cytotoxicity remain incomplete. In the current report, a plaque assay was developed using Brucella strains exhibiting varying degrees of cytotoxicity. Infected monolayers were observed daily using phase contrast microscopy for plaque formation while Brucella uptake and replication were monitored using an immunofluorescence assay (IFA). Visible plaques were detected at 4-5 days post infection (p.i.) with cytotoxic Brucella 16MΔmanBA at an MOI of 0.1. IFA staining demonstrated that the plaques consisted of macrophages with replicating Brucella. Visible plaques were not detected in monolayers infected with non-cytotoxic 16MΔmanBAΔvirB2 at an MOI of 0.1. However, IFA staining did reveal small groups of macrophages (foci) with replicating Brucella in the monolayers infected with 16MΔmanBAΔvirB2. The size of the foci observed in macrophage monolayers infected with rough Brucella correlated directly with cytotoxicity measured in liquid culture, suggesting that cytotoxicity was essential for Brucella egress and dissemination. In monolayers infected with 16M, small and large foci were observed. Double antibody staining revealed spontaneous rough mutants within the large, but not the small foci in 16M infected monolayers. Furthermore, plaque formation was observed in the large foci derived from 16M infections. Finally, the addition of gentamicin to the culture medium inhibited plaque formation, suggesting that cell-to-cell spread occurred only following release of the organisms from the cells. Taken together, these results demonstrate that Brucella-induced cytotoxicity is critical for Brucella egress and dissemination.

Comparative phylogenomics and evolution of the Brucellae reveal a path to virulence

Brucella species include important zoonotic pathogens that have a substantial impact on both agriculture and human health throughout the world. Brucellae are thought of as "stealth pathogens" that escape recognition by the host innate immune response, modulate the acquired immune response, and evade intracellular destruction. We analyzed the genome sequences of members of the family Brucellaceae to assess its evolutionary history from likely free-living soil-based progenitors into highly successful intracellular pathogens. Phylogenetic analysis split the genus into two groups: recently identified and early-dividing "atypical" strains and a highly conserved "classical" core clade containing the major pathogenic species. Lateral gene transfer events brought unique genomic regions into Brucella that differentiated them from Ochrobactrum and allowed the stepwise acquisition of virulence factors that include a type IV secretion system, a perosamine-based O antigen, and systems for sequestering metal ions that are absent in progenitors. Subsequent radiation within the core Brucella resulted in lineages that appear to have evolved within their preferred mammalian hosts, restricting their virulence to become stealth pathogens capable of causing long-term chronic infections.

In vivo-induced argininosuccinate lyase plays a role in the replication of Brucella abortus in RAW264.7 cells

Brucellosis caused by Brucella species is a zoonotic disease with a serious impact on public health and the livestock industry. To better understand the pathogenesis of the disease, in vivo-induced antigen technology (IVIAT) was used to investigate the in vivo-induced antigens of Brucella abortus in this study. A genomic expression library of B. abortus was constructed and screened using pooled bovine B. abortus-positive sera by IVIAT. In total, 33 antigens were identified. Five antigens were further expressed and tested for their seroreactivity against 33 individual bovine B. abortus-positive sera by Western blot analysis. The results showed a highest positive rate of 32/33 for argininosuccinate lyase (ASL), indicating that ASL may be used as a candidate marker for serodiagnosis of brucellosis. Furthermore, an asl gene-deleted mutant strain S2308ΔASL was constructed, and the intracellular survival and replication of the mutant strain in RAW264.7 cells were investigated. Interestingly, the numbers of bacteria recovered from cells infected with mutant strain S2308ΔASL were similar at all time points observed from 0 h to 96 h post-infection, suggesting the asl gene plays an important role in the bacterial replication in RAW264.7 cells. Real-time quantitative PCR (qPCR) analysis showed that the mRNA levels in S2308ΔASL were decreased for BvrR, BvrS and virB5 when compared with those in S2308 (P<0.05). Our results not only expand the knowledge of Brucella intracellular replication but also expand the list of candidates for serodiagnostic markers of brucellosis.

Purification of intracellular bacteria: isolation of viable Brucella abortus from host cells

The pathogenesis of brucellosis depends on the ability of bacteria from the genus Brucella to invade and replicate within animal cells. To understand the molecular pathways used by Brucella spp. to reach its intracellular niche, robust and reproducible bacteria purification protocols that provide enough material for biochemical and molecular biology studies are essential. Here, we describe a detailed methodology designed to extract and purify viable brucellae from mammalian host cells at different time periods of their intracellular cycle. The yield of proteins and nucleic acids is sufficient to perform immunochemical analysis, genetic studies, transcriptomics, and proteomics among others.

Brucella induces an unfolded protein response via TcpB that supports intracellular replication in macrophages

Brucella melitensis is a facultative intracellular bacterium that causes brucellosis, the most prevalent zoonosis worldwide. The Brucella intracellular replicative niche in macrophages and dendritic cells thwarts immune surveillance and complicates both therapy and vaccine development. Currently, host-pathogen interactions supporting Brucella replication are poorly understood. Brucella fuses with the endoplasmic reticulum (ER) to replicate, resulting in dramatic restructuring of the ER. This ER disruption raises the possibility that Brucella provokes an ER stress response called the Unfolded Protein Response (UPR). In this study, B. melitensis infection up regulated expression of the UPR target genes BiP, CHOP, and ERdj4, and induced XBP1 mRNA splicing in murine macrophages. These data implicate activation of all 3 major signaling pathways of the UPR. Consistent with previous reports, XBP1 mRNA splicing was largely MyD88-dependent. However, up regulation of CHOP, and ERdj4 was completely MyD88 independent. Heat killed Brucella stimulated significantly less BiP, CHOP, and ERdj4 expression, but induced XBP1 splicing. Although a Brucella VirB mutant showed relatively intact UPR induction, a TcpB mutant had significantly compromised BiP, CHOP and ERdj4 expression. Purified TcpB, a protein recently identified to modulate microtubules in a manner similar to paclitaxel, also induced UPR target gene expression and resulted in dramatic restructuring of the ER. In contrast, infection with the TcpB mutant resulted in much less ER structural disruption. Finally, tauroursodeoxycholic acid, a pharmacologic chaperone that ameliorates the UPR, significantly impaired Brucella replication in macrophages. Together, these results suggest Brucella induces a UPR, via TcpB and potentially other factors, that enables its intracellular replication. Thus, the UPR may provide a novel therapeutic target for the treatment of brucellosis. These results also have implications for other intracellular bacteria that rely on host physiologic stress responses for replication.

Impact of plasmids, including those encodingVirB4/D4 type IV secretion systems, on Salmonella enterica serovar Heidelberg virulence in macrophages and epithelial cells

Salmonella enterica serovar Heidelberg (S. Heidelberg) can cause foodborne illness in humans following the consumption of contaminated meat and poultry products. Recent studies from our laboratory have demonstrated that certain S. Heidelberg isolated from food-animal sources harbor multiple transmissible plasmids with genes that encode antimicrobial resistance, virulence and a VirB4/D4 type-IV secretion system. This study examines the potential role of these transmissible plasmids in bacterial uptake and survival in intestinal epithelial cells and macrophages, and the molecular basis of host immune system modulation that may be associated with disease progression. A series of transconjugant and transformant strains were developed with different combinations of the plasmids to determine the roles of the individual and combinations of plasmids on virulence. Overall the Salmonella strains containing the VirB/D4 T4SS plasmids entered and survived in epithelial cells and macrophages to a greater degree than those without the plasmid, even though they carried other plasmid types. During entry in macrophages, the VirB/D4 T4SS encoding genes are up-regulated in a time-dependent fashion. When the potential mechanisms for increased virulence were examined using an antibacterial Response PCR Array, the strain containing the T4SS down regulated several host innate immune response genes which likely contributed to the increased uptake and survival within macrophages and epithelial cells.

Regulation of virulence in Brucella: an eclectic repertoire of transcription factors defines the complex architecture of the virB promoter

Many intracellular bacterial pathogens use type IV secretion systems to deliver effector molecules and subvert the eukaryotic host cell defenses. The genus Brucella comprises facultative intracellular bacteria that cause brucellosis, a disease affecting a wide range of mammals including humans. The virB operon codes for a type IV secretion system that plays a central role in intracellular survival and replication of Brucella within the host. Expression of the virB genes is under the control of various transcription factors that allow this system to respond to different types of environmental signals, and display binding site structures and arrangements that define the intrinsic complexity of the virB promoter. This review focuses on summarizing the current state of research concerning regulation of the Brucella virB operon, with special emphasis on describing the nature and function of the implicated regulatory elements and examining the involved protein–DNA interactions.

Impact of Hfq on global gene expression and intracellular survival in Brucella melitensis

Brucella melitensis is a facultative intracellular bacterium that replicates within macrophages. The ability of brucellae to survive and multiply in the hostile environment of host macrophages is essential to its virulence. The RNA-binding protein Hfq is a global regulator that is involved in stress resistance and pathogenicity. Here we demonstrate that Hfq is essential for stress adaptation and intracellular survival in B. melitensis. A B. melitensis hfq deletion mutant exhibits reduced survival under environmental stresses and is attenuated in cultured macrophages and mice. Microarray-based transcriptome analyses revealed that 359 genes involved in numerous cellular processes were dysregulated in the hfq mutant. From these same samples the proteins were also prepared for proteomic analysis to directly identify Hfq-regulated proteins. Fifty-five proteins with significantly affected expression were identified in the hfq mutant. Our results demonstrate that Hfq regulates many genes and/or proteins involved in metabolism, virulence, and stress responses, including those potentially involved in the adaptation of Brucella to the oxidative, acid, heat stress, and antibacterial peptides encountered within the host. The dysregulation of such genes and/or proteins could contribute to the attenuated hfq mutant phenotype. These findings highlight the involvement of Hfq as a key regulator of Brucella gene expression and facilitate our understanding of the role of Hfq in environmental stress adaptation and intracellular survival of B. melitensis.

Comparative genomic analysis of Brucella melitensis vaccine strain M5 provides insights into virulence attenuation

The Brucella melitensis vaccine strain M5 is widely used to prevent and control brucellosis in animals. In this study, we determined the whole-genome sequence of M5, and conducted a comprehensive comparative analysis against the whole-genome sequence of the virulent strain 16 M and other reference strains. This analysis revealed 11 regions of deletion (RDs) and 2 regions of insertion (RIs) within the M5 genome. Among these regions, the sequences encompassed in 5 RDs and 1 RI showed consistent variation, with a large deletion between the M5 and the 16 M genomes. RD4 and RD5 showed the large diversity among all Brucella genomes, both in RD length and RD copy number. Thus, RD4 and RD5 are potential sites for typing different Brucella strains. Other RD and RI regions exhibited multiple single nucleotide polymorphisms (SNPs). In addition, a genome fragment with a 56 kb rearrangement was determined to be consistent with previous studies. Comparative genomic analysis indicated that genomic island inversion in Brucella was widely present. With the genetic pattern common among all strains analyzed, these 2 RDs, 1 RI, and one inversion region are potential sites for detection of genomic differences. Several SNPs of important virulence-related genes (motB, dhbC, sfuB, dsbAB, aidA, aroC, and lysR) were also detected, and may be used to determine the mechanism of virulence attenuation. Collectively, this study reveals that comparative analysis between wild-type and vaccine strains can provide resources for the study of virulence and microevolution of Brucella.

Bartonella and Brucella--weapons and strategies for stealth attack

Bartonella spp. and Brucella spp. are closely related α-proteobacterial pathogens that by distinct stealth-attack strategies cause chronic infections in mammals including humans. Human infections manifest by a broad spectrum of clinical symptoms, ranging from mild to fatal disease. Both pathogens establish intracellular replication niches and subvert diverse pathways of the host's immune system. Several virulence factors allow them to adhere to, invade, proliferate, and persist within various host-cell types. In particular, type IV secretion systems (T4SS) represent essential virulence factors that transfer effector proteins tailored to recruit host components and modulate cellular processes to the benefit of the bacterial intruders. This article puts the remarkable features of these two pathogens into perspective, highlighting the mechanisms they use to hijack signaling and trafficking pathways of the host as the basis for their stealthy infection strategies.

Autophagy and bacterial infection: an evolving arms race

Autophagy is an important membrane transport pathway that is conserved among eukaryotic cells. Although first described as an intracellular catabolic pathway used to break down self-components, autophagy has been found to play an important role in the elimination of intracellular pathogens. A variety of host mechanisms exist for recognizing and targeting intracellular bacteria to autophagosomes. Several intracellular bacteria have evolved ways to manipulate, inhibit, or avoid autophagy in order to survive in the cell. Thus, the autophagy pathway can be viewed as an evolutionarily conserved host response to infection.

Global Rsh-dependent transcription profile of Brucella suis during stringent response unravels adaptation to nutrient starvation and cross-talk with other stress responses

Background

In the intracellular pathogen Brucella spp., the activation of the stringent response, a global regulatory network providing rapid adaptation to growth-affecting stress conditions such as nutrient deficiency, is essential for replication in the host. A single, bi-functional enzyme Rsh catalyzes synthesis and hydrolysis of the alarmone (p)ppGpp, responsible for differential gene expression under stringent conditions.

Results

cDNA microarray analysis allowed characterization of the transcriptional profiles of the B. suis 1330 wild-type and Δrsh mutant in a minimal medium, partially mimicking the nutrient-poor intramacrophagic environment. A total of 379 genes (11.6% of the genome) were differentially expressed in a rsh-dependent manner, of which 198 were up-, and 181 were down-regulated. The pleiotropic character of the response was confirmed, as the genes encoded an important number of transcriptional regulators, cell envelope proteins, stress factors, transport systems, and energy metabolism proteins. Virulence genes such as narG and sodC, respectively encoding respiratory nitrate reductase and superoxide dismutase, were under the positive control of (p)ppGpp, as well as expression of the cbb3-type cytochrome c oxidase, essential for chronic murine infection. Methionine was the only amino acid whose biosynthesis was absolutely dependent on stringent response in B. suis.

Conclusions

The study illustrated the complexity of the processes involved in adaptation to nutrient starvation, and contributed to a better understanding of the correlation between stringent response and Brucella virulence. Most interestingly, it clearly indicated (p)ppGpp-dependent cross-talk between at least three stress responses playing a central role in Brucella adaptation to the host: nutrient, oxidative, and low-oxygen stress.

Host-Brucella interactions and the Brucella genome as tools for subunit antigen discovery and immunization against brucellosis

Vaccination is the most important approach to counteract infectious diseases. Thus, the development of new and improved vaccines for existing, emerging, and re-emerging diseases is an area of great interest to the scientific community and general public. Traditional approaches to subunit antigen discovery and vaccine development lack consideration for the critical aspects of public safety and activation of relevant protective host immunity. The availability of genomic sequences for pathogenic Brucella spp. and their hosts have led to development of systems-wide analytical tools that have provided a better understanding of host and pathogen physiology while also beginning to unravel the intricacies at the host-pathogen interface. Advances in pathogen biology, host immunology, and host-agent interactions have the potential to serve as a platform for the design and implementation of better-targeted antigen discovery approaches. With emphasis on Brucella spp., we probe the biological aspects of host and pathogen that merit consideration in the targeted design of subunit antigen discovery and vaccine development.

Small GTPases and Brucella entry into the endoplasmic reticulum

A key determinant for intracellular pathogenic bacteria to ensure their virulence within host cells is their ability to bypass the endocytic pathway and to reach a safe niche of replication. In the case of Brucella, the bacterium targets the ER (endoplasmic reticulum) to create a replicating niche called the BCV (Brucella-containing vacuole). The ER is a suitable strategic place for pathogenic Brucella. Indeed, bacteria can be hidden from host cell defences to persist within the host, and they can take advantage of the membrane reservoir delivered by the ER to replicate. Interaction with the ER leads to the presence on the BCV of the GAPDH (glyceraldehyde-3-phosphate dehydrogenase) and the small GTPase Rab2 known to be located on secretory vesicles that traffic between the ER and the Golgi apparatus. GAPDH and the small GTPase Rab2 controls Brucella replication at late times post-infection. A specific interaction between the human small GTPase Rab2 and a Brucella spp. protein named RicA was identified. Altered kinetics of intracellular trafficking and faster proliferation of the Brucella abortus ΔricA mutant was observed compared with the wild-type strain. RicA is the first reported effector with a proposed function for B. abortus.

Replication of Brucella melitensis inside primary human monocytes depends on mitogen activated protein kinase signaling

The clinical course of infections caused by Brucella is linked to its capacity to modulate the initial immune response of macrophages in order to ensure its intracellular replication. Signal transduction pathways implicated in the survival of Brucella in human cells are not completely elucidated. We herein investigated the involvement of the TLR-MAPK-dependent signaling pathways in the survival of Brucella in primary human monocytes using live clinical strains of Brucella melitensis. B. melitensis caused a delayed, TLR2 dependent MAPK activation. Specific MAPK inhibitors for p38 (SB203580), ERK1/2 (PD98059) and JNK (SP600125) or the anti-TLR2 blocking Ab inhibited both inflammatory and anti-inflammatory responses characterized by TNF-α, IL-6 and IL-10 production. Intracellular replication of B. melitensis was mainly dependent on p38 and JNK activation and not affected by IL-10 levels. These are the first evidence to support that survival of B. melitensis inside human monocytes depends on interplay among the different MAPK family members, activated through TLR2, in spite of an initial pro-inflammatory response.

Pathogenic brucellae replicate in human trophoblasts

Brucellae replicate in a vacuole derived from the endoplasmic reticulum (ER) in epithelial cells, macrophages, and dendritic cells. In animals, trophoblasts are also key cellular targets where brucellae efficiently replicate in association with the ER. Therefore, we investigated the ability of Brucella spp. to infect human trophoblasts using both immortalized and primary trophoblasts. Brucella extensively proliferated within different subpopulations of trophoblasts, suggesting that they constitute an important niche in cases where the fetal-maternal barrier is breached. In extravillous trophoblasts (EVTs), B. abortus and B. suis replicated within single-membrane acidic lysosomal membrane-associated protein 1-positive inclusions, whereas B. melitensis replicated in the ER-derived compartment. Furthermore, B. melitensis but not B. abortus nor B. suis interfered with the invasive capacity of EVT-like cells in vitro. Because EVTs are essential for implantation during early stages of pregnancy, the nature of the replication niche may have a central role during Brucella-associated abortion in infected women.

Sensing of bacterial type IV secretion via the unfolded protein response

Host cytokine responses to Brucella abortus infection are elicited predominantly by the deployment of a type IV secretion system (T4SS). However, the mechanism by which the T4SS elicits inflammation remains unknown. Here we show that translocation of the T4SS substrate VceC into host cells induces proinflammatory responses. Ectopically expressed VceC interacted with the endoplasmic reticulum (ER) chaperone BiP/Grp78 and localized to the ER of HeLa cells. ER localization of VceC required a transmembrane domain in its N terminus. Notably, the expression of VceC resulted in reorganization of ER structures. In macrophages, VceC was required for B. abortus-induced inflammation by induction of the unfolded protein response by a process requiring inositol-requiring transmembrane kinase/endonuclease 1. Altogether, these findings suggest that translocation of the T4SS effector VceC induces ER stress, which results in the induction of proinflammatory host cell responses during B. abortus infection. IMPORTANCE Brucella species are pathogens that require a type IV secretion system (T4SS) to survive in host cells and to maintain chronic infection. By as-yet-unknown pathways, the T4SS also elicits inflammatory responses in infected cells. Here we show that inflammation caused by the T4SS results in part from the sensing of a T4SS substrate, VceC, that localizes to the endoplasmic reticulum (ER), an intracellular site of Brucella replication. Possibly via binding of the ER chaperone BiP, VceC causes ER stress with concomitant expression of proinflammatory cytokines. Thus, induction of the unfolded protein response may represent a novel pathway by which host cells can detect pathogens deploying a T4SS.

Evidence of the presence of a functional Dot/Icm type IV-B secretion system in the fish bacterial pathogen Piscirickettsia salmonis

Piscirickettsia salmonis is a fish bacterial pathogen that has severely challenged the sustainability of the Chilean salmon industry since its appearance in 1989. As this Gram-negative bacterium has been poorly characterized, relevant aspects of its life cycle, virulence and pathogenesis must be identified in order to properly design prophylactic procedures. This report provides evidence of the functional presence in P. salmonis of four genes homologous to those described for Dot/Icm Type IV Secretion Systems. The Dot/Icm System, the major virulence mechanism of phylogenetically related pathogens Legionella pneumophila and Coxiella burnetii, is responsible for their intracellular survival and multiplication, conditions that may also apply to P. salmonis. Our results demonstrate that the four P. salmonis dot/icm homologues (dotB, dotA, icmK and icmE) are expressed both during in vitro tissue culture cells infection and growing in cell-free media, suggestive of their putative constitutive expression. Additionally, as it happens in other referential bacterial systems, temporal acidification of cell-free media results in over expression of all four P. salmonis genes, a well-known strategy by which SSTIV-containing bacteria inhibit phagosome-lysosome fusion to survive. These findings are very important to understand the virulence mechanisms of P. salmonis in order to design new prophylactic alternatives to control the disease.

Brucella T4SS: the VIP pass inside host cells

For many Gram-negative bacteria, like Brucella, the type IV secretion system (T4SS) has a critical role in bacterial virulence. In Brucella, the VirB T4SS permits the injection of bacterial effectors inside host cells, leading to subversion of signaling pathways and favoring bacterial growth and pathogenesis. The virB operon promoter is tightly regulated by a combination of transcriptional activators and repressors that are expressed according to the environmental conditions encountered by Brucella. Recent advances have shed light on the Brucella T4SS regulatory mechanisms and also its substrates. Characterization of the targets and functions of these translocated effectors is underway and will help understand the role of the T4SS in the establishment of a replication niche inside host cells.

The impact of Toll-like receptors on bacterial virulence strategies

The mammalian immune system has evolved in the presence of microbes, both pathogenic and commensal. The consequences of microbial recognition by the host has led to the development of compensatory mechanisms by both the host and microbe to either resist or tolerate the existence of the other. In this review we discuss examples of this co-evolutionary relationship. Because of space considerations and for conceptual clarity, we have focused on detection of bacteria by the Toll-like receptor (TLR) family and highlight examples of bacterial strategies to evade, subvert and in some cases even utilize these receptors.

A MarR-Type regulator directly activates transcription from the Brucella abortus virB promoter by sharing a redundant role with HutC

Type IV secretion systems (T4SS) are multiprotein structures that direct the translocation of specific molecules across the bacterial cell envelope. As in other bacteria, pathogenicity of the genus Brucella essentially depends on the integrity of the T4SS-encoding virB operon, whose expression is regulated by multiple transcription factors belonging to different families. Previously, we identified IHF and HutC, two direct regulators of the virB genes that were isolated from total protein extracts of Brucella. Here, we report the identification of MdrA, a third regulatory element that was isolated using the same screening procedure. This transcription factor, which belongs to the MarR-family of transcriptional regulators, binds at two different sites of the virB promoter and regulates expression in a growth phase-dependent manner. Like other members of the MarR family, specific ligands were able to dissociate MdrA from DNA in vitro. Determination of the MdrA-binding sites by DNase I footprinting and analyses of protein-DNA complexes by electrophoresis mobility shift assays (EMSAs) showed that MdrA competes with IHF and HutC for the binding to the promoter because their target DNA sequences overlap. Unlike IHF, both MdrA and HutC bound to the promoter without inducing bending of DNA. Moreover, the two latter transcription factors activated virB expression to similar extents, and in doing so, they are functionally redundant. Taken together, our results show that MdrA is a regulatory element that directly modulates the activity of the virB promoter and is probably involved in coordinating gene expression in response to specific environmental signals.

Bacterial Type IV secretion systems: versatile virulence machines

Many bacterial pathogens employ multicomponent protein complexes to deliver macromolecules directly into their eukaryotic host cell to promote infection. Some Gram-negative pathogens use a versatile Type IV secretion system (T4SS) that can translocate DNA or proteins into host cells. T4SSs represent major bacterial virulence determinants and have recently been the focus of intense research efforts designed to better understand and combat infectious diseases. Interestingly, although the two major classes of T4SSs function in a similar manner to secrete proteins, the translocated 'effectors' vary substantially from one organism to another. In fact, differing effector repertoires likely contribute to organism-specific host cell interactions and disease outcomes. In this review, we discuss the current state of T4SS research, with an emphasis on intracellular bacterial pathogens of humans and the diverse array of translocated effectors used to manipulate host cells.