Mycobacterium avium-triggered diseases: pathogenomics

Ecological and evolutionary perspectives advance understanding of mycobacterial diseases

Predicting the outbreak of infectious diseases and designing appropriate preventive health actions require interdisciplinary research into the processes that drive exposure to and transmission of disease agents. In the case of mycobacterial diseases, the epidemiological understanding of the scientific community hitherto was based on the clinical studies of infections in vertebrates. To evaluate the information gained by comprehensively accounting for the ecological and evolutionary constraints, we conducted literature searches assessing the role of mycobacteria interactions with non-vertebrate species in the origin of their pathogenicity and variations in disease risk. The reviewed literature challenges the current theory of person-to-person transmission for several mycobacterial infections. Furthermore, the findings suggest that diverse non-vertebrate organisms influence virulence, mediate transmission, and contribute to pathogen abundance in relation to vertebrate exposure. We advocate that an ecological and evolutionary framework provides novel insights to support a more comprehensive understanding of the prevention and management of diseases in vertebrates.

Nontuberculous Mycobacteria in Horses: A Narrative Review

Nontuberculous mycobacteria (NTM) infections are increasing in human and veterinary medicine. Although horses were initially thought to be resistant to NTM infection, reports of horses suffering from gastrointestinal, respiratory, and reproductive diseases associated with NTM have increased in the last few decades. The aim of this literature review is to summarize the mycobacteria species found in horses, describe clinical manifestations, diagnostic and treatment approaches, and public health concerns of NTM infection in horses. Clinical manifestations of NTM in horses include pulmonary disease, lymphadenitis, soft tissue, bone infections, and disseminated disease. NTM are also linked to granulomatous enteritis, placentitis, and abortions. Currently, diagnostic methods for NTM are limited and include acid-fast microscopy, bacterial cultures, species-specific PCR assays, and gene sequencing. In humans, NTM treatment guidelines are available, but their application appears inadequate and inconsistent. In horses, treatment guidelines for NTM infections are not available. NTM are a serious public health threat as 70% of people with untreated acquired immunodeficiency syndrome (AIDS) have a chronic pulmonary disease caused by NTM. Thus, it is essential that we gain a better understanding of NTM infections in horses and their zoonotic potential.

Mycobacterium avium Subsp. hominissuis Interactions with Macrophage Killing Mechanisms

Non-tuberculosis mycobacteria (NTM) are ubiquitously found throughout the environment. NTM can cause respiratory infections in individuals with underlying lung conditions when inhaled, or systemic infections when ingested by patients with impaired immune systems. Current therapies can be ineffective at treating NTM respiratory infections, even after a long course or with multidrug treatment regimens. NTM, such as Mycobacterium avium subspecies hominissuis (M. avium), is an opportunistic pathogen that shares environments with ubiquitous free-living amoeba and other environmental hosts, possibly their evolutionary hosts. It is highly likely that interactions between M. avium and free-living amoeba have provided selective pressure on the bacteria to acquire survival mechanisms, which are also used against predation by macrophages. In macrophages, M. avium resides inside phagosomes and has been shown to exit it to infect other cells. M. avium's adaptation to the hostile intra-phagosomal environment is due to many virulence mechanisms. M. avium is able to switch the phenotype of the macrophage to be anti-inflammatory (M2). Here, we have focused on and discussed the bacterial defense mechanisms associated with the intra-phagosome phase of infection. M. avium possesses a plethora of antioxidant enzymes, including the superoxide dismutases, catalase and alkyl hydroperoxide reductase. When these defenses fail or are overtaken by robust oxidative burst, many other enzymes exist to repair damage incurred on M. avium proteins, including thioredoxin/thioredoxin reductase. Finally, M. avium has several oxidant sensors that induce transcription of antioxidant enzymes, oxidation repair enzymes and biofilm- promoting genes. These expressions induce physiological changes that allow M. avium to survive in the face of leukocyte-generated oxidative stress. We will discuss the strategies used by M. avium to infect human macrophages that evolved during its evolution from free-living amoeba. The more insight we gain about M. avium's mode of pathogenicity, the more targets we can have to direct new anti-virulence therapies toward.

Hsp16.3 of mycobacterium tuberculosis in exosomes as a biomarker of tuberculosis

Exosomes are selectively packaged cell-derived vesicles that contain a rich cargo of nucleic acids and proteins. The small heat shock protein, Hsp16.3, is an important capsule protein produced by Mycobacterium tuberculosis (MTB). Exploring the distribution of Hsp16.3 in exosomes is valuable to tuberculosis biomarker development. Our results showed that Hsp16.3 protein overexpressed in cells can be efficiently packaged into exosomes. U937 cells infected with MTB secreted abnormally excessive amounts of Hsp16.3 protein in exosomes. Finally, a substantial number of Hsp16.3 proteins were detected in blood exosomes of tuberculosis patients. The research provides a potential exosome-based tuberculosis biomarker for MTB diagnosis.

Genetic Involvement of Mycobacterium avium Complex in the Regulation and Manipulation of Innate Immune Functions of Host Cells

Mycobacterium avium complex (MAC), a collection of mycobacterial species representing nontuberculous mycobacteria, are characterized as ubiquitous and opportunistic pathogens. The incidence and prevalence of infectious diseases caused by MAC have been emerging globally due to complications in the treatment of MAC-pulmonary disease (PD) in humans and the lack of understating individual differences in genetic traits and pathogenesis of MAC species or subspecies. Despite genetically close one to another, mycobacteria species belonging to the MAC cause diseases to different host range along with a distinct spectrum of disease. In addition, unlike Mycobacterium tuberculosis, the underlying mechanisms for the pathogenesis of MAC infection from environmental sources of infection to their survival strategies within host cells have not been fully elucidated. In this review, we highlight unique genetic and genotypic differences in MAC species and the virulence factors conferring the ability to MAC for the tactics evading innate immune attacks of host cells based on the recent advances in genetic analysis by exemplifying M. avium subsp. hominissuis, a major representative pathogen causing MAC-PD in humans. Further understanding of the genetic link between host and MAC may contribute to enhance host anti-MAC immunity, but also provide novel therapeutic approaches targeting the pangenesis-associated genes of MAC.

Lesions of Mycobacterium avium spp. hominissuis Infection Resembling M. bovis Lesions in a Wild Mule Deer, Canada1

We used molecular analyses to confirm Mycobacterium avium spp. hominissuis infection in lung granulomas and pyogranulomas in the tracheobronchial lymph node in a wild mule deer in Banff, Canada. These lesions are similar to those found in M. bovis–infected animals, emphasizing the critical need for disease surveillance in wildlife populations.

Mycobacterium avium sp. paratuberculosis (MAP) induces IL-17a production in bovine peripheral blood mononuclear cells (PBMCs) and enhances IL-23R expression in-vivo and in-vitro

Johne's disease (JD) is a chronic inflammatory gastrointestinal disease of ruminants caused by Mycobacterium avium subspecies paratuberculosis (MAP). Control of JD is difficult largely due to insensitive diagnostic tools, a long subclinical stage of infection, and lack of effective vaccines. Correlates of protection are lacking in model systems of JD and the sources of inflammation due to JD are not well characterized. Commonly studied immune responses, such as the Th1/Th2 paradigm, do not adequately explain host responses to MAP. A potential role for non-classical immune responses to MAP, such as that mediated by Th17 cells, has been suggested. Indeed, MAP antigens induce mRNAs encoding the cytokines IL-23 and IL-17a in bovine peripheral blood mononuclear cells (PBMCs). IL-23 and IL-17a production have both been associated with Th17-like immune responses. Th17 cells are also defined by surface expression of the IL-23 receptor (IL-23R). To determine the relative prevalence of potential Th17 cells in PBMCs from MAP test positive and MAP test negative cows, PBMCs were isolated and analyzed by immunostaining and flow cytometry. Fresh PBMCs from MAP test positive cows (n = 12) contained a significantly higher proportion of IL-23R positive cells in populations of CD4+, CD8+, and Yδ + T cells than in cells from MAP test negative cows (n = 12; p < 0.05). Treatment with MAP antigens increased the percentage of all T cell subsets with surface expression of IL-23R when compared to untreated (n = 12; p < 0.05) cells. ELISA results for IL-17a secretion revealed a higher concentration of IL-17a secreted from PBMCs treated with MAP antigen (n = 20) than from PBMCs not treated with MAP antigens (n = 20) (p < 0.001), regardless of the JD test status of source cows. Also, we observed a moderate negative correlation between JD diagnostic scores for JD + cows and plasma IL-17a concentration (n = 42; r = -0.437; p-value < 0.004). Plasma with low and mid JD- scores (n = 31; n = 9; 0.1 ≤ X < 0.3) had significantly more IL-17a when compared to plasma with high JD- scores (n = 10; 0.3 ≤ X < 0.46; p-values < 0.05). Similarly, plasma with low JD + score values (0.55 ≤ X < 1.0; n = 9) had significantly more IL-17a when compared to plasma with high JD + score values (X ≥ 2.0; n = 21; p < 0.05). Overall, plasma from JD + cows (0.55 < X ≤ 2.86; n = 41) had significantly less IL-17a than plasma from JD- cows (0 < X ≤ 0.46; n = 70). Our data suggests that Th17-like cells may indeed play a role in early immune responses to MAP infection and development or control of JD.

Proteome Analysis of a M. avium Mutant Exposes a Novel Role of the Bifunctional Protein LysX in the Regulation of Metabolic Activity

Lysyl-phosphatidylglycerol is one of the components of the mycobacterial membrane that contributes to the resistance to cationic antimicrobial peptides, a host-induced frontline defense against invading pathogens. Its production is catalyzed by LysX, a bifunctional protein with lysyl transferase and lysyl transfer RNA synthetase activity. Comparative proteome analysis of a lysX mutant of Mycobacterium avium strain 104 and the wild type indicated that the lysX mutant strain undergoes a transition in phenotype by switching the carbon metabolism to β-oxidation of fatty acids, along with accumulation of lipid inclusions. Surprisingly, proteins associated with intracellular survival were upregulated in the lysX mutant, even during extracellular growth, preparing bacteria for the conditions occurring inside host cells. In line with this, the lysX mutant exhibited enhanced intracellular growth in human-blood-derived monocytes. Thus, our study exposes the significance of lysX in the metabolism and virulence of the environmental pathogen M. avium hominissuis.

Current topics of molecular mycobacteriology

The 2nd St. Petersburg Symposium on Tuberculosis and Mycobacteria: Molecular Approach, was held in St. Petersburg, Russia on 5-6 December 2018. A special issue of Infection, Genetics and Evolution will publish articles based on the selected presentations. In this paper, I will discuss some of the hot topics of molecular mycobacteriology highlighted at this meeting that I had the pleasure to organize and honor to chair. The symposium addressed interrelated fundamental and applied issues of modern mycobacteriology such as molecular evolution and phylogenomics, host-microbe interactions and pathogenesis, coevolution of M. tuberculosis with humans, new genomic and postgenomic technologies. Molecular methods for TB diagnostics and drug resistance detection are supported by WHO and whole genome/next generation sequencing presents a comprehensive approach. At the same time, cost and implementation of new methods for direct analysis of clinical samples and/or in low-resource settings remain a great challenge. A due attention was also given to the medically important nontuberculous mycobacteria. Assessment of spectrum of the circulating mycobacterial species in the Russian Federation and the countries of the European Union was presented and the underlying reasons of the observed diversity were discussed. To conclude, the symposium became a multidisciplinary event that was useful to promote networking and exchange of knowledge and experience. The next (third) symposium was planned to be organized in 2021.

Presence of Infected Gr-1intCD11bhiCD11cint Monocytic Myeloid Derived Suppressor Cells Subverts T Cell Response and Is Associated With Impaired Dendritic Cell Function in Mycobacterium avium-Infected Mice

Myeloid-derived suppressor cells (MDSC) are immature myeloid cells with immunomodulatory function. To study the mechanism by which MDSC affect antimicrobial immunity, we infected mice with two M. avium strains of differential virulence, highly virulent Mycobacterium avium subsp. avium strain 25291 (MAA) and low virulent Mycobacterium avium subsp. hominissuis strain 104 (MAH). Intraperitoneal infection with MAA, but not MAH, caused severe disease and massive splenic infiltration of monocytic MDSC (M-MDSC; Gr-1^intCD11b^hiCD11c^int) expressing inducible NO synthase (Nos2) and bearing high numbers of mycobacteria. Depletion experiments demonstrated that M-MDSC were essential for disease progression. NO production by M-MDSC influenced antigen-uptake and processing by dendritic cells and proliferation of CD4⁺ T cells. M-MDSC were also induced in MAA-infected mice lacking Nos2. In these mice CD4⁺ T cell expansion and control of infection were restored. However, T cell inhibition was only partially relieved and arginase (Arg) 1-expressing M-MDSC were accumulated. Likewise, inhibition of Arg1 also partially rescued T cell proliferation. Thus, mycobacterial virulence results in the induction of M-MDSC that block the T cell response in a Nos2- and Arg1-dependent manner.

Mycobacterium avium complex infection in pigs: A review

Mycobacterial infections in pigs are caused particularly by the Mycobacterium avium complex (MAC) and these infections lead to great economic losses mainly within the countries with high pork meat production. The importance of the MAC infections in humans is rising because of its higher prevalence and also higher mortality rates particularly in advanced countries. In addition, treatment of the MAC infections in humans tends to be complicated because of its increasing resistance to antimicrobial agents. Several studies across Europe have documented the MAC occurrence in the slaughtered pigs - not only in their lymph nodes and tonsils, which are the most frequent, but also in the diaphragmas, other organs and not least in meat. This is why we need both more specific and more sensitive methods for the MAC infection detection. Different PCR assays were established as well as advanced intravital testing by the gamma interferon release test. On the other hand, tuberculin skin test is still one of the cheapest methods of mycobacterial infections detection.

Phylogenetic uniqueness of Mycobacterium avium subspecies hominissuis isolated from an abnormal pulmonary bovine case

Mycobacterium avium subspecies hominissuis (MAH) is an important cause of infection in human pulmonary and swine intestinal cases. Although MAH is isolated from environmental sources frequently, infections of other animals have rarely been analysed. Recently, we detected granulomatous inflammation in bovine lung as an abnormal postmortem inspection case. To ascertain its genetic profile, we conducted a variable numbers of tandem repeats (VNTR) analysis and genomic characterization using deep sequencing. The VNTR type was a unique profile that differed from reported genotypes, but it was assigned within a broad genotypic complex of isolates from human patients and bathrooms. Genomic comparison with 116 registered genome sequences of the subspecies revealed that the strain was separate from five major genetic population groups proposed previously. Although the infection source remains unclear, its isolation from various resources such as animal infection cases should be elucidated more extensively to reveal its genetic diversity and ecological context.

Insights from the Genome Sequence of Mycobacterium lepraemurium: Massive Gene Decay and Reductive Evolution

Mycobacterium lepraemurium is the causative agent of murine leprosy, a chronic, granulomatous disease similar to human leprosy. Due to the similar clinical manifestations of human and murine leprosy and the difficulty of growing both bacilli axenically, Mycobacterium leprae and M. lepraemurium were once thought to be closely related, although it was later suggested that M. lepraemurium might be related to Mycobacterium avium. In this study, the complete genome of M. lepraemurium was sequenced using a combination of PacBio and Illumina sequencing. Phylogenomic analyses confirmed that M. lepraemurium is a distinct species within the M. avium complex (MAC). The M. lepraemurium genome is 4.05 Mb in length, which is considerably smaller than other MAC genomes, and it comprises 2,682 functional genes and 1,139 pseudogenes, which indicates that M. lepraemurium has undergone genome reduction. An error-prone repair homologue of the DNA polymerase III α-subunit was found to be nonfunctional in M. lepraemurium, which might contribute to pseudogene formation due to the accumulation of mutations in nonessential genes. M. lepraemurium has retained the functionality of several genes thought to influence virulence among members of the MAC.

Mycobacterium lepraemurium seems to be evolving toward a minimal set of genes required for an obligatory intracellular lifestyle within its host, a niche seldom adopted by most mycobacteria, as they are free-living. M. lepraemurium could be used as a model to elucidate functions of genes shared with other members of the MAC. Its reduced gene set can be exploited for studying the essentiality of genes in related pathogenic species, which might lead to discovery of common virulence factors or clarify host-pathogen interactions. M. lepraemurium can be cultivated in vitro only under specific conditions and even then with difficulty. Elucidating the metabolic (in)capabilities of M. lepraemurium will help develop suitable axenic media and facilitate genetic studies.

Systemic mycobacteriosis caused by 'Mycobacterium avium subspecies hominissuis' in a 14-month-old Japanese black beef steer

A 14-month-old Japanese black beef steer presented with severe chronic diarrhea and emaciation and was euthanized. Postmortem examination showed thickened and corrugated intestinal mucosa and enlarged granulomatous mesenteric lymph nodes with caseating necrosis. Numerous epithelioid cells and multinucleated giant cells infiltrated in the lamina propria and the submucosal tissue of the intestines. These cells were also observed in the systemic organs. Many acid-fast bacilli were detected in the cytoplasm of these cells and were identified as 'Mycobacterium avium subsp. hominissuis' (Mah) on the basis of the results of molecular examinations and immunohistochemistry. These findings indicate that Mah can cause systemic mycobacteriosis, and this unique infection needs to be distinguished from Johne's disease and tuberculosis in cattle.

Complete Genome Sequence of Mycobacterium avium subsp. hominissuis Strain H87 Isolated from an Indoor Water Sample

Mycobacterium avium subsp. hominissuis is an environmentally acquired bacterium known to cause pulmonary and soft tissue infections, lymphadenitis, and disseminated disease in humans. We report here the complete genome sequence of strain H87, isolated from an indoor water sample, as a single circular chromosome of 5,626,623 bp with a G+C content of 68.8%.

Genotyping and strain distribution of Mycobacterium avium subspecies hominissuis isolated from humans and pigs in Belgium, 2011-2013

Mycobacterium avium represents a health concern for both humans and pigs. The characterisation of its subspecies is an important step improving the understanding of the epidemiology and the control of this pathogen. Ninety-two human M. avium strains were selected for a retrospective study. Subspecies determination by rpoB sequencing and IS1245/IS901 analysis showed that 98.9% of Belgian human M. avium strains belong to the subspecies hominissuis (MAH). Some of these MAH strains present particular IS1245/IS901 profiles (absence of IS1245 and false IS901 detection provoked by the presence of ISMav6). In addition, 54 MAH strains isolated from submandibular lymph nodes of Belgian pigs with lymphadenitis were included in this study. Genotyping of human and porcine isolates was performed using multispacer sequence typing (MST). In total, 49 different MST types were identified among pig (n = 11) and human (n = 43) MA isolates, with only five shared by both hosts. Among these MST types, 34 were newly identified. Our findings demonstrate the extensive genetic diversity among MAH isolates. Some genotypes were more prevalent in human or pigs but no correlation was observed between MST type and place of residence or the farm of origin for human and porcine isolates respectively, suggesting an environmental source of infection.

Genetic diversity of clinical Mycobacterium avium subsp. hominissuis and Mycobacterium intracellulare isolates causing pulmonary diseases recovered from different geographical regions

Mycobacterium avium complex (MAC) infections are increasing annually in many countries. MAC strains are the most common nontuberculous mycobacterial pathogens isolated from respiratory samples and predominantly consist of two species, Mycobacterium avium and Mycobacterium intracellulare. The aim of this study was to analyze the molecular epidemiology and genetic backgrounds of clinical MAC isolates collected from The Netherlands, Germany, United States, Korea and Japan. Variable numbers of tandem repeats (VNTR) analysis was used to examine the genetic relatedness of clinical isolates of M. avium subsp. hominissuis (n=261) and M. intracellulare (n=116). Minimum spanning tree and unweighted pair group method using arithmetic averages analyses based on the VNTR data indicated that M. avium subsp. hominissuis isolates from Japan shared a high degree of genetic relatedness with Korean isolates, but not with isolates from Europe or the United States, whereas M. intracellulare isolates did not show any specific clustering by geographic origin. The findings from the present study indicate that strains of M. avium subsp. hominissuis, but not M. intracellulare, exhibit geographical differences in genetic diversity and imply that MAC strains may have different sources, routes of transmission and perhaps clinical manifestations.

The Mycobacterium avium ssp. paratuberculosis specific mptD gene is required for maintenance of the metabolic homeostasis necessary for full virulence in mouse infections

Mycobacterium avium subspecies paratuberculosis (MAP) causes Johne's disease, a chronic granulomatous enteritis in ruminants. Furthermore, infections of humans with MAP have been reported and a possible association with Crohn's disease and diabetes type I is currently discussed. MAP owns large sequence polymorphisms (LSPs) that were exclusively found in this mycobacteria species. The relevance of these LSPs in the pathobiology of MAP is still unclear. The mptD gene (MAP3733c) of MAP belongs to a small group of functionally uncharacterized genes, which are not present in any other sequenced mycobacteria species. mptD is part of a predicted operon (mptABCDEF), encoding a putative ATP binding cassette-transporter, located on the MAP-specific LSP14. In the present study, we generated an mptD knockout strain (MAPΔmptD) by specialized transduction. In order to investigate the potential role of mptD in the host, we performed infection experiments with macrophages. By this, we observed a significantly reduced cell number of MAPΔmptD early after infection, indicating that the mutant was hampered with respect to adaptation to the early macrophage environment. This important role of mptD was supported in mouse infection experiments where MAPΔmptD was significantly attenuated after peritoneal challenge. Metabolic profiling was performed to determine the cause for the reduced virulence and identified profound metabolic disorders especially in the lipid metabolism of MAPΔmptD. Overall our data revealed the mptD gene to be an important factor for the metabolic adaptation of MAP required for persistence in the host.

Facts, myths and hypotheses on the zoonotic nature of Mycobacterium avium subspecies paratuberculosis

Mycobacterium avium subspecies paratuberculosis (MAP) is the causative agent of paratuberculosis (Johne's disease [JD]), a chronic granulomatous enteritis in ruminants. JD is one of the most widespread bacterial diseases of domestic animals with significant economic impact. The histopathological picture of JD resembles that of Crohn's disease (CD), a human chronic inflammatory bowel disease of still unresolved aetiology. An aetiological relevance of MAP for CD has been proposed. This and the ambiguity of other published epidemiological findings raise the question whether MAP represents a zoonotic agent. In this review, we will discuss evidence that MAP has zoonotic capacity.

Relatedness of Mycobacterium avium subspecies hominissuis clinical isolates of human and porcine origins assessed by MLVA

Mycobacterium avium subsp. hominissuis (MAH) is an important opportunistic pathogen, infecting humans and animals, notably pigs. Several methods have been used to characterize MAH strains. RFLP and PFGE typing techniques have been used as standard methods but are technically demanding. In contrast, the analysis of VNTR loci is a simpler, affordable and highly reliable PCR-based technique, allowing a numerical and reproductive digitalization of typing data. In this study, the analysis of Mycobacterium avium tandem repeats (MATRs) loci was adapted to evaluate the genetic diversity of epidemiological unrelated MAH clinical strains of human (n=28) and porcine (n=69) origins, collected from diverse geographical regions across mainland Portugal. These MAH isolates were found to be genetically diverse and genotypes are randomly distributed across the country. Some of the human strains shared identical VNTR profiles with porcine isolates. Our study shows that the VNTR genotyping using selected MATR loci is a useful analysis technique for assessing the genetic diversity of MAH isolates from Portugal. This typing method could be successfully applied in other countries toward the implementation of a worldwide open-access database of MATR-VNTR profiles of MAH isolates, allowing a better assessment of the global epidemiology traits of this important pathogenic species.

Mycobacterium avium genotype is associated with the therapeutic response to lung infection

Factors that can interfere with the successful treatment of Mycobacterium avium lung infection have been inadequately studied. To identify a potent predictor of therapeutic responses of M. avium lung infection, we analyzed variable number tandem repeats (VNTR) at 16 minisatellite loci of M. avium clinical isolates. Associations between the VNTR profiling data and a therapeutic response were evaluated in 59 subjects with M. avium lung infection. M. avium lung infection of 30 subjects in whom clarithromycin-containing regimens produced microbiological and radiographic improvement was defined as responsive disease, while that of the remaining 29 subjects was defined as refractory disease. In phylogenetic analysis using the genotypic distance aggregated from 16-dimensional VNTR data, 59 M. avium isolates were divided into three clusters, which showed a nearly significant association with therapeutic responses (p 0.06). We then subjected the raw 16-dimensional VNTR data directly to principal component analysis, and identified the genetic features that were significantly associated with the therapeutic response (p <0.05). By further analysis of logistic regression with a stepwise variable-selection, we constructed the highest likelihood multivariate model, adjusted for age, to predict a therapeutic response, using VNTR data from only four minisatellite loci. In conclusion, we identified four mycobacterial minisatellite loci that together were associated with the therapeutic response of M. avium lung infections.

Bacterial pathogens commandeer Rab GTPases to establish intracellular niches

Intracellular bacterial pathogens deploy virulence factors termed effectors to inhibit degradation by host cells and to establish intracellular niches where growth and differentiation take place. Here, we describe mechanisms by which human bacterial pathogens (including Chlamydiae; Coxiella burnetii; Helicobacter pylori; Legionella pneumophila; Listeria monocytogenes; Mycobacteria; Pseudomonas aeruginosa, Salmonella enterica) modulate endocytic and exocytic Rab GTPases in order to thrive in host cells. Host cell Rab GTPases are critical for intracellular transport following pathogen phagocytosis or endocytosis. At the molecular level bacterial effectors hijack Rab protein function to: evade degradation, direct transport to particular intracellular locations and monopolize host vesicles carrying molecules that are needed for a stable niche and/or bacterial growth and differentiation. Bacterial effectors may serve as specific receptors for Rab GTPases or as enzymes that post-translationally modify Rab proteins or endosomal membrane lipids required for Rab function. Emerging data indicate that bacterial effector expression is temporally and spatially regulated and multiple virulence factors may act concertedly to usurp Rab GTPase function, alter signaling and ensure niche establishment and intracellular bacterial growth, making this field an exciting area for further study.