The Brucella abortus Cu,Zn superoxide dismutase is required for optimal resistance to oxidative killing by murine macrophages and wild-type virulence in experimentally infected mice

Unraveling the Role of the Zinc-Dependent Metalloproteinase/HTH-Xre Toxin/Antitoxin (TA) System of Brucella abortus in the Oxidative Stress Response: Insights into the Stress Response and Virulence

Toxin/antitoxin (TA) systems have been scarcely studied in Brucella abortus, the causative agent of brucellosis, which is one of the most prevalent zoonotic diseases worldwide. In this study, the roles of a putative type II TA system composed by a Zinc-dependent metalloproteinase (ZnMP) and a transcriptional regulator HTH-Xre were evaluated. The deletion of the open reading frame (ORF) BAB1_0270, coding for ZnMP, used to produce a mutant strain, allowed us to evaluate the survival and gene expression of B. abortus 2308 under oxidative conditions. Our results showed that the B. abortus mutant strain exhibited a significantly reduced capacity to survive under hydrogen peroxide-induced oxidative stress. Furthermore, this mutant strain showed a decreased expression of genes coding for catalase (katE), alkyl hydroperoxide reductase (ahpC) and transcriptional regulators (oxyR and oxyR-like), as well as genes involved in the general stress response, phyR and rpoE1, when compared to the wild-type strain. These findings suggest that this type II ZnMP/HTH-Xre TA system is required by B. abortus to resist oxidative stress. Additionally, previous evidence has demonstrated that this ZnMP also participates in the acidic stress resistance and virulence of B. abortus 2308. Therefore, we propose a hypothetical regulatory function for this ZnMP/HTH-Xre TA system, providing insight into the stress response and its potential roles in the pathogenesis of B. abortus.

N,N-Dimethyldithiocarbamate Elicits Pneumococcal Hypersensitivity to Copper and Macrophage-Mediated Clearance

Streptococcus pneumoniae is a Gram-positive, encapsulated bacterium that is a significant cause of disease burden in pediatric and elderly populations. The rise in unencapsulated disease-causing strains and antimicrobial resistance in S. pneumoniae has increased the need for developing new antimicrobial strategies. Recent work by our laboratory has identified N,N-dimethyldithiocarbamate (DMDC) as a copper-dependent antimicrobial against bacterial, fungal, and parasitic pathogens. As a bactericidal antibiotic against S. pneumoniae, DMDC's ability to work as a copper-dependent antibiotic and its ability to work in vivo warranted further investigation. Here, our group studied the mechanisms of action of DMDC under various medium and excess-metal conditions and investigated DMDC's interactions with the innate immune system in vitro and in vivo. Of note, we found that DMDC plus copper significantly increased the internal copper concentration, hydrogen peroxide stress, nitric oxide stress, and the in vitro macrophage killing efficiency and decreased capsule. Furthermore, we found that in vivo DMDC treatment increased the quantity of innate immune cells in the lung during infection. Taken together, this study provides mechanistic insights regarding DMDC's activity as an antibiotic at the host-pathogen interface.

Lysine Acylation Modification Landscape of Brucella abortus Proteome and its Virulent Proteins

The myriad of posttranslational modifications (PTMs) of proteins that occur in all living cells are crucial to all kinds of biological processes. Brucella is an intracellular parasitic bacterium that can cause chronic diseases in both humans and livestock. To reveal the relationship between PTMs and the virulence and survival of Brucella, we described the first comprehensive multiple PTM-omics atlas of B. abortus 2308. Five PTMs involving lysine, namely 2-hydroxyisobutyrylation, succinylation, crotonylation, acetylation, and malonylation were identified. Nearly 2,000 modified proteins were observed, and these proteins took part in many biological processes, with a variety of molecular functions. In addition, we detected many significant virulence factors of Brucella among the modified proteins. 10 of the 15 T4SS effector proteins were detected with one or more PTMs. Moreover, abundant PTMs were detected in other typical virulence factors. Considering the role of PTMs in various biological processes of Brucella virulence and survival, we propose that the virulence of Brucella is associated with the PTMs of proteins. Taken together, this study provides the first global survey of PTMs in Brucella. This is a prospective starting point for further functional analysis of PTMs during the survival of Brucella in hosts, interpretation of the function of Brucella proteins, and elucidation of the pathogenic mechanism of Brucella.

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.

An ArsR Transcriptional Regulator Facilitates Brucella sp. Survival via Regulating Self and Outer Membrane Protein

The arsenic acid-resistant (ArsR) family transcriptional regulators are widely distributed in microorganisms, including in the facultative intracellular pathogen Brucella spp. ArsR proteins are implicated in numerous biological processes. However, the specific roles of ArsR family members in Brucella remain obscure. Here, we show that ArsR6 (BSS2_RS07325) is required for Brucella survival both under heat, oxidative, and osmotic stress and in a murine infection model in vivo. RNA-seq and ChIP-seq reveal that 34 potential target genes for ArsR6 protein were identified, among which eight genes were up-regulated and 26 genes were down-regulated, including outer membrane protein 25D (Omp25D). ArsR6 autoregulates its own expression to maintain bacterial intracellular Cu/Ni homeostasis to benefit bacterial survival in hostile environments. Moreover, ArsR6 also regulates the production of virulence factor Omp25D, which is important for the survival of Brucella under stress conditions. Significantly, Omp25D deletion strain attenuated in a murine infection model in vivo. Altogether, our findings reveal a unique mechanism in which the ArsR family member ArsR6 autoregulates its expression and also modulates Omp25D expression to maintain metal ion homeostasis and virulence in Brucella.

An extracytoplasmic function (ECF) sigma/anti-sigma factor system regulates hypochlorous acid resistance and impacts expression of the type IV secretion system in Brucella melitensis

The intracellular bacterial pathogen Brucella causes persistent infections in various mammalian species. To survive and replicate within macrophages, these bacteria must be able to withstand oxidative stresses and express the type IV secretion system (T4SS) to evade host immune responses. The extracytoplasmic function (ECF) sigma factor system is a major signal transduction mechanism in bacteria that senses environmental cues and responds by regulating gene expression. In this study, we defined an ECF σ bcrS and its cognate anti-σ factor abcS in Brucella melitensis M28 by conserved domain analysis and a protein interaction assay. BcrS directly activates an adjacent operon, bcrXQP, that encodes a methionine-rich peptide and a putative methionine sulfoxide reductase system, whereas AbcS is a negative regulator of bcrS and bcrXQP. The bcrS-abcS and bcrXQP operons can be induced by hypochlorous acid and contribute to hypochlorous acid resistance in vitro. Next, RNA sequencing analysis and genome-wide recognition sequence search identified the regulons of BcrS and AbcS. Interestingly, we found that BcrS positively influences T4SS expression in an AbcS-dependent manner and that AbcS also affects T4SS expression independently of BcrS. Last, we demonstrate that abcS is required for the maintenance of persistent infection, while bcrS is dispensable in a mouse infection model. Collectively, we conclude that BcrS and AbcS influence expression of multiple genes responsible for Brucella virulence traits.

IMPORTANCEBrucella is a notorious intracellular pathogen that induces chronic infections in animals and humans. To survive and replicate within macrophages, these bacteria require a capacity to withstand oxidative stresses and to express the type IV secretion system (T4SS) to combat host immune responses. In this study, we characterized an extracytoplasmic function sigma/anti-sigma factor system that regulates resistance to reactive chlorine species and T4SS expression, thereby establishing a potential link between two crucial virulence traits of Brucella. Furthermore, the anti-sigma factor AbcS contributes to Brucella persistent infection of mice. Thus, this work provides novel insights into Brucella virulence regulation as well as a potential drug target for fighting Brucella infections.

Uncovering the Hidden Credentials of Brucella Virulence

Bacteria in the genus Brucella are important human and veterinary pathogens. The abortion and infertility they cause in food animals produce economic hardships in areas where the disease has not been controlled, and human brucellosis is one of the world’s most common zoonoses. Brucella strains have also been isolated from wildlife, but we know much less about the pathobiology and epidemiology of these infections than we do about brucellosis in domestic animals.

Characterizing the transport and utilization of the neurotransmitter GABA in the bacterial pathogen Brucella abortus

The neurotransmitter gamma-aminobutyric acid (GABA) is the most abundant inhibitory neurotransmitter in the human brain; however, it is becoming more evident that this non-proteinogenic amino acid plays multiple physiological roles in biology. In the present study, the transport and function of GABA is studied in the highly infectious intracellular bacterium Brucella abortus. The data show that 3H-GABA is imported by B. abortus under nutrient limiting conditions and that the small RNAs AbcR1 and AbcR2 negatively regulate this transport. A specific transport system, gts, is responsible for the transport of GABA as determined by measuring 3H-GABA transport in isogenic deletion strains of known AbcR1/2 regulatory targets; however, this locus is unnecessary for Brucella infection in BALB/c mice. Similar assays revealed that 3H-GABA transport is uninhibited by the 20 standard proteinogenic amino acids, representing preference for the transport of 3H-GABA. Metabolic studies did not show any potential metabolic utilization of GABA by B. abortus as a carbon or nitrogen source, and RNA sequencing analysis revealed limited transcriptional differences between B. abortus 2308 with or without exposure to GABA. While this study provides evidence for GABA transport by B. abortus, questions remain as to why and when this transport is utilized during Brucella pathogenesis.

Pan-Proteomic Analysis and Elucidation of Protein Abundance among the Closely Related Brucella Species, Brucella abortus and Brucella melitensis

Brucellosis is a zoonotic infection caused by bacteria of the genus Brucella. The species, B. abortus and B. melitensis, major causative agents of human brucellosis, share remarkably similar genomes, but they differ in their natural hosts, phenotype, antigenic, immunogenic, proteomic and metabolomic properties. In the present study, label-free quantitative proteomic analysis was applied to investigate protein expression level differences. Type strains and field strains were each cultured six times, cells were harvested at a midlogarithmic growth phase and proteins were extracted. Following trypsin digestion, the peptides were desalted, separated by reverse-phase nanoLC, ionized using electrospray ionization and transferred into an linear trap quadrapole (LTQ) Orbitrap Velos mass spectrometer to record full scan MS spectra (m/z 300–1700) and tandem mass spectrometry (MS/MS) spectra of the 20 most intense ions. Database matching with the reference proteomes resulted in the identification of 826 proteins. The Cluster of Gene Ontologies of the identified proteins revealed differences in bimolecular transport and protein synthesis mechanisms between these two strains. Among several other proteins, antifreeze proteins, Omp10, superoxide dismutase and 30S ribosomal protein S14 were predicted as potential virulence factors among the proteins differentially expressed. All mass spectrometry data are available via ProteomeXchange with identifier PXD006348.

The Cation Diffusion Facilitator Family Protein EmfA Confers Resistance to Manganese Toxicity in Brucella abortus 2308 and Is an Essential Virulence Determinant in Mice

The gene designated bab_rs23470 in the Brucella abortus 2308 genome encodes an ortholog of the cation diffusion facilitator family protein EmfA which has been linked to resistance to Mn toxicity in Rhizobium etli A B. abortus emfA null mutant derived from strain 2308 displays increased sensitivity to elevated levels of Mn in the growth medium compared to that of the parent strain but wild-type resistance to Fe, Mg, Zn, Cu, Co, and Ni. Inductively coupled plasma mass spectroscopy also indicates that the B. abortus emfA mutant retains significantly higher levels of cellular Mn after exposure to this metal than the parent strain, which is consistent with the proposed role of EmfA as a Mn exporter. Phenotypic analysis of mutants indicates that EmfA plays a much more important role in maintaining Mn homeostasis and preventing the toxicity of this metal in Brucella than does the Mn-responsive transcriptional regulator Mur. EmfA is also an essential virulence determinant for B. abortus 2308 in C57BL/6 and C57BL/6^Nramp1+/+ mice, which suggests that avoiding Mn toxicity plays a critical role in Brucella pathogenesis.IMPORTANCE Mn nutrition is essential for the basic physiology and virulence of Brucella strains. The results of the study presented here demonstrate that the cation diffusion facilitator (CDF)-type metal exporter EmfA plays critical roles in maintaining Mn homeostasis and preventing Mn toxicity in Brucella and is an essential virulence determinant for these bacteria. EmfA and other cellular components involved in Mn homeostasis represent attractive targets for the development of improved vaccines and chemotherapeutic strategies for preventing and treating brucellosis in humans and animals.

Transcriptomic analysis of smooth versus rough Brucella melitensis Rev.1 vaccine strains reveals insights into virulence attenuation

Brucella melitensis Rev.1 is the live attenuated Elberg-originated vaccine strain of the facultative intracellular Brucella species, and is widely used to control brucellosis in small ruminants. However, Rev.1 may cause abortions in small ruminants that have been vaccinated during the last trimester of gestation, it is pathogenic to humans, and it induces antibodies directed at the O-polysaccharide (O-PS) of the smooth lipopolysaccharide, thus making it difficult to distinguish between vaccinated and infected animals. Rough Brucella strains, which lack O-PS and are considered less pathogenic, have been introduced to address these drawbacks; however, as Rev.1 confers a much better immunity than the rough mutants, it is still considered the reference vaccine for the prophylaxis of brucellosis in small ruminants. Therefore, developing an improved vaccine strain, which lacks the Rev.1 drawbacks, is a highly evaluated task, which requires a better understanding of the molecular mechanisms underlying the virulence attenuation of Rev.1 smooth strains and of natural Rev.1 rough strains, which are currently only partly understood. As the acidification of the Brucella-containing vacuole during the initial stages of infection is crucial for their survival, identifying the genes that contribute to their survival in an acidic environment versus a normal environment will greatly assist our understanding of the molecular pathogenic mechanisms and the attenuated virulence of the Rev.1 strain. Here, we compared the transcriptomes of the smooth and natural rough Rev.1 strains, each grown under either normal or acidic conditions. We found 12 key genes that are significantly downregulated in the Rev.1 rough strains under normal pH, as compared with Rev.1 smooth strains, and six highly important genes that are significantly upregulated in the smooth strains under acidic conditions, as compared with Rev.1 rough strains. All 18 differentially expressed genes are associated with bacterial virulence and survival and may explain the attenuated virulence of the rough Rev.1 strains versus smooth Rev.1 strains, thus providing new insights into the virulence attenuation mechanisms of Brucella. These highly important candidate genes may facilitate the design of new and improved brucellosis vaccines.

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).

Immunogenic and protective antigens of Brucella as vaccine candidates

Brucella is an intracellular pathogen that causes abortion in domestic animals and undulant fever in humans. Due to the lack of a human vaccine against brucellosis, animal vaccines play an important role in the management of animal and human brucellosis for decades. Strain 19, RB51 and Rev1 are the approved Brucella spp. vaccine strains that are most commonly used to protect livestock against infection and abortion. However, due to some disadvantages of these vaccines, numerous studies have been conducted for the development of effective vaccines that could also be used in other susceptible animals. In this review, we compare different aspects of immunogenic antigens that have been a candidate for the brucellosis vaccine.

Assessment of Survival and Replication of Brucella spp. in Murine Peritoneal Macrophages

Brucella spp. are bacteria that naturally infect a variety of domesticated and wild animals leading to abortions and infertility, and these bacteria are also capable of causing debilitating human infections, which often result from human exposure to infected animals and animal products. The brucellae are intracellular pathogens that reside in host cells, including macrophages and dendritic cells, and it is paramount for the pathogenesis of Brucella that the bacteria are able to survive and replicate in these host cells. The methods outlined in this chapter can be employed to study the interactions between Brucella strains and primary murine peritoneal macrophages.

Substantial Virulence Genes among Brucella melitensis Field Strains Isolated from Cattle in Egypt

BACKGROUND AND OBJECTIVE

The economic losses due to brucellosis as well as its potential public health in human worldwide encourage more researches to find novel pathways for effective control methods of the disease. The objective of this study was to investigate the most prevalent Brucella strains obtained from cattle and their virulence genes.

MATERIALS AND METHODS

Three hundred small-holders cows in Menoufia governorate, Egypt, were screened for brucellosis using rose bengal test (RBT) and confirmed by complement fixation test (CFT). Milk samples and supra-mammary lymph nodes of serologically positive cows were collected for bacteriological isolation and identification. The obtained isolates were genotyped using PCR and their virulence genes (omp25, omp31, manA, manB, virB and znuA) were screened.

RESULTS

The prevalence rate of bovine brucellosis was 15 (5%), 11 (3.6%) and 7 (2.33%) by RBT, CFT and bacteriological examination, respectively. The seven isolates were identified and genotyped as Brucella melitensis biotype3. Furthermore, the molecular detection of substantial virulence genes revealed that manA, manB, omp25 and omp31 genes were detected in all tested B. melitensis strains. Meanwhile, the virB genes were detected in 4 strains and the znuA genes were detected in 3 strains among the isolated B. melitensis strains.

CONCLUSION

It was concluded that B. melitensis biotype3 was the pre-dominant Brucella spp. as well as omp25, omp31, manA and manB were the most common related-virulence genes which assumed to play a worthy function in the pathogenesis of brucellosis.

Identification of Cross-Protective Potential Antigens against Pathogenic Brucella spp. through Combining Pan-Genome Analysis with Reverse Vaccinology

Brucellosis is a zoonotic infectious disease caused by bacteria of the genus Brucella. Brucella melitensis, Brucella abortus, and Brucella suis are the most pathogenic species of this genus causing the majority of human and domestic animal brucellosis. There is a need to develop a safe and potent subunit vaccine to overcome the serious drawbacks of the live attenuated Brucella vaccines. The aim of this work was to discover antigen candidates conserved among the three pathogenic species. In this study, we employed a reverse vaccinology strategy to compute the core proteome of 90 completed genomes: 55 B. melitensis, 17 B. abortus, and 18 B. suis. The core proteome was analyzed by a metasubcellular localization prediction pipeline to identify surface-associated proteins. The identified proteins were thoroughly analyzed using various in silico tools to obtain the most potential protective antigens. The number of core proteins obtained from analyzing the 90 proteomes was 1939 proteins. The surface-associated proteins were 177. The number of potential antigens was 87; those with adhesion score ≥ 0.5 were considered antigen with "high potential," while those with a score of 0.4-0.5 were considered antigens with "intermediate potential." According to a cumulative score derived from protein antigenicity, density of MHC-I and MHC-II epitopes, MHC allele coverage, and B-cell epitope density scores, a final list of 34 potential antigens was obtained. Remarkably, most of the 34 proteins are associated with bacterial adhesion, invasion, evasion, and adaptation to the hostile intracellular environment of macrophages which is adjusted to deprive Brucella of required nutrients. Our results provide a manageable list of potential protective antigens for developing a potent vaccine against brucellosis. Moreover, our elaborated analysis can provide further insights into novel Brucella virulence factors. Our next step is to test some of these antigens using an appropriate antigen delivery system.

The Manganese-Dependent Pyruvate Kinase PykM Is Required for Wild-Type Glucose Utilization by Brucella abortus 2308 and Its Virulence in C57BL/6 Mice

Pyruvate kinase plays a central role in glucose catabolism in bacteria, and efficient utilization of this hexose has been linked to the virulence of Brucella strains in mice. The brucellae produce a single pyruvate kinase which is an ortholog of the Bradyrhizobium manganese (Mn)-dependent pyruvate kinase PykM. A biochemical analysis of the Brucella pyruvate kinase and phenotypic analysis of a Brucella abortus mutant defective in high-affinity Mn import indicate that this enzyme is an authentic PykM ortholog which functions as a Mn-dependent enzyme in vivo The loss of PykM has a negative impact on the capacity of the parental 2308 strain to utilize glucose, fructose, and galactose but not on its ability to utilize ribose, xylose, arabinose, or erythritol, and a pykM mutant displays significant attenuation in C57BL/6 mice. Although the enzyme pyruvate phosphate dikinase (PpdK) can substitute for the loss of pyruvate kinase in some bacteria and is also an important virulence determinant in Brucella, a phenotypic analysis of B. abortus 2308 and isogenic pykM, ppdK, and pykM ppdK mutants indicates that PykM and PpdK make distinctly different contributions to carbon metabolism and virulence in these bacteria.IMPORTANCE Mn plays a critical role in the physiology and virulence of Brucella strains, and the results presented here suggest that one of the important roles that the high-affinity Mn importer MntH plays in the pathogenesis of these strains is supporting the function of the Mn-dependent kinase PykM. A better understanding of how the brucellae adapt their physiology and metabolism to sustain their intracellular persistence in host macrophages will provide knowledge that can be used to design improved strategies for preventing and treating brucellosis, a disease that has a significant impact on both the veterinary and public health communities worldwide.

The microbial contribution to reactive oxygen species dynamics in marine ecosystems

This review surveys the current state of knowledge of the concentrations, sources and sinks of reactive oxygen species (ROS) in the ocean. Both abiotic and biotic factors contribute to ROS dynamics in seawater, and ROS can feature prominently in marine microbe-microbe interactions. The sun plays a key role in the production of ROS in the ocean, and consequently ROS concentrations are typically maximal in the sun-exposed surface. However, microbes can also contribute significantly to extracellular ROS. Production of superoxide is widespread within the microbial community, and may benefit the producers as antimicrobial agents or perhaps more generally, as a means of nutrient scavenging. Decomposition of hydrogen peroxide is a community-wide activity, though some members may play less significant roles in this process. The more reactive forms of ROS, singlet oxygen and the hydroxyl radical, may be less important as microbial stressors, as they tend to react with the chemicals in seawater before they can contact the cells. However, exceptions may exist for microbes attached to singlet oxygen-generating sinking particulate matter. Extracellular ROS thus plays an important role in the ecology of marine microbes, the full extent to which we are only beginning to appreciate.

Chemical Warfare at the Microorganismal Level: A Closer Look at the Superoxide Dismutase Enzymes of Pathogens

Superoxide anion radical is generated as a natural byproduct of aerobic metabolism but is also produced as part of the oxidative burst of the innate immune response design to kill pathogens. In living systems, superoxide is largely managed through superoxide dismutases (SODs), families of metalloenzymes that use Fe, Mn, Ni, or Cu cofactors to catalyze the disproportionation of superoxide to oxygen and hydrogen peroxide. Given the bursts of superoxide faced by microbial pathogens, it comes as no surprise that SOD enzymes play important roles in microbial survival and virulence. Interestingly, microbial SOD enzymes not only detoxify host superoxide but also may participate in signaling pathways that involve reactive oxygen species derived from the microbe itself, particularly in the case of eukaryotic pathogens. In this Review, we will discuss the chemistry of superoxide radicals and the role of diverse SOD metalloenzymes in bacterial, fungal, and protozoan pathogens. We will highlight the unique features of microbial SOD enzymes that have evolved to accommodate the harsh lifestyle at the host-pathogen interface. Lastly, we will discuss key non-SOD superoxide scavengers that specific pathogens employ for defense against host superoxide.

Endoribonuclease YbeY Is Linked to Proper Cellular Morphology and Virulence in Brucella abortus

The YbeY endoribonuclease is one of the best-conserved proteins across the kingdoms of life. In the present study, we demonstrated that YbeY in Brucella abortus is linked to a variety of important activities, including proper cellular morphology, mRNA transcript levels, and virulence. Deletion of ybeY in B. abortus led to a small-colony phenotype when the bacteria were grown on agar medium, as well as to significant aberrations in the morphology of the bacterial cell as evidenced by electron microscopy. Additionally, compared to the parental strain, the ΔybeY strain was significantly attenuated in both macrophage and mouse models of infection. The ΔybeY strain also showed increased sensitivities to several in vitro-applied stressors, including bile acid, hydrogen peroxide, SDS, and paraquat. Transcriptomic analysis revealed that a multitude of mRNA transcripts are dysregulated in the ΔybeY strain, and many of the identified mRNAs encode proteins involved in metabolism, nutrient transport, transcriptional regulation, and flagellum synthesis. We subsequently constructed gene deletion strains of the most highly dysregulated systems, and several of the YbeY-linked gene deletion strains exhibited defects in the ability of the bacteria to survive and replicate in primary murine macrophages. Taken together, these data establish a clear role for YbeY in the biology and virulence of Brucella; moreover, this work further illuminates the highly varied roles of this widely conserved endoribonuclease in bacteria.IMPORTANCEBrucella spp. are highly efficient bacterial pathogens of animals and humans, causing significant morbidity and economic loss worldwide, and relapse of disease often occurs following antibiotic treatment of human brucellosis. As such, novel therapeutic strategies to combat Brucella infections are needed. Ribonucleases in the brucellae are understudied, and these enzymes represent elements that may be potential targets for future treatment approaches. The present work demonstrates the importance of the YbeY endoribonuclease for cellular morphology, efficient control of mRNA levels, and virulence in B. abortus Overall, the results of this study advance our understanding of the critical roles of YbeY in the pathogenesis of the intracellular brucellae and expand our understanding of this highly conserved RNase.

Overexpression of Cu-Zn SOD in Brucella abortus suppresses bacterial intracellular replication via down-regulation of Sar1 activity

Brucella Cu-Zn superoxide dismutase (Cu-Zn SOD) is a periplasmic protein, and immunization of mice with recombinant Cu-Zn SOD protein confers protection against Brucella abortus infection. However, the role of Cu-Zn SOD during the process of Brucella infection remains unknown. Here, we report that Cu-Zn SOD is secreted into culture medium and is translocated into host cells independent of type IV secretion systems (T4SS). Furthermore, co-immunoprecipitation and immunofluorescence studies reveal that Brucella abortus Cu-Zn SOD interacts with the small GTPase Sar1. Overexpression of Cu-Zn SOD in Brucella abortus inhibits bacterial intracellular growth by abolishing Sar1 activity in a manner independent of reactive oxygen species (ROS) production.

Ml proteins from Mesorhizobium loti and MucR from Brucella abortus: an AT-rich core DNA-target site and oligomerization ability

Mesorhizobium loti contains ten genes coding for proteins sharing high amino acid sequence identity with members of the Ros/MucR transcription factor family. Five of these Ros/MucR family members from Mesorhizobium loti (Ml proteins) have been recently structurally and functionally characterized demonstrating that Ml proteins are DNA-binding proteins. However, the DNA-binding studies were performed using the Ros DNA-binding site with the Ml proteins. Currently, there is no evidence as to when the Ml proteins are expressed during the Mesorhizobium lo ti life cycle as well as no information concerning their natural DNA-binding site. In this study, we examine the ml genes expression profile in Mesorhizobium loti and show that ml1, ml2, ml3 and ml5 are expressed during planktonic growth and in biofilms. DNA-binding experiments show that the Ml proteins studied bind a conserved AT-rich site in the promoter region of the exoY gene from Mesorhizobium loti and that the proteins make important contacts with the minor groove of DNA. Moreover, we demonstrate that the Ml proteins studied form higher-order oligomers through their N-terminal region and that the same AT-rich site is recognized by MucR from Brucella abortus using a similar mechanism involving contacts with the minor groove of DNA and oligomerization.

Simultaneous RNA-seq based transcriptional profiling of intracellular Brucella abortus and B. abortus-infected murine macrophages

Brucella is a zoonotic pathogen that survives within macrophages; however the replicative mechanisms involved are not fully understood. We describe the isolation of sufficient Brucella abortus RNA from primary host cell environment using modified reported methods for RNA-seq analysis, and simultaneously characterize the transcriptional profiles of intracellular B. abortus and bone marrow-derived macrophages (BMM) from BALB/c mice at 24 h (replicative phase) post-infection. Our results revealed that 25.12% (801/3190) and 16.16% (515/3190) of the total B. abortus genes were up-regulated and down-regulated at >2-fold, respectively as compared to the free-living B. abortus. Among >5-fold differentially expressed genes, the up-regulated genes are mostly involved in DNA, RNA manipulations as well as protein biosynthesis and secretion while the down-regulated genes are mainly involved in energy production and metabolism. On the other hand, the host responses during B. abortus infection revealed that 14.01% (6071/43,346) of BMM genes were reproducibly transcribed at >5-fold during infection. Transcription of cytokines, chemokines and transcriptional factors, such as tumor necrosis factor (Tnf), interleukin-1α (Il1α), interleukin-1β (Il1β), interleukin-6 (Il6), interleukin-12 (Il12), chemokine C-X-C motif (CXCL) family, nuclear factor kappa B (Nf-κb), signal transducer and activator of transcription 1 (Stat1), that may contribute to host defense were markedly induced while transcription of various genes involved in cell proliferation and metabolism were suppressed upon B. abortus infection. In conclusion, these data suggest that Brucella modulates gene expression in hostile intracellular environment while simultaneously alters the host pathways that may lead to the pathogen's intracellular survival and infection.

Proline utilization system is required for infection by the pathogenic α-proteobacterium Brucella abortus

Proline utilization (Put) systems have been described in a number of bacteria; however, the importance and functionality of the Put system in the intracellular pathogen Brucellaabortus has not been explored. Generally, bacterial Put systems are composed of the bifunctional enzyme proline dehydrogenase PutA and its transcriptional activator PutR. Here, we demonstrate that the genes putA (bab2_0518) and putR (bab2_0517) are critical for the chronic infection of mice by B. abortus, but putA and putR are not required for the survival and replication of the bacteria in naive macrophages. Additionally, in vitro experiments revealed that putR is necessary for the ability of the bacteria to withstand oxidative stress, as a ΔputR deletion strain is hypersensitive to hydrogen peroxide exposure. Quantitative reverse transcription-PCR and putA-lacZ transcriptional reporter studies revealed that PutR acts as a transcriptional activator of putA in Brucella, and electrophoretic mobility shift assays confirmed that PutR binds directly to the putA promoter region. Biochemical analyses demonstrated that a purified recombinant B. abortus PutA protein possesses quintessential proline dehydrogenase activity, as PutA is capable of catalysing the conversion of proline to glutamate. Altogether, these data are the first to reveal that the Put system plays a significant role in the ability of B. abortus to replicate and survive within its host, as well as to describe the genetic regulation and biochemical activity of the Put system in Brucella.

A 6-Nucleotide Regulatory Motif within the AbcR Small RNAs of Brucella abortus Mediates Host-Pathogen Interactions

In Brucella abortus, two small RNAs (sRNAs), AbcR1 and AbcR2, are responsible for regulating transcripts encoding ABC-type transport systems. AbcR1 and AbcR2 are required for Brucella virulence, as a double chromosomal deletion of both sRNAs results in attenuation in mice. Although these sRNAs are responsible for targeting transcripts for degradation, the mechanism utilized by the AbcR sRNAs to regulate mRNA in Brucella has not been described. Here, two motifs (M1 and M2) were identified in AbcR1 and AbcR2, and complementary motif sequences were defined in AbcR-regulated transcripts. Site-directed mutagenesis of M1 or M2 or of both M1 and M2 in the sRNAs revealed transcripts to be targeted by one or both motifs. Electrophoretic mobility shift assays revealed direct, concentration-dependent binding of both AbcR sRNAs to a target mRNA sequence. These experiments genetically and biochemically characterized two indispensable motifs within the AbcR sRNAs that bind to and regulate transcripts. Additionally, cellular and animal models of infection demonstrated that only M2 in the AbcR sRNAs is required for Brucella virulence. Furthermore, one of the M2-regulated targets, BAB2_0612, was found to be critical for the virulence of B. abortus in a mouse model of infection. Although these sRNAs are highly conserved among Alphaproteobacteria, the present report displays how gene regulation mediated by the AbcR sRNAs has diverged to meet the intricate regulatory requirements of each particular organism and its unique biological niche.

Small RNAs (sRNAs) are important components of bacterial regulation, allowing organisms to quickly adapt to changes in their environments. The AbcR sRNAs are highly conserved throughout the Alphaproteobacteria and negatively regulate myriad transcripts, many encoding ABC-type transport systems. In Brucella abortus, AbcR1 and AbcR2 are functionally redundant, as only a double abcR1 abcR2 (abcR1/2) deletion results in attenuation in vitro and in vivo. In the present study, we confirmed that the AbcR sRNAs have redundant regulatory functions and defined two six-nucleotide motifs, M1 and M2, that the AbcR sRNAs utilize to control gene expression. Importantly, only M2 was linked to B. abortus virulence. Further investigation of M2-regulated targets identified BAB2_0612 as critical for colonization of B. abortus in mice, highlighting the significance of AbcR M2-regulated transcripts for Brucella infection. Overall, our findings define the molecular mechanism of the virulence-associated AbcR system in the pathogenic bacterium B. abortus.

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.

Contrasting Lifestyles Within the Host Cell

Intracellular bacterial pathogens have evolved to exploit the protected niche provided within the boundaries of a eukaryotic host cell. Upon entering a host cell, some bacteria can evade the adaptive immune response of its host and replicate in a relatively nutrient-rich environment devoid of competition from other host flora. Growth within a host cell is not without their hazards, however. Many pathogens enter their hosts through receptor-mediated endocytosis or phagocytosis, two intracellular trafficking pathways that terminate in a highly degradative organelle, the phagolysosome. This usually deadly compartment is maintained at a low pH and contains degradative enzymes and reactive oxygen species, resulting in an environment to which few bacterial species are adapted. Some intracellular pathogens, such as Shigella, Listeria, Francisella, and Rickettsia, escape the phagosome to replicate within the cytosol of the host cell. Bacteria that remain within a vacuole either alter the trafficking of their initial phagosomal compartment or adapt to survive within the harsh environment it will soon become. In this chapter, we focus on the mechanisms by which different vacuolar pathogens either evade lysosomal fusion, as in the case of Mycobacterium and Chlamydia, or allow interaction with lysosomes to varying degrees, such as Brucella and Coxiella, and their specific adaptations to inhabit a replicative niche.

Vaccination with a ΔnorD ΔznuA Brucella abortus mutant confers potent protection against virulent challenge

There remains a need for an improved livestock vaccine for brucellosis since conventional vaccines are only ∼70% efficacious, making some vaccinated animals susceptible to Brucella infections. To address this void, a vaccine capable of evoking protective immunity, while still being sufficiently attenuated to produce minimal disease, is sought. In this pursuit, the ΔnorD ΔznuA B. abortus-lacZ (termed as znBAZ) was developed to be devoid of functional norD and znuA B. abortus genes, and to contain the lacZ as a marker gene. The results show that znBAZ is highly attenuated in mouse and human macrophages, and completely cleared from mouse spleens within eight weeks post-vaccination. Producing less splenic inflammation, znBAZ is significantly more protective than the conventional RB51 vaccine by more than four orders of magnitude. Vaccination with znBAZ elicits elevated numbers of IFN-γ⁺, TNF-α⁺, and polyfunctional IFN-γ⁺ TNF-α⁺ CD4⁺ and CD8⁺ T cells in contrast to RB51-vaccinated mice, which show reduced numbers of proinflammatory cytokine-producing T cells. These results demonstrate that znBAZ is a highly efficacious vaccine candidate capable of eliciting diverse T cell subsets that confer protection against parenteral challenge with virulent, wild-type B. abortus.

Proteomic Profile of Brucella abortus-Infected Bovine Chorioallantoic Membrane Explants

Brucella abortus is the etiological agent of bovine brucellosis, a zoonotic disease that causes significant economic losses worldwide. The differential proteomic profile of bovine chorioallantoic membrane (CAM) explants at early stages of infection with B. abortus (0.5, 2, 4, and 8 h) was determined. Analysis of CAM explants at 0.5 and 4 h showed the highest differences between uninfected and infected CAM explants, and therefore were used for the Differential Gel Electrophoresis (DIGE). A total of 103 spots were present in only one experimental group and were selected for identification by mass spectrometry (MALDI/ToF-ToF). Proteins only identified in extracts of CAM explants infected with B. abortus were related to recognition of PAMPs by TLR, production of reactive oxygen species, intracellular trafficking, and inflammation.

Structural, Functional, and Immunogenic Insights on Cu,Zn Superoxide Dismutase Pathogenic Virulence Factors from Neisseria meningitidis and Brucella abortus

Bacterial pathogens Neisseria meningitidis and Brucella abortus pose threats to human and animal health worldwide, causing meningococcal disease and brucellosis, respectively. Mortality from acute N. meningitidis infections remains high despite antibiotics, and brucellosis presents alimentary and health consequences. Superoxide dismutases are master regulators of reactive oxygen and general pathogenicity factors and are therefore therapeutic targets. Cu,Zn superoxide dismutases (SODs) localized to the periplasm promote survival by detoxifying superoxide radicals generated by major host antimicrobial immune responses. We discovered that passive immunization with an antibody directed at N. meningitidis SOD (NmSOD) was protective in a mouse infection model. To define the relevant atomic details and solution assembly states of this important virulence factor, we report high-resolution and X-ray scattering analyses of NmSOD and of SOD from B. abortus (BaSOD). The NmSOD structures revealed an auxiliary tetrahedral Cu-binding site bridging the dimer interface; mutational analyses suggested that this metal site contributes to protein stability, with implications for bacterial defense mechanisms. Biochemical and structural analyses informed us about electrostatic substrate guidance, dimer assembly, and an exposed C-terminal epitope in the NmSOD dimer. In contrast, the monomeric BaSOD structure provided insights for extending immunogenic peptide epitopes derived from the protein. These collective results reveal unique contributions of SOD to pathogenic virulence, refine predictive motifs for distinguishing SOD classes, and suggest general targets for antibacterial immune responses. The identified functional contributions, motifs, and targets distinguishing bacterial and eukaryotic SOD assemblies presented here provide a foundation for efforts to develop SOD-specific inhibitors of or vaccines against these harmful pathogens.

IMPORTANCE

By protecting microbes against reactive oxygen insults, SODs aid survival of many bacteria within their hosts. Despite the ubiquity and conservation of these key enzymes, notable species-specific differences relevant to pathogenesis remain undefined. To probe mechanisms that govern the functioning of Neisseria meningitidis and Brucella abortus SODs, we used X-ray structures, enzymology, modeling, and murine infection experiments. We identified virulence determinants common to the two homologs, assembly differences, and a unique metal reservoir within meningococcal SOD that stabilizes the enzyme and may provide a safeguard against copper toxicity. The insights reported here provide a rationale and a basis for SOD-specific drug design and an extension of immunogen design to target two important pathogens that continue to pose global health threats.

Transcriptome-Wide Identification of Hfq-Associated RNAs in Brucella suis by Deep Sequencing

Recent breakthroughs in next-generation sequencing technologies have led to the identification of small noncoding RNAs (sRNAs) as a new important class of regulatory molecules. In prokaryotes, sRNAs are often bound to the chaperone protein Hfq, which allows them to interact with their partner mRNA(s). We screened the genome of the zoonotic and human pathogen Brucella suis 1330 for the presence of this class of RNAs. We designed a coimmunoprecipitation strategy that relies on the use of Hfq as a bait to enrich the sample with sRNAs and eventually their target mRNAs. By deep sequencing analysis of the Hfq-bound transcripts, we identified a number of mRNAs and 33 sRNA candidates associated with Hfq. The expression of 10 sRNAs in the early stationary growth phase was experimentally confirmed by Northern blotting and/or reverse transcriptase PCR.

IMPORTANCE

Brucella organisms are facultative intracellular pathogens that use stealth strategies to avoid host defenses. Adaptation to the host environment requires tight control of gene expression. Recently, small noncoding RNAs (sRNAs) and the sRNA chaperone Hfq have been shown to play a role in the fine-tuning of gene expression. Here we have used RNA sequencing to identify RNAs associated with the B. suis Hfq protein. We have identified a novel list of 33 sRNAs and 62 Hfq-associated mRNAs for future studies aiming to understand the intracellular lifestyle of this pathogen.

A LysR-family transcriptional regulator required for virulence in Brucella abortus is highly conserved among the α-proteobacteria

Small RNAs are principal elements of bacterial gene regulation and physiology. Two small RNAs in Brucella abortus, AbcR1 and AbcR2, are required for wild-type virulence. Examination of the abcR loci revealed the presence of a gene encoding a LysR-type transcriptional regulator flanking abcR2 on chromosome 1. Deletion of this lysR gene (bab1_1517) resulted in the complete loss of abcR2 expression while no difference in abcR1 expression was observed. The B. abortus bab1_1517 mutant strain was significantly attenuated in macrophages and mice, and bab1_1517 was subsequently named vtlR for virulence-associated transcriptional LysR-family regulator. Microarray analysis revealed three additional genes encoding small hypothetical proteins also under the control of VtlR. Electrophoretic mobility shift assays demonstrated that VtlR binds directly to the promoter regions of abcR2 and the three hypothetical protein-encoding genes, and DNase I footprint analysis identified the specific nucleotide sequence in these promoters that VtlR binds to and drives gene expression. Strikingly, orthologs of VtlR are encoded in a wide range of host-associated α-proteobacteria, and it is likely that the VtlR genetic system represents a common regulatory circuit critical for host-bacterium interactions.

Structural asymmetry in a conserved signaling system that regulates division, replication, and virulence of an intracellular pathogen

We have functionally and structurally defined an essential protein phosphorelay that regulates expression of genes required for growth, division, and intracellular survival of the global zoonotic pathogen Brucella abortus. Our study delineates phosphoryl transfer through this molecular pathway, which initiates from the sensor kinase CckA and proceeds through the ChpT phosphotransferase to two regulatory substrates: CtrA and CpdR. Genetic perturbation of this system results in defects in cell growth and division site selection, and a specific viability deficit inside human phagocytic cells. Thus, proper control of B. abortus division site polarity is necessary for survival in the intracellular niche. We further define the structural foundations of signaling from the central phosphotransferase, ChpT, to its response regulator substrate, CtrA, and provide evidence that there are at least two modes of interaction between ChpT and CtrA, only one of which is competent to catalyze phosphoryltransfer. The structure and dynamics of the active site on each side of the ChpT homodimer are distinct, supporting a model in which quaternary structure of the 2:2 ChpT-CtrA complex enforces an asymmetric mechanism of phosphoryl transfer between ChpT and CtrA. Our study provides mechanistic understanding, from the cellular to the atomic scale, of a conserved transcriptional regulatory system that controls the cellular and infection biology of B. abortus. More generally, our results provide insight into the structural basis of two-component signal transduction, which is broadly conserved in bacteria, plants, and fungi.

Comparative genomic analysis of Brucella abortus vaccine strain 104M reveals a set of candidate genes associated with its virulence attenuation

The Brucella abortus strain 104M, a spontaneously attenuated strain, has been used as a vaccine strain in humans against brucellosis for 6 decades in China. Despite many studies, the molecular mechanisms that cause the attenuation are still unclear. Here, we determined the whole-genome sequence of 104M and conducted a comprehensive comparative analysis against the whole genome sequences of the virulent strain, A13334, and other reference strains. This analysis revealed a highly similar genome structure between 104M and A13334. The further comparative genomic analysis between 104M and A13334 revealed a set of genes missing in 104M. Some of these genes were identified to be directly or indirectly associated with virulence. Similarly, a set of mutations in the virulence-related genes was also identified, which may be related to virulence alteration. This study provides a set of candidate genes associated with virulence attenuation in B.abortus vaccine strain 104M.

Coordinated zinc homeostasis is essential for the wild-type virulence of Brucella abortus

Metal homeostasis in bacterial cells is a highly regulated process requiring intricately coordinated import and export, as well as precise sensing of intracellular metal concentrations. The uptake of zinc (Zn) has been linked to the virulence of Brucella abortus; however, the capacity of Brucella strains to sense Zn levels and subsequently coordinate Zn homeostasis has not been described. Here, we show that expression of the genes encoding the zinc uptake system ZnuABC is negatively regulated by the Zn-sensing Fur family transcriptional regulator, Zur, by direct interactions between Zur and the promoter region of znuABC. Moreover, the MerR-type regulator, ZntR, controls the expression of the gene encoding the Zn exporter ZntA by binding directly to its promoter. Deletion of zur or zntR alone did not result in increased zinc toxicity in the corresponding mutants; however, deletion of zntA led to increased sensitivity to Zn but not to other metals, such as Cu and Ni, suggesting that ZntA is a Zn-specific exporter. Strikingly, deletion of zntR resulted in significant attenuation of B. abortus in a mouse model of chronic infection, and subsequent experiments revealed that overexpression of zntA in the zntR mutant is the molecular basis for its decreased virulence.

IMPORTANCE

The importance of zinc uptake for Brucella pathogenesis has been demonstrated previously, but to date, there has been no description of how overall zinc homeostasis is maintained and genetically controlled in the brucellae. The present work defines the predominant zinc export system, as well as the key genetic regulators of both zinc uptake and export in Brucella abortus. Moreover, the data show the importance of precise coordination of the zinc homeostasis systems as disregulation of some elements of these systems leads to the attenuation of Brucella virulence in a mouse model. Overall, this study advances our understanding of the essential role of zinc in the pathogenesis of intracellular 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.

Proteomic analyses of the time course responses of mice infected with Brucella abortus 544 reveal immunogenic antigens

Brucellosis is a major zoonotic disease caused by pathogens of the genus Brucella. The eradication of brucellosis in domestic animals, associated with the prevention of human infection, can be attained through accurate diagnosis. However, the conventional serological diagnosis of brucellosis has limitations, particularly in detecting the infection period. Accordingly, the aim of this study was to determine reliable immunogenic proteins to detect Brucella abortus infection according to time course responses to aid in the appropriate management of this disease. Proteomic identification through two-dimensional electrophoresis (2DE), followed by immunoblotting, revealed 13, 24, and 55 immunodominant B. abortus 544 proteins that were reactive to sera from experimentally infected mice at early (10 days), middle (30 days), and late (60 days) infection periods, respectively. After excluding several spots reactive to sera from Yersinia enterocolitica O:9-infected and noninfected mice, 17 of the 67 immunodominant proteins were identified through MALDI-TOF MS. Consequently, the identified proteins showed time course-dependent immunogenicity against Brucella infection. Thus, the results of this study suggest that the production of immunogenic proteins during infection periods improves the diagnosis and discovery of vaccine candidates.

Cupric yersiniabactin is a virulence-associated superoxide dismutase mimic

Many Gram-negative bacteria interact with extracellular metal ions by expressing one or more siderophore types. Among these, the virulence-associated siderophore yersiniabactin (Ybt) is an avid copper chelator, forming stable cupric (Cu(II)-Ybt) complexes that are detectable in infected patients. Here we show that Ybt-expressing E. coli are protected from intracellular killing within copper-replete phagocytic cells. This survival advantage is highly dependent upon the phagocyte respiratory burst, during which superoxide is generated by the NADPH oxidase complex. Chemical fractionation links this phenotype to a previously unappreciated superoxide dismutase (SOD)-like activity of Cu(II)-Ybt. Unlike previously described synthetic copper-salicylate (Cu(II)-SA) SOD mimics, the salicylate-based natural product Cu(II)-Ybt retains catalytic activity at physiologically plausible protein concentrations. These results reveal a new virulence-associated adaptation based upon spontaneous assembly of a non-protein catalyst.

A genome-wide MeSH-based literature mining system predicts implicit gene-to-gene relationships and networks

Background

The large amount of literature in the post-genomics era enables the study of gene interactions and networks using all available articles published for a specific organism. MeSH is a controlled vocabulary of medical and scientific terms that is used by biomedical scientists to manually index articles in the PubMed literature database. We hypothesized that genome-wide gene-MeSH term associations from the PubMed literature database could be used to predict implicit gene-to-gene relationships and networks. While the gene-MeSH associations have been used to detect gene-gene interactions in some studies, different methods have not been well compared, and such a strategy has not been evaluated for a genome-wide literature analysis. Genome-wide literature mining of gene-to-gene interactions allows ranking of the best gene interactions and investigation of comprehensive biological networks at a genome level.

Results

The genome-wide GenoMesh literature mining algorithm was developed by sequentially generating a gene-article matrix, a normalized gene-MeSH term matrix, and a gene-gene matrix. The gene-gene matrix relies on the calculation of pairwise gene dissimilarities based on gene-MeSH relationships. An optimized dissimilarity score was identified from six well-studied functions based on a receiver operating characteristic (ROC) analysis. Based on the studies with well-studied Escherichia coli and less-studied Brucella spp., GenoMesh was found to accurately identify gene functions using weighted MeSH terms, predict gene-gene interactions not reported in the literature, and cluster all the genes studied from an organism using the MeSH-based gene-gene matrix. A web-based GenoMesh literature mining program is also available at: http://genomesh.hegroup.org. GenoMesh also predicts gene interactions and networks among genes associated with specific MeSH terms or user-selected gene lists.

Conclusions

The GenoMesh algorithm and web program provide the first genome-wide, MeSH-based literature mining system that effectively predicts implicit gene-gene interaction relationships and networks in a genome-wide scope.

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.

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.

The RNA chaperone Hfq promotes fitness of Actinobacillus pleuropneumoniae during porcine pleuropneumonia

Actinobacillus pleuropneumoniae is the etiological agent of porcine pleuropneumonia, an economically important disease of pigs. The hfq gene in A. pleuropneumoniae, encoding the RNA chaperone and posttranscriptional regulator Hfq, is upregulated during infection of porcine lungs. To investigate the role of this in vivo-induced gene in A. pleuropneumoniae, an hfq mutant strain was constructed. The hfq mutant was defective in biofilm formation on abiotic surfaces. The level of pgaC transcript, encoding the biosynthesis of poly-β-1,6-N-acetylglucosamine (PNAG), a major biofilm matrix component, was lower and PNAG content was 10-fold lower in the hfq mutant than in the wild-type strain. When outer membrane proteins were examined, cysteine synthase, implicated in resistance to oxidative stress and tellurite, was not found at detectable levels in the absence of Hfq. The hfq mutant displayed enhanced sensitivity to superoxide generated by methyl viologen and tellurite. These phenotypes were readily reversed by complementation with the hfq gene expressed from its native promoter. The role of Hfq in the fitness of A. pleuropneumoniae was assessed in a natural host infection model. The hfq mutant failed to colonize porcine lungs and was outcompeted by the wild-type strain (median competitive index of 2 × 10(-5)). Our data demonstrate that the in vivo-induced gene hfq is involved in the regulation of PNAG-dependent biofilm formation, resistance to superoxide stress, and the fitness and virulence of A. pleuropneumoniae in pigs and begin to elucidate the role of an in vivo-induced gene in the pathogenesis of pleuropneumonia.

Diverse genetic regulon of the virulence-associated transcriptional regulator MucR in Brucella abortus 2308

The Ros-type regulator MucR is one of the few transcriptional regulators that have been linked to virulence in Brucella. Here, we show that a Brucella abortus in-frame mucR deletion strain exhibits a pronounced growth defect during in vitro cultivation and, more importantly, that the mucR mutant is attenuated in cultured macrophages and in mice. The genetic basis for the attenuation of Brucella mucR mutants has not been defined previously, but in the present study the genes regulated by MucR in B. abortus have been elucidated using microarray analysis and real-time reverse transcription-PCR (RT-PCR). In B. abortus 2308, MucR regulates a wide variety of genes whose products may function in establishing and maintaining cell envelope integrity, polysaccharide biosynthesis, iron homeostasis, genome plasticity, and transcriptional regulation. Particularly notable among the MucR-regulated genes identified is arsR6 (nolR), which encodes a transcriptional regulator previously linked to virulence in Brucella melitensis 16 M. Importantly, electrophoretic mobility shift assays (EMSAs) determined that a recombinant MucR protein binds directly to the promoter regions of several genes repressed by MucR (including arsR6 [nolR]), and in Brucella, as in other alphaproteobacteria, MucR binds to its own promoter to repress expression of the gene that encodes it. Overall, these studies have uncovered the diverse genetic regulon of MucR in Brucella, and in doing so this work has begun to define the MucR-controlled genetic circuitry whose misregulation contributes to the virulence defect of Brucella mucR mutants.

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.

Characterization of the organic hydroperoxide resistance system of Brucella abortus 2308

The organic hydroperoxide resistance protein Ohr has been identified in numerous bacteria where it functions in the detoxification of organic hydroperoxides, and expression of ohr is often regulated by a MarR-type regulator called OhrR. The genes annotated as BAB2_0350 and BAB2_0351 in the Brucella abortus 2308 genome sequence are predicted to encode OhrR and Ohr orthologs, respectively. Using isogenic ohr and ohrR mutants and lacZ promoter fusions, it was determined that Ohr contributes to resistance to organic hydroperoxide, but not hydrogen peroxide, in B. abortus 2308 and that OhrR represses the transcription of both ohr and ohrR in this strain. Moreover, electrophoretic mobility shift assays and DNase I footprinting revealed that OhrR binds directly to a specific region in the intergenic region between ohr and ohrR that shares extensive nucleotide sequence similarity with so-called "OhrR boxes" described in other bacteria. While Ohr plays a prominent role in protecting B. abortus 2308 from organic hydroperoxide stress in in vitro assays, this protein is not required for the wild-type virulence of this strain in cultured murine macrophages or experimentally infected mice.

Identification of two small regulatory RNAs linked to virulence in Brucella abortus 2308

Hfq is an RNA-binding protein that functions in post-transcriptional gene regulation by mediating interactions between mRNAs and small regulatory RNAs (sRNAs). Two proteins encoded by BAB1_1794 and BAB2_0612 are highly over-produced in a Brucella abortus hfq mutant compared with the parental strain, and recently, expression of orthologues of these proteins in Agrobacterium tumefaciens was shown to be regulated by two sRNAs, called AbcR1 and AbcR2. Orthologous sRNAs (likewise designated AbcR1 and AbcR2) have been identified in B. abortus 2308. In Brucella, abcR1 and abcR2 single mutants are not defective in their ability to survive in cultured murine macrophages, but an abcR1 abcR2 double mutant exhibits significant attenuation in macrophages. Additionally, the abcR1 abcR2 double mutant displays significant attenuation in a mouse model of chronic Brucella infection. Quantitative proteomics and microarray analyses revealed that the AbcR sRNAs predominantly regulate genes predicted to be involved in amino acid and polyamine transport and metabolism, and Northern blot analyses indicate that the AbcR sRNAs accelerate the degradation of the target mRNAs. In an Escherichia coli two-plasmid reporter system, overexpression of either AbcR1 or AbcR2 was sufficient for regulation of target mRNAs, indicating that the AbcR sRNAs from B. abortus 2308 perform redundant regulatory functions.