Brucella abortus transits through the autophagic pathway and replicates in the endoplasmic reticulum of nonprofessional phagocytes

The putative type 4 secretion system effector BspD is involved in maintaining envelope integrity of the pathogen Brucella

Brucellosis is a debilitating disease caused by the Gram-negative, facultative intracellular zoonotic pathogen Brucella. En route to its intracellular replicative niche, Brucella encounters various stressful environments that may compromise envelope integrity. Here we show that the proposed type 4 secretion system (T4SS) effector BspD is a conserved protein of the Rhizobiales, which does not show signs of co-evolution with the presence of a T4SS or a certain lifestyle. We further present data indicating that BspD is critical for the envelope integrity of Brucella abortus in the stationary phase and in the presence of EDTA, a compound known to destabilize the outer membrane. Deletion of bspD resulted in abnormal bacterial morphologies, indicating its involvement in maintaining envelope integrity. Additionally, the absence of BspD led to the formation of fewer and smaller intracellular microcolonies in a macrophage infection model. From our observations, we propose that BspD of B. abortus is critical for preserving the integrity of the bacterial envelope, particularly under stressful conditions, which may enhance Brucella's ability to survive within host cells.

IMPORTANCE

Brucellosis, caused by the intracellular pathogen Brucella, poses a significant health threat. Understanding how Brucella adapts to stressful environments is crucial. This study unveils BspD, a conserved protein within the Rhizobiales order, as a key player in maintaining Brucella's envelope integrity. Remarkably, BspD's presence within the Rizobiales appears independent of the presence of a T4SS or a specific lifestyle. Deletion of bspD resulted in compromised envelope integrity, abnormal bacterial morphologies, and reduced intracellular microcolony formation. These findings underscore BspD's critical role, particularly in stressful conditions like the stationary phase and EDTA exposure, and highlight its significance for the survival of Brucella within host cells. This elucidation deepens our understanding of Brucella pathogenesis and may inform future therapeutic strategies against brucellosis.

Brucella mediates autophagy, inflammation, and apoptosis to escape host killing

Brucellosis is a serious zoonosis caused by Brucella spp. infection, which not only seriously jeopardizes the health of humans and mammals, but also causes huge economic losses to the livestock industry. Brucella is a Gram-negative intracellular bacterium that relies primarily on its virulence factors and a variety of evolved survival strategies to replicate and proliferate within cells. Currently, the mechanisms of autophagy, inflammation, and apoptosis in Brucella-infected hosts are not fully understood and require further research and discussion. This review focuses on the relationship between Brucella and autophagy, inflammation, and apoptosis to provide the scientific basis for revealing the pathogenesis of Brucella.

A Review on the Methodology and Use of the Pregnant Mouse Model in the Study of Brucella Reproductive Pathogenesis and Its Abortifacient Effect

Brucellosis is one of the most common and widespread bacterial zoonoses and is caused by Gram-negative bacteria belonging to the genus Brucella. These organisms are able to infect and replicate within the placenta, resulting in abortion, one of the main clinical signs of brucellosis. Although the mouse model is widely used to study Brucella virulence and, more recently, to evaluate the protection of new vaccines, there is no clear consensus on the experimental conditions (e.g., mouse strains, doses, routes of inoculation, infection/pregnancy time) and the natural host reproducibility of the pregnant mouse model for reproductive brucellosis. This lack of consensus calls for a review that integrates the major findings regarding the effect of Brucella wild-type and vaccine strains infections on mouse pregnancy. We found sufficient evidence on the utility of the pregnant mouse model to study Brucella-induced placentitis and abortion and propose suitable experimental conditions (dose, time of infection) and pregnancy outcome readouts for B. abortus and B. melitensis studies. Finally, we discuss the utility and limitations of the pregnant mouse as a predictive model for the abortifacient effect of live Brucella vaccines.

A genome-wide CRISPR screen identified host genes essential for intracellular Brucella survival

Brucella is a zoonotic intracellular bacterium that poses threats to human health and economic security. Intracellular infection is a hallmark of the agent Brucella and a primary cause of distress, through which the bacterium regulates the host intracellular environment to promote its own colonization and replication, evading host immunity and pharmaceutical killing. Current studies of Brucella intracellular processes are typically premised on bacterial phenotype such as intracellular bacterial survival, followed by biochemical or molecular biological approaches to reveal detailed mechanisms. While such processes can deepen the understanding of Brucella-host interaction, the insights into host alterations in infection would be easily restricted to known pathways. In the current study, we applied CRISPR Cas9 screen to identify host genes that are most affected by Brucella infection on cell viability at the genomic level. As a result of CRISPR screening, we firstly identified that knockout of the negatively selected genes GOLGA6L6, DEFB103B, OR4F29, and ERCC6 attenuate the viability of both the host cells and intracellular Brucella, suggesting these genes to be potential therapeutic targets for Brucella control. In particular, knockout of DEFB103B diminished Brucella intracellular survival by altering host cell autophagy. Conversely, knockout of positive screening genes promoted intracellular proliferation of Brucella. In summary, we screened host genes at the genomic level throughout Brucella infection, identified host genes that are previously not recognized to be involved in Brucella infection, and provided targets for intracellular infection control.IMPORTANCEBrucella is a Gram-negative bacterium that infects common mammals causing arthritis, myalgia, neuritis, orchitis, or miscarriage and is difficult to cure with antibiotics due to its intracellular parasitism. Therefore, unraveling the mechanism of Brucella-host interactions will help controlling Brucella infections. CRISPR-Cas9 is a gene editing technology that directs knockout of individual target genes by guided RNA, from which genome-wide gene-knockout cell libraries can be constructed. Upon infection with Brucella, the cell library would show differences in viability as a result of the knockout and specific genes could be revealed by genomic DNA sequencing. As a result, genes affecting cell viability during Brucella infection were identified. Further testing of gene function may reveal the mechanisms of Brucella-host interactions, thereby contributing to clinical therapy.

Effector Proteins of Type IV Secretion System: Weapons of Brucella Used to Fight Against Host Immunity

Brucella is an intracellular bacterial pathogen capable of long-term persistence in the host, resulting in chronic infections in livestock and wildlife. The type IV secretion system (T4SS) is an important virulence factor of Brucella and is composed of 12 protein complexes encoded by the VirB operon. T4SS exerts its function through its secreted 15 effector proteins. The effector proteins act on important signaling pathways in host cells, inducing host immune responses and promoting the survival and replication of Brucella in host cells to promote persistent infection. In this article, we describe the intracellular circulation of Brucella-infected cells and survey the role of Brucella VirB T4SS in regulating inflammatory responses and suppressing host immune responses during infection. In addition, the important mechanisms of these 15 effector proteins in resisting the host immune response during Brucella infection are elucidated. For example, VceC and VceA assist in achieving sustained survival of Brucella in host cells by affecting autophagy and apoptosis. BtpB, together with BtpA, controls the activation of dendritic cells during infection, induces inflammatory responses, and controls host immunity. This article reviews the effector proteins secreted by Brucella T4SS and their involvement in immune responses, which can provide a reliable theoretical basis for the subsequent mechanism of hijacking the host cell signaling pathway by bacteria and contribute to the development of better vaccines to effectively treat Brucella bacterial infection.

A sensor histidine kinase from a plant-endosymbiont bacterium restores the virulence of a mammalian intracellular pathogen

Alphaproteobacteria include organisms living in close association with plants or animals. This interaction relies partly on orthologous two-component regulatory systems (TCS), with sensor and regulator proteins modulating the expression of conserved genes related to symbiosis/virulence. We assessed the ability of the exoS+Sm gene, encoding a sensor protein from the plant endosymbiont Sinorhizobium meliloti to substitute its orthologous bvrS in the related animal/human pathogen Brucella abortus. ExoS phosphorylated the B. abortus regulator BvrR in vitro and in cultured bacteria, showing conserved biological function. Production of ExoS in a B. abortus bvrS mutant reestablished replication in host cells and the capacity to infect mice. Bacterial outer membrane properties, the production of the type IV secretion system VirB, and its transcriptional regulators VjbR and BvrR were restored as compared to parental B. abortus. These results indicate that conserved traits of orthologous TCS from bacteria living in and sensing different environments are sufficient to achieve phenotypic plasticity and support bacterial survival. The knowledge of bacterial genetic networks regulating host interactions allows for an understanding of the subtle differences between symbiosis and parasitism. Rewiring these networks could provide new alternatives to control and prevent bacterial infection.

Meta-Analysis of the Prevalence of Porcine Zoonotic Bacterial Pathogens in India: A 13-Year (2010-2023) Study

The presence of bacterial pathogens such as Brucella spp., Clostridium spp., E. coli, Listeria monocytogenes, Salmonella spp., Staphylococcus spp., and Streptococcus suis not only hampers pig production but also carries significant zoonotic implications. The present study aims to conduct a comprehensive meta-analysis spanning over 13 years (2010-2023) to ascertain the prevalence of these zoonotic bacterial pathogens in Indian pig populations. The study seeks to synthesize data from diverse geographic regions within India and underscores the relevance of the One Health framework. A systematic search of electronic databases was meticulously performed. Inclusion criteria encompassed studies detailing zoonotic bacterial pathogen prevalence in pigs within India during the specified timeframe. Pertinent information including authors, publication year, geographical location, sampling techniques, sample sizes, and pathogen-positive case counts were meticulously extracted. The meta-analysis of zoonotic bacterial pathogens in Indian pig populations (2010-2023) unveiled varying prevalence rates: 9% Brucella spp., 22% Clostridium spp., 19% E. coli, 12% Listeria monocytogenes, 10% Salmonella spp. and Streptococcus suis, and 24% Staphylococcus spp. The application of random effects further revealed additional variability: 6% Brucella spp., 23% Clostridium spp., 24% E. coli, 14% Listeria monocytogenes, 10% Salmonella spp. and Streptococcus suis, and 35% Staphylococcus spp. Notably, the observed heterogeneity (I2) varied significantly from 87% to 99%. The meta-analysis findings underscore the pervasive nature of these diseases throughout India's pig populations, accentuating the substantial impact of these pathogens on pig health and the potential for zoonotic transmission. The present study reinforces the importance of the adoption of a comprehensive One Health approach that acknowledges the intricate interplay between animal, human and environmental health.

The Brucella abortus Type IV Effector BspA Inhibits MARCH6-Dependent ERAD To Promote Intracellular Growth

Brucella abortus, the intracellular causative agent of brucellosis, relies on type IV secretion system (T4SS) effector-mediated modulation of host cell functions to establish a replicative niche, the Brucella-containing vacuole (BCV). Brucella exploits the host's endocytic, secretory, and autophagic pathways to modulate the nature and function of its vacuole from an endocytic BCV (eBCV) to an endoplasmic reticulum (ER)-derived replicative BCV (rBCV) to an autophagic egress BCV (aBCV). A role for the host ER-associated degradation pathway (ERAD) in the B. abortus intracellular cycle was recently uncovered, as it is enhanced by the T4SS effector BspL to control the timing of aBCV-mediated egress. Here, we show that the T4SS effector BspA also interferes with ERAD, yet to promote B. abortus intracellular proliferation. BspA was required for B. abortus replication in bone marrow-derived macrophages and interacts with membrane-associated RING-CH-type finger 6 (MARCH6), a host E3 ubiquitin ligase involved in ERAD. Pharmacological inhibition of ERAD and small interfering RNA (siRNA) depletion of MARCH6 did not affect the replication of wild-type B. abortus but rescued the replication defect of a bspA deletion mutant, while depletion of the ERAD component UbxD8 affected replication of B. abortus and rescued the replication defect of the bspA mutant. BspA affected the degradation of ERAD substrates and destabilized the MARCH6 E3 ligase complex. Taken together, these findings indicate that BspA inhibits the host ERAD pathway via targeting of MARCH6 to promote B. abortus intracellular growth. Our data reveal that targeting ERAD components by type IV effectors emerges as a multifaceted theme in Brucella pathogenesis.

Cell and Tissue Tropism of Brucella spp

Brucella spp. are facultatively intracellular bacteria that can infect, survive, and multiply in various host cell types in vivo and/or in vitro. The genus Brucella has markedly expanded in recent years with the identification of novel species and hosts, which has revealed additional information about the cell and tissue tropism of these pathogens. Classically, Brucella spp. are considered to have tropism for organs that contain large populations of phagocytes such as lymph nodes, spleen, and liver, as well as for organs of the genital system, including the uterus, epididymis, testis, and placenta. However, experimental infections of several different cultured cell types indicate that Brucella may actually have a broader cell tropism than previously thought. Indeed, recent studies indicate that certain Brucella species in particular hosts may display a pantropic distribution in vivo. This review discusses the available knowledge on cell and tissue tropism of Brucella spp. in natural infections of various host species, as well as in experimental animal models and cultured cells.

Association of endoplasmic reticulum associated degradation (ERAD) with the transmission of Liberibacter solanacearum by its psyllid vector

Candidatus Liberibacter solanacearum (CLso) is a complex of gram negative plant pathogenic and fastidious bacterial haplotypes restricted to the phloem and transmitted by several psyllid species. In Israel, the carrot psyllid Bactericera trigonica transmits CLso haplotype D in a persistent and propagative manner and causes the carrot yellows disease, inflicting significant economic losses in many countries. Understanding the transmission of CLso is fundamental to devising sustainable management strategies. Persistent transmission of vector-borne pathogens involves the critical steps of adhesion, cell invasion and replication inside the insect gut cells before passage to the hemolymph. Using microscopy and expression analyses, we have previously confirmed a role for the endoplasmic reticulum (ER) in inducing immune responses and subsequent molecular pathways resulting in programmed cell death (apoptosis) upon CLso-infection in the midgut. In the current study, we confirm that the ER-associated degradation (ERAD) machinery and its associated marker genes were upregulated in CLso infected insects, including Derlin-1, Selenoprotein-1 and Ubiquitin Ligase RNF-185. Silencing Derlin-1, which acts on the ER membrane by regulating the degradation of unfolded proteins upon ER stress, revealed its role in CLso persistence and transmission. Molecular pathways initiated in the ER membrane upon bacterial infection are well documented in human, animal and insect systems, and this study confirms the role of the ER in CLso-psyllid interactions.

Brucella Egresses from Host Cells Exploiting Multivesicular Bodies

Host cell egress is a critical step in the life cycle of intracellular pathogens, especially in microbes capable of establishing chronic infections. The Gram-negative bacterium Brucella belongs to such a group of pathogens. Even though much has been done to understand how Brucella avoids killing and multiplies in its intracellular niche, the mechanism that this bacterium deploys to egress from the cell to complete its cycle has been poorly studied. In the manuscript, we quantify the kinetics of bacterial egress and show that Brucella exploits multivesicular bodies to exit host cells. For the first time, we visualized the process of egress in real time by live video microscopy and showed that a population of intracellular bacteria exit from host cells in vacuoles containing multivesicular body-like features. We observed the colocalization of Brucella with two multivesicular markers, namely, CD63 and LBPA, both during the final stages of the intracellular life cycle and in egressed bacteria. Moreover, drugs that either promote or inhibit multivesicular bodies either increased or decreased the number of extracellular bacteria, respectively. Our results strongly suggest that Brucella hijacks multivesicular bodies to exit the host cells to initiate new infection events. IMPORTANCE How intracellular bacterial pathogens egress from host cells has been poorly studied. This is particularly important because this stage of the infectious cycle can have a strong impact on how the host resolves the infection. Brucella is an intracellular pathogen that infects mammals, including humans, and causes a chronic debilitating illness. The bacterium has evolved a plethora of mechanisms to invade host cells, avoid degradation in the endocytic pathway, and actively multiply within a specialized intracellular compartment. However, how this pathogen exits from infected cells to produce reinfection and complete its life cycle is poorly understood. In the manuscript, we shed some light on the mechanisms that are exploited by Brucella to egress from host cells. We observed for the first time the egress of Brucella from infected cells by time-lapse video microscopy, and we found that the bacterium exits in vesicles containing multivesicular bodies (MVBs) features. Moreover, the drug manipulation of MVBs resulted in the alteration of bacterial egress efficiency. Our results indicate that Brucella hijacks MVBs to exit host cells and that this strongly contributes to the reinfection cycle.

The mechanism of chronic intracellular infection with Brucella spp

Globally, brucellosis is a widespread zoonotic disease. It is prevalent in more than 170 countries and regions. It mostly damages an animal's reproductive system and causes extreme economic losses to the animal husbandry industry. Once inside cells, Brucella resides in a vacuole, designated the BCV, which interacts with components of the endocytic and secretory pathways to ensure bacterial survival. Numerous studies conducted recently have revealed that Brucella's ability to cause a chronic infection depends on how it interacts with the host. This paper describes the immune system, apoptosis, and metabolic control of host cells as part of the mechanism of Brucella survival in host cells. Brucella contributes to both the body's non-specific and specific immunity during chronic infection, and it can aid in its survival by causing the body's immune system to become suppressed. In addition, Brucella regulates apoptosis to avoid being detected by the host immune system. The BvrR/BvrS, VjbR, BlxR, and BPE123 proteins enable Brucella to fine-tune its metabolism while also ensuring its survival and replication and improving its ability to adapt to the intracellular environment.

Development and evaluation of the Galleria mellonella (greater wax moth) infection model to study Brucella host-pathogen interaction

Brucellosis is a zoonotic disease caused by Gram-negative bacteria of the genus Brucella. These pathogens cause long-lasting infections, a process in which Brucella modifications in the lipopolysaccharide (LPS) and envelope lipids reduce pathogen-associated molecular pattern (PAMP) recognition, thus hampering innate immunity activation. In vivo models are essential to investigate bacterial virulence, mice being the most used model. However, ethical and practical considerations impede their use in high-throughput screening studies. Although lacking the complexity of the mammalian immune system, insects share key-aspects of innate immunity with mammals, and Galleria mellonella has been used increasingly as a model. G. mellonella larvae have been shown useful in virulence analyses, including Gram-negative pathogens like Klebsiella pneumoniae and Legionella pneumophila. To assess its potential to study Brucella virulence, we first evaluated larva survival upon infection with representative Brucella species (i.e.B. abortus 2308W, B. microti CCM4915 and B. suis biovar 2) and mutants in the VirB type-IV secretion system (T4SS) or in the LPS-O-polysaccharide (O-PS). As compared to K.pneumoniae, the Brucella spp. tested induced a delayed and less severe mortality profile consistent with an escape of innate immunity detection. Brucella replication within larvae was affected by the lack of O-PS, which is reminiscent of their attenuation in natural hosts. On the contrary, replication was not affected by T4SS dysfunction and the mutant induced only slightly less mortality (not statistically significant) than its parental strain. We also evaluated G. mellonella to efficiently recognise Brucella and their LPS by quantification of the pro-phenoloxidase system and melanisation activation, using Pseudomonas LPS as a positive control. Among the brucellae, only B. microti LPS triggered an early-melanisation response consistent with the slightly increased endotoxicity of this species in mice. Therefore, G. mellonella represents a tool to screen for potential Brucella factors modulating innate immunity, but its usefulness to investigate other mechanisms relevant in Brucella intracellular life is limited.

Activation of mucosal immunity as a novel therapeutic strategy for combating brucellosis

Brucellosis is a disease of livestock that is commonly asymptomatic until an abortion occurs. Disease in humans results from contact of infected livestock or consumption of contaminated milk or meat. Brucella zoonosis is primarily caused by one of three species that infect livestock, Bacillus abortus in cattle, B. melitensis in goats and sheep, and B. suis in pigs. To aid in disease prophylaxis, livestock vaccines are available, but are only 70% effective; hence, improved vaccines are needed to mitigate disease, particularly in countries where disease remains pervasive. The absence of knowing which proteins confer complete protection limits development of subunit vaccines. Instead, efforts are focused on developing new and improved live, attenuated Brucella vaccines, since these mimic attributes of wild-type Brucella, and stimulate host immune, particularly T helper 1-type responses, required for protection. In considering their development, the new mutants must address Brucella's defense mechanisms normally active to circumvent host immune detection. Vaccination approaches should also consider mode and route of delivery since disease transmission among livestock and humans is believed to occur via the naso-oropharyngeal tissues. By arming the host's mucosal immune defenses with resident memory T cells (TRMs) and by expanding the sources of IFN-γ, brucellae dissemination from the site of infection to systemic tissues can be prevented. In this review, points of discussion focus on understanding the various immune mechanisms involved in disease progression and which immune players are important in fighting disease.

A comprehensive review of small regulatory RNAs in Brucella spp

Brucella spp. are Gram-negative bacteria that naturally infect a variety of domesticated and wild animals, often resulting in abortions and sterility. Humans exposed to these animals or animal products can also develop debilitating, flu-like disease. The brucellae are intracellular pathogens that reside predominantly within immune cells, typically macrophages, where they replicate in a specialized compartment. This capacity of Brucella to survive and replicate within macrophages is essential to their ability to cause disease. In recent years, several groups have identified and characterized small regulatory RNAs (sRNAs) as critical factors in the control of Brucella physiology within macrophages and overall disease virulence. sRNAs are generally < 300 nucleotides in length, and these independent sRNA transcripts are encoded either next to (i.e., cis-encoded) or at a distant location to (i.e., trans-encoded) the genes that they regulate. Trans-encoded sRNAs interact with the mRNA transcripts through short stretches of imperfect base pairing that often require the RNA chaperone Hfq to facilitate sRNA-mRNA interaction. In many instances, these sRNA-mRNA interactions inhibit gene expression, usually by occluding the ribosome-binding site (RBS) and/or by decreasing the stability of the mRNA, leading to degradation of the transcript. A number of sRNAs have been predicted and authenticated in Brucella strains, and a variety of approaches, techniques, and means of validation have been employed in these efforts. Nonetheless, some important issues and considerations regarding the study of sRNA regulation in Brucella need to be addressed. For example, the lack of uniform sRNA nomenclature in Brucella has led to difficulty in comparisons of sRNAs across the different Brucella species, and there exist multiple names in the literature for what are functionally the same sRNA. Moreover, even though bona fide sRNAs have been discovered in Brucella, scant functional information is known about the regulatory activities of these sRNAs, or the extent to which these sRNAs are required for the intracellular life and/or host colonization by the brucellae. Therefore, this review summarizes the historical context of Hfq and sRNAs in Brucella; our current understanding of Brucella sRNAs; and some future perspectives and considerations for the field of sRNA biology in the brucellae.

Plasmodium falciparum Nicotinamidase as A Novel Antimalarial Target

Inhibition of Plasmodium falciparum nicotinamidase could represent a potential antimalarial since parasites require nicotinic acid to successfully recycle nicotinamide to NAD⁺, and importantly, humans lack this biosynthetic enzyme. Recently, mechanism-based inhibitors of nicotinamidase have been discovered. The most potent compound inhibits both recombinant P. falciparum nicotinamidase and parasites replication in infected human red blood cells (RBCs). These studies provide evidence for the importance of nicotinamide salvage through nicotinamidase as a central master player of NAD⁺ homeostasis in P. falciparum.

Antibiotic persistence of intracellular Brucella abortus

Background

Human brucellosis caused by the facultative intracellular pathogen Brucella spp. is an endemic bacterial zoonosis manifesting as acute or chronic infections with high morbidity. Treatment typically involves a combination therapy of two antibiotics for several weeks to months, but despite this harsh treatment relapses occur at a rate of 5–15%. Although poor compliance and reinfection may account for a fraction of the observed relapse cases, it is apparent that the properties of the infectious agent itself may play a decisive role in this phenomenon.

Methodology/Principal findings

We used B. abortus carrying a dual reporter in a macrophage infection model to gain a better understanding of the efficacy of recommended therapies in cellulo. For this we used automated fluorescent microscopy as a prime read-out and developed specific CellProfiler pipelines to score infected macrophages at the population and the single cell level. Combining microscopy of constitutive and induced reporters with classical CFU determination, we quantified the protective nature of the Brucella intracellular lifestyle to various antibiotics and the ability of B. abortus to persist in cellulo despite harsh antibiotic treatments.

Conclusion/Significance

We demonstrate that treatment of infected macrophages with antibiotics at recommended concentrations fails to fully prevent growth and persistence of B. abortus in cellulo, which may be explained by a protective nature of the intracellular niche(s). Moreover, we show the presence of bona fide intracellular persisters upon antibiotic treatment, which are metabolically active and retain the full infectious potential, therefore constituting a plausible reservoir for reinfection and relapse. In conclusion, our results highlight the need to extend the spectrum of models to test new antimicrobial therapies for brucellosis to better reflect the in vivo infection environment, and to develop therapeutic approaches targeting the persister subpopulation.

Brucellosis is a zoonosis endemic to many low- and middle-income countries around the world. Therapies recommended by the WHO are comprised of at least two antibiotics for several weeks, sometimes months. Relapses are frequent despite these harsh treatments. The underlying reasons for these relapses, besides reinfection and non-compliance to treatment, are unknown. Our study shows that Brucella abortus can form so called “persisters” in rich broth but also inside macrophages. This small bacterial subpopulation survives antibiotic treatment and resumes growth after removal of the antibiotics and could therefore serve as a reservoir for relapses in human brucellosis. Furthermore, we show that the intracellular lifestyle of Brucella has protective properties against recommended antibiotics as observed for other intracellular pathogens, highlighting the necessity to develop new infection models to assess antibiotic efficacy.

Immunosuppressive Mechanisms in Brucellosis in Light of Chronic Bacterial Diseases

Brucellosis is considered one of the major zoonoses worldwide, constituting a critical livestock and human health concern with a huge socio-economic burden. Brucella genus, its etiologic agent, is composed of intracellular bacteria that have evolved a prodigious ability to elude and shape host immunity to establish chronic infection. Brucella's intracellular lifestyle and pathogen-associated molecular patterns, such as its specific lipopolysaccharide (LPS), are key factors for hiding and hampering recognition by the immune system. Here, we will review the current knowledge of evading and immunosuppressive mechanisms elicited by Brucella species to persist stealthily in their hosts, such as those triggered by their LPS and cyclic β-1,2-d-glucan or involved in neutrophil and monocyte avoidance, antigen presentation impairment, the modulation of T cell responses and immunometabolism. Attractive strategies exploited by other successful chronic pathogenic bacteria, including Mycobacteria, Salmonella, and Chlamydia, will be also discussed, with a special emphasis on the mechanisms operating in brucellosis, such as granuloma formation, pyroptosis, and manipulation of type I and III IFNs, B cells, innate lymphoid cells, and host lipids. A better understanding of these stratagems is essential to fighting bacterial chronic infections and designing innovative treatments and vaccines.

Brucella activates the host RIDD pathway to subvert BLOS1-directed immune defense

The phagocytosis and destruction of pathogens in lysosomes constitute central elements of innate immune defense. Here, we show that Brucella, the causative agent of brucellosis, the most prevalent bacterial zoonosis globally, subverts this immune defense pathway by activating regulated IRE1α-dependent decay (RIDD) of Bloc1s1 mRNA encoding BLOS1, a protein that promotes endosome-lysosome fusion. RIDD-deficient cells and mice harboring a RIDD-incompetent variant of IRE1α were resistant to infection. Inactivation of the Bloc1s1 gene impaired the ability to assemble BLOC-1-related complex (BORC), resulting in differential recruitment of BORC-related lysosome trafficking components, perinuclear trafficking of Brucella-containing vacuoles (BCVs), and enhanced susceptibility to infection. The RIDD-resistant Bloc1s1 variant maintains the integrity of BORC and a higher-level association of BORC-related components that promote centrifugal lysosome trafficking, resulting in enhanced BCV peripheral trafficking and lysosomal destruction, and resistance to infection. These findings demonstrate that host RIDD activity on BLOS1 regulates Brucella intracellular parasitism by disrupting BORC-directed lysosomal trafficking. Notably, coronavirus murine hepatitis virus also subverted the RIDD-BLOS1 axis to promote intracellular replication. Our work establishes BLOS1 as a novel immune defense factor whose activity is hijacked by diverse pathogens.

Effect of the endoplasmic reticulum stressor tunicamycin in Angomonas deanei heat-shock protein expression and on the association with the endosymbiotic bacterium

The endoplasmic reticulum (ER) presents unique properties to establishing bacterium symbiosis in eukaryotic cells since it synthesizes and glycosylates essential molecules like proteins and lipids. Tunicamycin (TM) is an antibiotic that inhibits the first step in the N-linked glycosylation in eukaryotes and has been used as an ER stress inducer to activate the Unfolded Protein Response (UPR). Mutualistic symbiosis in trypanosomatids is characterized by structural adaptations and intense metabolic exchanges, thus we investigated the effects of TM in the association between Angomonas deanei and its symbiotic bacterium, through ultrastructural and proteomic approaches. Cells treated with the inhibitor showed a decrease in proliferation, enlargement of the ER and Golgi cisternae and an increased distance between the symbiont and the ER. TM proved to be an important tool to better understand ER stress in trypanosomatids, since changes in protein composition were observed in the host protozoan, especially the expression of the Hsp90 chaperone. Furthermore, data obtained indicates the importance of the ER for the adaptation and maintenance of symbiotic associations between prokaryotes and eukaryotes, considering that this organelle has recognized importance in the biogenesis and division of cell structures.

Intracellular Life Cycle of 'Candidatus Liberibacter asiaticus' Inside Psyllid Gut Cells

'Candidatus Liberibacter asiaticus' (CLas), the devastating pathogen related to Huanglongbing (HLB), is a phloem-limited, fastidious, insect-borne bacterium. Rapid spread of HLB disease relies on CLas-efficient propagation in the vector, the Asian citrus psyllid Diaphorina citri, in a circulative manner. Understanding the intracellular lifecycle of CLas in psyllid midgut, the major organ for CLas transmission, is fundamental to improving current management strategies. Using a microscopic approach within CLas-infected insect midgut, we observed the entry of CLas into gut cells inside vesicles, termed Liberibacter-containing vacuoles (LCVs), by endocytosis. Endocytosis is followed by the formation of endoplasmic reticulum-related and replication permissive vacuoles (rLCVs). Additionally, we observed the formation of double membrane autophagosome-like structure, termed autophagy-related vacuole (aLCV). Vesicles containing CLas egress from aLCV and fuse with the cell membrane. Immunolocalization studies showed that CLas uses endocytosis- and exocytosis-like mechanisms that mediates bacterial invasion and egress. Upregulation of autophagy-related genes indicated subversion of host autophagy by CLas in psyllid vector to promote infection. These results indicate that CLas interacts with host cellular machineries to undergo a multistage intracellular cycle through endocytic, secretory, autophagic, and exocytic pathways via complex machineries. Potential tactics for HLB control can be made depending on further investigations on the knowledge of the molecular mechanisms of CLas intracellular cycle.

The VirB System Plays a Crucial Role in Brucella Intracellular Infection

Brucellosis is a highly prevalent zoonotic disease caused by Brucella. Brucella spp. are gram-negative facultative intracellular parasitic bacteria. Its intracellular survival and replication depend on a functional virB system, an operon encoded by VirB1–VirB12. Type IV secretion system (T4SS) encoded by the virB operon is an important virulence factor of Brucella. It can subvert cellular pathway and induce host immune response by secreting effectors, which promotes Brucella replication in host cells and induce persistent infection. Therefore, this paper summarizes the function and significance of the VirB system, focusing on the structure of the VirB system where VirB T4SS mediates biogenesis of the endoplasmic reticulum (ER)-derived replicative Brucella-containing vacuole (rBCV), the effectors of T4SS and the cellular pathways it subverts, which will help better understand the pathogenic mechanism of Brucella and provide new ideas for clinical vaccine research and development.

Idiosyncratic Biogenesis of Intracellular Pathogens-Containing Vacuoles

While most bacterial species taken up by macrophages are degraded through processing of the bacteria-containing vacuole through the endosomal-lysosomal degradation pathway, intravacuolar pathogens have evolved to evade degradation through the endosomal-lysosomal pathway. All intra-vacuolar pathogens possess specialized secretion systems (T3SS-T7SS) that inject effector proteins into the host cell cytosol to modulate myriad of host cell processes and remodel their vacuoles into proliferative niches. Although intravacuolar pathogens utilize similar secretion systems to interfere with their vacuole biogenesis, each pathogen has evolved a unique toolbox of protein effectors injected into the host cell to interact with, and modulate, distinct host cell targets. Thus, intravacuolar pathogens have evolved clear idiosyncrasies in their interference with their vacuole biogenesis to generate a unique intravacuolar niche suitable for their own proliferation. While there has been a quantum leap in our knowledge of modulation of phagosome biogenesis by intravacuolar pathogens, the detailed biochemical and cellular processes affected remain to be deciphered. Here we discuss how the intravacuolar bacterial pathogens Salmonella, Chlamydia, Mycobacteria, Legionella, Brucella, Coxiella, and Anaplasma utilize their unique set of effectors injected into the host cell to interfere with endocytic, exocytic, and ER-to-Golgi vesicle traffic. However, Coxiella is the main exception for a bacterial pathogen that proliferates within the hydrolytic lysosomal compartment, but its T4SS is essential for adaptation and proliferation within the lysosomal-like vacuole.

A Brucella effector modulates the Arf6-Rab8a GTPase cascade to promote intravacuolar replication

Remodeling of host cellular membrane transport pathways is a common pathogenic trait of many intracellular microbes that is essential to their intravacuolar life cycle and proliferation. The bacterium Brucella abortus generates a host endoplasmic reticulum‐derived vacuole (rBCV) that supports its intracellular growth, via VirB Type IV secretion system‐mediated delivery of effector proteins, whose functions and mode of action are mostly unknown. Here, we show that the effector BspF specifically promotes Brucella replication within rBCVs by interfering with vesicular transport between the trans‐Golgi network (TGN) and recycling endocytic compartment. BspF targeted the recycling endosome, inhibited retrograde traffic to the TGN, and interacted with the Arf6 GTPase‐activating Protein (GAP) ACAP1 to dysregulate Arf6‐/Rab8a‐dependent transport within the recycling endosome, which resulted in accretion of TGN‐associated vesicles by rBCVs and enhanced bacterial growth. Altogether, these findings provide mechanistic insight into bacterial modulation of membrane transport used to promote their own proliferation within intracellular vacuoles.

Edwardsiella piscicida Interferes with Classical Endocytic Trafficking and Replicates in a Specialized Replication-Permissive Niche in Nonphagocytic Cells

Edwardsiella piscicida is an intracellular pathogen within a broad spectrum of hosts. Essential to E. piscicida's virulence is its ability to invade and replicate inside host cells, yet the survival mechanisms and the nature of the replicative compartment remain unknown. Here, we characterized its intracellular lifestyle in nonphagocytic cells and showed that the intracellular replication of E. piscicida in nonphagocytic cells is dependent on its type III secretion system (T3SS) but not its type VI secretion system. Following internalization, E. piscicida is contained in vacuoles that transiently mature into early endosomes but subsequently bypasses the classical endosome pathway and fusion with lysosomes, which depend on its T3SS. Following rapid escape from the degradative pathway, E. piscicida was found to create a specialized replication-permissive niche characterized by endoplasmic reticulum (ER) markers. Furthermore, we found that a T3SS effector, EseJ, is responsible for the intracellular replication of E. piscicida by preventing endosome/lysosome fusion. In vivo experiments also confirmed that EseJ is necessary for bacterial colonization by E. piscicida in the epithelial layer, followed by systemic dissemination in both zebrafish and mice. Thus, this work elucidates the tactics used by E. piscicida to survive and proliferate within host nonphagocytic cells. IMPORTANCE E. piscicida is a facultative intracellular bacterium associated with septicemia and fatal infections in many animals, including fish and humans. However, little is known about its intracellular life, which is important for successful invasion of the host. The present study is the first comprehensive characterization of E. piscicida's intracellular lifestyle in host cells. Upon internalization, E. piscicida is transiently contained in Rab5-positive vacuoles, but the pathogen prevents further endosome maturation and fusion with lysosomes by utilizing a T3SS effector, EseJ. In addition, the bacterium creates a specialized replication niche for rapid growth via an interaction with the ER. Our study provides new insights into the strategies used by E. piscicida to successfully establish an intracellular lifestyle that contributes to its survival and dissemination during infection.

Intracellular Passage Triggers a Molecular Response in Brucella abortus That Increases Its Infectiousness

Brucella abortus is a facultatively extracellular-intracellular pathogen that encounters a diversity of environments within the host cell. We report that bacteria extracted from infected cells at late stages (48 h postinfection) of the intracellular life cycle significantly increase their ability to multiply in new target cells. This increase depends on early interaction with the cell surface, since the bacteria become more adherent and penetrate more efficiently than in vitro-grown bacteria. At this late stage of infection, the bacterium locates within an autophagosome-like compartment, facing starvation and acidic conditions. At this point, the BvrR/BvrS two-component system becomes activated, and the expression of the transcriptional regulator VjbR and the type IV secretion system component VirB increases. Using bafilomycin to inhibit BvrR/BvrS activation and using specific inhibitors for VjbR and VirB, we showed that the BvrR/BvrS and VjbR systems correlate with increased interaction with new host cells, while the VirB system does not. Bacteria released from infected cells under natural conditions displayed the same phenotype as intracellular bacteria. We propose a model in which the B. abortus BvrR/BvrS system senses the transition from its replicative niche at the endoplasmic reticulum to the autophagosome-like exit compartment. This activation leads to the expression of VirB, which participates in the release of the bacterium from the cells, and an increase in VjbR expression that results in a more efficient interaction with new host cells.

Regulatory Networks of the T4SS Control: From Host Cell Sensing to the Biogenesis and the Activity during the Infection

Delivery of effectors, DNA or proteins, that hijack host cell processes to the benefit of bacteria is a mechanism widely used by bacterial pathogens. It is achieved by complex effector injection devices, the secretion systems, among which Type 4 Secretion Systems (T4SSs) play a key role in bacterial virulence of numerous animal and plant pathogens. Considerable progress has recently been made in the structure-function analyses of T4SSs. Nevertheless, the signals and processes that trigger machine assembly and activity during infection, as well as those involved in substrate recognition and transfer, are complex and still poorly understood. In this review, we aim at summarizing the last updates of the knowledge on signaling pathways that regulate the biogenesis and the activity of T4SSs in important bacterial pathogens.

Brucella: Reservoirs and Niches in Animals and Humans

Brucella is an intracellular bacterium that causes abortion, reproduction failure in livestock and leads to a debilitating flu-like illness with serious chronic complications if untreated in humans. As a successful intracellular pathogen, Brucella has developed strategies to avoid recognition by the immune system of the host and promote its survival and replication. In vivo, Brucellae reside mostly within phagocytes and other cells including trophoblasts, where they establish a preferred replicative niche inside the endoplasmic reticulum. This process is central as it gives Brucella the ability to maintain replicating-surviving cycles for long periods of time, even at low bacterial numbers, in its cellular niches. In this review, we propose that Brucella takes advantage of the environment provided by the cellular niches in which it resides to generate reservoirs and disseminate to other organs. We will discuss how the favored cellular niches for Brucella infection in the host give rise to anatomical reservoirs that may lead to chronic infections or persistence in asymptomatic subjects, and which may be considered as a threat for further contamination. A special emphasis will be put on bone marrow, lymph nodes, reproductive and for the first time adipose tissues, as well as wildlife reservoirs.

Characteristics of Brucella abortus vaccine strain A19 reveals its potential mechanism of attenuated virulence

Brucella vaccination is one of the most important strategies for controlling brucellosis in livestock. The A19 strain was the effective vaccine used to control brucellosis in China. However, the characteristics of physiological and attenuated virulence of the A19 strain are not investigated in detail. In this study, we compared the phenotypic characteristics of the A19 to the wild-type strain S2308. Virulence test showed that the A19 was significantly attenuated at chronic infection stage in infected mouse model. In growth analysis, the A19 exhibited a quick growth at exponential phase and premature at stationary phase. The inflammatory response of macrophages infected by the A19 was detected using TaqMan qPCR assay, indicating that the inflammatory level of the A19-infected macrophages was higher than that of the S2308 infection. Cell death analysis showed that the A19 was not cytotoxic for macrophages. Cell infection showed that the A19 reduced its ability to invade, survive and traffic within host cells, and the intracellular A19 hardly excludes lysosome-associated marker LAMP-1, suggesting that the A19 can't escape the lysosome degradation within host cells. In further study, the sensitivity test exhibited that the A19 is more sensitive to stress and bactericidal factors than the S2308 strain, Western blot and silver staining analysis exhibited that the A19 has a different expression pattern of OMPs and reduces LPS O-antigen expression relative to the S2308 strain. Those data give us a more detailed understanding about the A19 vaccine strain, which will be beneficial for improvement of current Brucella vaccine and overcoming its defects.

Adhesins of Brucella: Their Roles in the Interaction with the Host

A central aspect of Brucella pathogenicity is its ability to invade, survive, and replicate in diverse phagocytic and non-phagocytic cell types, leading to chronic infections and chronic inflammatory phenomena. Adhesion to the target cell is a critical first step in the invasion process. Several Brucella adhesins have been shown to mediate adhesion to cells, extracellular matrix components (ECM), or both. These include the sialic acid-binding proteins SP29 and SP41 (binding to erythrocytes and epithelial cells, respectively), the BigA and BigB proteins that contain an Ig-like domain (binding to cell adhesion molecules in epithelial cells), the monomeric autotransporters BmaA, BmaB, and BmaC (binding to ECM components, epithelial cells, osteoblasts, synoviocytes, and trophoblasts), the trimeric autotransporters BtaE and BtaF (binding to ECM components and epithelial cells) and Bp26 (binding to ECM components). An in vivo role has also been shown for the trimeric autotransporters, as deletion mutants display decreased colonization after oral and/or respiratory infection in mice, and it has also been suggested for BigA and BigB. Several adhesins have shown unipolar localization, suggesting that Brucella would express an adhesive pole. Adhesin-based vaccines may be useful to prevent brucellosis, as intranasal immunization in mice with BtaF conferred high levels of protection against oral challenge with B. suis.

A genomic island in Brucella involved in the adhesion to host cells: Identification of a new adhesin and a translocation factor

Adhesion to host cells is the first step in the virulence cycle of any pathogen. In Gram-negative bacteria, adhesion is mediated, among other virulence factors such as the lipopolysaccharides, by specific outer-membrane proteins generally termed adhesins that belong to a wide variety of families and have different evolutionary origins. In Brucella, a widespread zoonotic pathogen of animal and human health concern, adhesion is central as it may determine the intracellular fate of the bacterium, an essential stage in its pathogenesis. In the present paper, we further characterised a genomic locus that we have previously reported encodes an adhesin (BigA) with a bacterial immunoglobulin-like domain (BIg-like). We found that this region encodes a second adhesin, which we have named BigB; and PalA, a periplasmic protein necessary for the proper display in the outer membrane of BigA and BigB. Deletion of bigB or palA diminishes the adhesion of the bacterium and overexpression of BigB dramatically increases it. Incubation of cells with the recombinant BIg-like domain of BigB induced important cytoskeletal rearrangements and affected the focal adhesion sites indicating that the adhesin targets cell-cell or cell-matrix proteins. We additionally show that PalA has a periplasmic localisation and is completely necessary for the proper display of BigA and BigB, probably avoiding their aggregation and facilitating their transport to the outer membrane. Our results indicate that this genomic island is entirely devoted to the adhesion of Brucella to host cells.

Intracellular Growth and Cell Cycle Progression are Dependent on (p)ppGpp Synthetase/Hydrolase in Brucella abortus

Brucella abortus is a pathogenic bacterium able to proliferate inside host cells. During the first steps of its trafficking, it is able to block the progression of its cell cycle, remaining at the G1 stage for several hours, before it reaches its replication niche. We hypothesized that starvation mediated by guanosine tetra- or penta-phosphate, (p)ppGpp, could be involved in the cell cycle arrest. Rsh is the (p)ppGpp synthetase/hydrolase. A B. abortus ∆rsh mutant is unable to grow in minimal medium, it is unable to survive in stationary phase in rich medium and it is unable to proliferate inside RAW 264.7 macrophages. A strain producing the heterologous constitutive (p)ppGpp hydrolase Mesh1b is also unable to proliferate inside these macrophages. Altogether, these data suggest that (p)ppGpp is necessary to allow B. abortus to adapt to its intracellular growth conditions. The deletion of dksA, proposed to mediate a part of the effect of (p)ppGpp on transcription, does not affect B. abortus growth in culture or inside macrophages. Expression of a gene coding for a constitutively active (p)ppGpp synthetase slows down growth in rich medium and inside macrophages. Using an mCherry–ParB fusion able to bind to the replication origin of the main chromosome of B. abortus, we observed that expression of the constitutive (p)ppGpp synthetase gene generates an accumulation of bacteria at the G1 phase. We thus propose that (p)ppGpp accumulation could be one of the factors contributing to the G1 arrest observed for B. abortus in RAW 264.7 macrophages.

Identification of novel targets for host-directed therapeutics against intracellular Staphylococcus aureus

During patient colonization, Staphylococcus aureus is able to invade and proliferate within human cells to evade the immune system and last resort drugs such as vancomycin. Hijacking specific host molecular factors and/or pathways is necessary for pathogens to successfully establish an intracellular infection. In this study, we employed an unbiased shRNA screening coupled with ultra-fast sequencing to screen 16,000 human genes during S. aureus infection and we identified several host genes important for this intracellular pathogen. In addition, we interrogated our screening results to find novel host-targeted therapeutics against intracellular S. aureus. We found that silencing the human gene TRAM2 resulted in a significant reduction of intracellular bacterial load while host cell viability was restored, showing its importance during intracellular infection. Furthermore, TRAM2 is an interactive partner of the endoplasmic reticulum SERCA pumps and treatment with the SERCA-inhibitor Thapsigargin halted intracellular MRSA survival. Our results suggest that Thapsigargin could be repurposed to tackle S. aureus host cell infection in combination with conventional antibiotics.

Occurrence and repair of alkylating stress in the intracellular pathogen Brucella abortus

It is assumed that intracellular pathogenic bacteria have to cope with DNA alkylating stress within host cells. Here we use single-cell reporter systems to show that the pathogen Brucella abortus does encounter alkylating stress during the first hours of macrophage infection. Genes encoding direct repair and base-excision repair pathways are required by B. abortus to face this stress in vitro and in a mouse infection model. Among these genes, ogt is found to be under the control of the conserved cell-cycle transcription factor GcrA. Our results highlight that the control of DNA repair in B. abortus displays distinct features that are not present in model organisms such as Escherichia coli.

It is assumed that intracellular pathogenic bacteria must cope with DNA alkylating stress within host cells. Here, Poncin et al. show that the pathogen Brucella abortus does encounter alkylating stress within macrophages, and shed light into the pathways required for DNA repair in this organism.

Bacteria Exploit Autophagy For Their Own Benefit

Autophagy is a lysosomal degradation pathway to clear long-lived proteins, protein aggregates, and damaged organelles. Certain microorganisms can be eliminated by an autophagic degradation process termed xenophagy. However, many pathogens deploy highly evolved mechanisms to evade autophagic degradation. What is more, series of pathogens have developed different strategies to exploit autophagy to ensure their survival. These bacteria could induce autophagy and/or prevent autophagosomes fusion with lysosomes through secreted effector proteins or utilizing host components, thereby maintaining the localization of the bacteria within the autophagosomes where they replicate. Here, we review the current knowledge of the mechanisms developed by the bacteria to benefit from autophagy for their survival.

Immune Response to Mucosal Brucella Infection

Brucellosis is one of the most prevalent bacterial zoonosis of worldwide distribution. The disease is caused by Brucella spp., facultative intracellular pathogens. Brucellosis in animals results in abortion of fetuses, while in humans, it frequently manifests flu-like symptoms and a typical undulant fever, being osteoarthritis a common complication of the chronic infection. The two most common ways to acquire the infection in humans are through the ingestion of contaminated dairy products or by inhalation of contaminated aerosols. Brucella spp. enter the body mainly through the gastrointestinal and respiratory mucosa; however, most studies of immune response to Brucella spp. are performed analyzing models of systemic immunity. It is necessary to better understand the mucosal immune response induced by Brucella infection since this is the main entry site for the bacterium. In this review, some virulence factors and the mechanisms needed for pathogen invasion and persistence are discussed. Furthermore, some aspects of local immune responses induced during Brucella infection will be reviewed. With this knowledge, better vaccines can be designed focused on inducing protective mucosal immune response.

A Chemical Genetics Screen Reveals Influence of p38 Mitogen-Activated Protein Kinase and Autophagy on Phagosome Development and Intracellular Replication of Brucella neotomae in Macrophages

Brucella is an intracellular bacterial pathogen that causes chronic systemic infection in domesticated livestock and poses a zoonotic infectious risk to humans. The virulence of Brucella is critically dependent on its ability to replicate and survive within host macrophages. Brucella modulates host physiological pathways and cell biology in order to establish a productive intracellular replicative niche. Conversely, the host cell presumably activates pathways that limit infection. To identify host pathways contributing to this yin and yang during host cell infection, we performed a high-throughput chemical genetics screen of known inhibitors and agonists of host cell targets to identify host factors that contribute to intracellular growth of the model pathogen Brucella neotomae Using this approach, we identified the p38 mitogen-activated protein (MAP) kinase pathway and autophagy machinery as both a linchpin and an Achilles' heel in B. neotomae's ability to coopt host cell machinery and replicate within macrophages. Specifically, B. neotomae induced p38 MAP kinase phosphorylation and autophagy in a type IV secretion system-dependent fashion. Both p38 MAP kinase stimulation and an intact autophagy machinery in turn were required for phagosome maturation and intracellular replication. These findings contrasted with those for Legionella pneumophila, where chemical inhibition of the p38 MAP kinase pathway and autophagy factor depletion failed to block intracellular replication. Therefore, results from a chemical genetics screen suggest that intersections of the MAP kinase pathways and autophagy machinery are critical components of Brucella's intracellular life cycle.

Intra-amoebic localization of Arcobacter butzleri as an endocytobiont of Acanthamoeba castellanii

Acanthamoeba castellanii is a free-living amoeba found mainly in humid environments and Arcobacter butzleri is an emerging zoonotic pathogen, both can establish in vitro endosymbiotic relationships in the absence of bacterial replication. We analyzed the localization of A. butzleri within A. castellanii establishing their association with endoplasmic reticulum vesicles and mitochondria. Through confocal microscopy, we observed that during the early stages of endosymbiosis, there is not colocalization between amoebic vacuoles containing A. butzleri and mitochondria or ER vesicles of A. castellanii. Considering that energy production of this bacterium occurs via metabolism of amino acids or the tricarboxylic acid cycle, these results contribute to explain the absence of bacterial replication, since A. butzleri would not have access to the nutrients found in endoplasmic reticulum vesicles and mitochondria. In addition, we observe that A. butzleri induces significantly the actin polymerization of A. castellanii during the early stages of endosymbiosis.

A Role for the VPS Retromer in Brucella Intracellular Replication Revealed by Genomewide siRNA Screening

Brucella, the agent causing brucellosis, is a major zoonotic pathogen with worldwide distribution. Brucella resides and replicates inside infected host cells in membrane-bound compartments called Brucella-containing vacuoles (BCVs). Following uptake, Brucella resides in endosomal BCVs (eBCVs) that gradually mature from early to late endosomal features. Through a poorly understood process that is key to the intracellular lifestyle of Brucella, the eBCV escapes fusion with lysosomes by transitioning to the replicative BCV (rBCV), a replicative niche directly connected to the endoplasmic reticulum (ER). Despite the notion that this complex intracellular lifestyle must depend on a multitude of host factors, a holistic view on which of these components control Brucella cell entry, trafficking, and replication is still missing. Here we used a systematic cell-based small interfering RNA (siRNA) knockdown screen in HeLa cells infected with Brucella abortus and identified 425 components of the human infectome for Brucella infection. These include multiple components of pathways involved in central processes such as the cell cycle, actin cytoskeleton dynamics, or vesicular trafficking. Using assays for pathogen entry, knockdown complementation, and colocalization at single-cell resolution, we identified the requirement of the VPS retromer for Brucella to escape the lysosomal degradative pathway and to establish its intracellular replicative niche. We thus validated the VPS retromer as a novel host factor critical for Brucella intracellular trafficking. Further, our genomewide data shed light on the interplay between central host processes and the biogenesis of the Brucella replicative niche.IMPORTANCE With >300,000 new cases of human brucellosis annually, Brucella is regarded as one of the most important zoonotic bacterial pathogens worldwide. The agent causing brucellosis resides inside host cells within vacuoles termed Brucella-containing vacuoles (BCVs). Although a few host components required to escape the degradative lysosomal pathway and to establish the ER-derived replicative BCV (rBCV) have already been identified, the global understanding of this highly coordinated process is still partial, and many factors remain unknown. To gain deeper insight into these fundamental questions, we performed a genomewide RNA interference (RNAi) screen aiming at discovering novel host factors involved in the Brucella intracellular cycle. We identified 425 host proteins that contribute to Brucella cellular entry, intracellular trafficking, and replication. Together, this study sheds light on previously unknown host pathways required for the Brucella infection cycle and highlights the VPS retromer components as critical factors for the establishment of the Brucella intracellular replicative niche.

Brucella - Virulence Factors, Pathogenesis and Treatment

Brucellae are Gram-negative, small rods infecting mammals and capable of causing disease called brucellosis. The infection results in abortion and sterility in domestic animals (sheeps, pigs, rams etc). Especially dangerous for humans are: Brucella melitensis, Brucella suis, Brucella abortus, and Brucella canis that trigger unspecific symptoms (flu-like manifestation). Brucella rods are introduced via host cells, by inhalation, skin abrasions, ingestion or mucosal membranes. The most important feature of Brucella is the ability to survive and multiply within both phagocytic and non-phagocytic cells. Brucella does not produce classical virulence factors: exotoxin, cytolisins, exoenzymes, plasmids, fimbria, and drug resistant forms. Major virulence factors are: lipopolysaccharide (LPS), T4SS secretion system and BvrR/BvrS system, which allow interaction with host cell surface, formation of an early, late BCV (Brucella Containing Vacuole) and interaction with endoplasmic reticulum (ER) when the bacteria multiply. The treatment of brucellosis is based on two-drug therapy, the most common combinations of antibiotics are: doxycycline with rifampicin or fluoroquinolones with rifampicin. Currently, also other methods are used to disrupt Brucella intracellular replication (tauroursodeoxycholic acid or ginseng saponin fraction A).

Brucella abortus Cyclic Dinucleotides Trigger STING-Dependent Unfolded Protein Response That Favors Bacterial Replication

Brucella abortus is a facultative intracellular bacterium that causes brucellosis, a prevalent zoonosis that leads to abortion and infertility in cattle, and undulant fever, debilitating arthritis, endocarditis, and meningitis in humans. Signaling pathways triggered by B. abortus involves stimulator of IFN genes (STING), which leads to production of type I IFNs. In this study, we evaluated the pathway linking the unfolded protein response (UPR) and the endoplasmic reticulum-resident transmembrane molecule STING, during B. abortus infection. We demonstrated that B. abortus infection induces the expression of the UPR target gene BiP and XBP1 in murine macrophages through a STING-dependent pathway. Additionally, we also observed that STING activation was dependent on the bacterial second messenger cyclic dimeric GMP. Furthermore, the Brucella-induced UPR is crucial for induction of multiple molecules linked to type I IFN signaling pathway, such as IFN-β, IFN regulatory factor 1, and guanylate-binding proteins. Furthermore, IFN-β is also important for the UPR induction during B. abortus infection. Indeed, IFN-β shows a synergistic effect in inducing the IRE1 axis of the UPR. In addition, priming cells with IFN-β favors B. abortus survival in macrophages. Moreover, Brucella-induced UPR facilitates bacterial replication in vitro and in vivo. Finally, these results suggest that B. abortus-induced UPR is triggered by bacterial cyclic dimeric GMP, in a STING-dependent manner, and that this response supports bacterial replication. In summary, association of STING and IFN-β signaling pathways with Brucella-induced UPR unravels a novel link between innate immunity and endoplasmic reticulum stress that is crucial for bacterial infection outcome.

The Intracellular Life Cycle of Brucella spp

Bacteria of the genus Brucella colonize a wide variety of mammalian hosts, in which their infectious cycle and ability to cause disease predominantly rely on an intracellular lifestyle within phagocytes. Upon entry into host cells, Brucella organisms undergo a complex, multistage intracellular cycle in which they sequentially traffic through, and exploit functions of, the endocytic, secretory, and autophagic compartments via type IV secretion system (T4SS)-mediated delivery of bacterial effectors. These effectors modulate an array of host functions and machineries to first promote conversion of the initial endosome-like Brucella-containing vacuole (eBCV) into a replication-permissive organelle derived from the host endoplasmic reticulum (rBCV) and then to an autophagy-related vacuole (aBCV) that mediates bacterial egress. Here we detail and discuss our current knowledge of cellular and molecular events of the Brucella intracellular cycle. We discuss the importance of the endosomal stage in determining T4SS competency, the roles of autophagy in rBCV biogenesis and aBCV formation, and T4SS-driven mechanisms of modulation of host secretory traffic in rBCV biogenesis and bacterial egress.

Learning from the master: targets and functions of the CtrA response regulator in Brucella abortus and other alpha-proteobacteria

The α-proteobacteria are a fascinating group of free-living, symbiotic and pathogenic organisms, including the Brucella genus, which is responsible for a worldwide zoonosis. One common feature of α-proteobacteria is the presence of a conserved response regulator called CtrA, first described in the model bacterium Caulobacter crescentus, where it controls gene expression at different stages of the cell cycle. Here, we focus on Brucella abortus and other intracellular α-proteobacteria in order to better assess the potential role of CtrA in the infectious context. Comparative genomic analyses of the CtrA control pathway revealed the conservation of specific modules, as well as the acquisition of new factors during evolution. The comparison of CtrA regulons also suggests that specific clades of α-proteobacteria acquired distinct functions under its control, depending on the essentiality of the transcription factor. Other CtrA-controlled functions, for instance motility and DNA repair, are proposed to be more ancestral. Altogether, these analyses provide an interesting example of the plasticity of a regulation network, subject to the constraints of inherent imperatives such as cell division and the adaptations to diversified environmental niches.

Tiny architects: biogenesis of intracellular replicative niches by bacterial pathogens

Co-evolution of bacterial pathogens with their hosts led to the emergence of a stunning variety of strategies aiming at the evasion of host defences, colonisation of host cells and tissues and, ultimately, the establishment of a successful infection. Pathogenic bacteria are typically classified as extracellular and intracellular; however, intracellular lifestyle comes in many different flavours: some microbes rapidly escape to the cytosol whereas other microbes remain within vacuolar compartments and harness membrane trafficking pathways to generate their host-derived, pathogen-specific replicative niche. Here we review the current knowledge on a variety of vacuolar lifestyles, the effector proteins used by bacteria as tools to take control of the host cell and the main membrane trafficking signalling pathways targeted by vacuolar pathogens as source of membranes and nutrients. Finally, we will also discuss how host cells have developed countermeasures to sense the biogenesis of the aberrant organelles harbouring bacteria. Understanding the dialogue between bacterial and eukaryotic proteins is the key to unravel the molecular mechanisms of infection and in turn, this may lead to the identification of new targets for the development of new antimicrobials.

Ehrlichia type IV secretion system effector Etf-2 binds to active RAB5 and delays endosome maturation

Ehrlichia chaffeensis, an obligatory intracellular bacterium, infects monocytes/macrophages by sequestering a regulator of endosomal traffic, the small GTPase RAB5, on its membrane-bound inclusions to avoid routing to host-cell phagolysosomes. How RAB5 is sequestered on ehrlichial inclusions is poorly understood, however. We found that native Ehrlichia translocated factor-2 (Etf-2), a previously predicted effector of the Ehrlichia type IV secretion system, and recombinant Etf-2 (cloned into the Ehrlichia genome) are secreted into the host-cell cytoplasm and localize to ehrlichial inclusions. Ectopically expressed Etf-2-GFP also localized to inclusions and membranes of early endosomes marked with RAB5 and interacted with GTP-bound RAB5 but not with a GDP-bound RAB5. Etf-2, although lacking a RAB GTPase-activating protein (GAP) Tre2-Bub2-Cdc16 (TBC) domain, contains two conserved TBC domain motifs, namely an Arg finger and a Gln finger, and site-directed mutagenesis revealed that both Arg188 and Gln245 are required for Etf-2 localization to early endosomes. The yeast two-hybrid assay and microscale thermophoresis revealed that Etf-2 binds tightly to GTP-bound RAB5 but not to GDP-bound RAB5. However, Etf-2 lacks RAB5-specific GAP activity. Etf-2 localized to bead-containing phagosomes as well as endosomes containing beads coated with the C-terminal fragment of EtpE (entry-triggering protein of Ehrlichia), an Ehrlichia outer-membrane invasin, and significantly delayed RAB5 dissociation from and RAB7 localization to phagosomes/endosomes and RABGAP5 localization to endosomes. Thus, binding of Etf-2 to RAB5-GTP appears to delay RAB5 inactivation by impeding RABGAP5 localization to endosomes. This suggests a unique mechanism by which RAB5 is sequestered on ehrlichial inclusions to benefit bacterial survival and replication.

Analysis of protein expression in Brucella abortus mutants with different growth rates by two-dimensional gel electrophoresis and LC-MS/MS peptide analysis

Brucella abortus is a bacterium that causes brucellosis and is the causative agent of worldwide zoonoses. Pathogenesis of the B. abortus infection is complicated, and several researchers have attempted to elucidate the infection mechanism of B. abortus. While several proteins have been revealed as pathogenic factors by previous researchers, the underlying mechanism of B. abortus infection is unresolved. In this study, we identified proteins showing different expression levels in B. abortus mutants with different biological characteristics that were generated by random insertion of a transposon. Five mutants were selected based on biological characteristics, in particular, their growth features. Total proteins of mutant and wild-type B. abortus were purified and subjected to two-dimensional gel electrophoresis. Thirty protein spots of each mutant with expression increases or decreases were selected; those with a change of more than 2-fold were compared with the wild-type. Selected spots underwent liquid chromatography tandem mass spectrometry for peptide analysis. DnaK and ClpB, involved in protein aggregation, increased. SecA and GAPDH, associated with energy metabolism, decreased in some mutants with a growth rate slower than that of the wild-type. Mutants with slower growth showed a decrease in energy metabolism-related proteins, while mutants with faster growth showed an increase in pathogenicity-related proteins.