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Category: Bacterial Pathogenesis
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A current review of basic research on Rickettsiales biology and pathogenesis in one comprehensive volume. The text details the scientific knowledge about how these obligate intracellular bacteria invade, survive and replicate inside eukaryotic cells. It also describes the spectrum of disease caused by an infection and the role of vectors in tramsmission, and discusses protective and pathologic immune responses and establishment of persisten infection. In addition, the text describes the latest developments including genomics and progress in vaccine development.
The title serves as a significant research book for scientists, physicians, medical students, public health professionals, epidemiologists, biocomputational scientists and government policy makers.
Hardcover, 443 pages, full-color illustrations, index.
This chapter reviews and discusses (i) the human disease manifestations for major pathogenic groups, with some attention to detail on how these differ between genera and species within a single genus; (ii) the evolution of treatment for rickettsial infections; (iii) the existing challenges for treatment and management in human infections; and (iv) corroborated or suspected treatment failures or persistent clinical complaints. The underlying theme for disease manifestations in the Rickettsiaceae is systemic infection of endothelial cells, leading to increased vascular permeability. Once diagnosed, rickettsioses, ehrlichiosis, and anaplasmosis are primarily managed with specific antimicrobial agent therapy. The major advantage of doxycycline is its twice-daily oral dosing, as opposed to three or four doses daily for tetracycline. In general, tetracycline antibiotics are considered rickettsiastatic, not rickettsiacidal. This concept has led to several specific recommendations regarding treatment in rickettsial disease. First, treatment should be provided for long enough that immune recognition and clearance of active infection can proceed to eliminate viable although nonreplicating rickettsia surviving within intracellular niches. Second, since proper immune induction is in part dependent on rickettsial antigen mass, early therapy can sometimes simply suppress growth in the absence of waxing immunity, leading to relapse after apparent appropriate treatment doses and intervals. The identification of innate resistance owing to naturally occurring genetic mutations that confer resistance to currently useful antibiotics underscores the degree of genetic diversity observed in Rickettsiales genome projects.
Rickettsial diseases have had a profound impact on human lives for centuries but were poorly understood prior to the beginning of the 20th century. The study of the pathogens is challenging and requires an intimate understanding of arthropod vectors, mammalian hosts, and human nature. This chapter focuses on rickettsial infections of public health importance to humans, primarily focusing on rickettsiae in the United States. It also briefly addresses the global importance of rickettsial pathogens. An understanding of the epidemiology of rickettsial diseases requires not only a look at contemporary public health issues, but also an understanding of the history of these infections and classic approaches to control and prevention. Since the early investigations of Ricketts and others into the origins and ecology of Rocky Mountain spotted fever (RMSF), public health interest in rickettsiae has remained strong in the United States. The chapter concentrates on the historical and current epidemiology and public health importance of rickettsial pathogens known to occur in the United States. R. parkeri infection can be difficult to distinguish from infection with Rickettsia rickettsii, as extensive serologic cross-reactivity occurs between this and other spotted fever group rickettsial (SFGR) species. From 2000 to 2007, 3,126 cases of presumed Ehrlichia chaffeensis and an additional 824 cases classified as "Ehrlichiosis Undetermined/Unspecified/Other Agent (UUOA)" were reported within the United States. Most of these Ehrlichiosis UUOA cases were probably E. chaffeensis or Anaplasma phagocytophilum cases that could not be properly classified.
In light of the age of genomics, a compilation of small-subunit rRNA-encoding genes (16S rRNA genes) from Rickettsiales is provided, with a phylogeny estimated from a subset of the sequences that spans the diversity of Rickettsiales. Robust phylogeny estimation based on whole-genome sequences supports the close relationship of Bartonellaceae with another lineage of facultative intracellular bacteria, Brucellaceae. The first genome sequence from the Anaplasmataceae also reveals reductive genome evolution, with a genome size of 1.3 Mb and 1,195 predicted open reading frames (ORFs). Significant reclassification of the species within the Anaplasmataceae was proposed based on data. Subsequent reevaluation of the trace file archives determined that the Wolbachia sequences discovered in D. mojavensis were an artifact; however, 2,291 novel Wolbachia sequences were found in another fly genome, Drosophila willistoni. This work underscores the utility of eukaryotic genomes for discovering Rickettsiales endosymbionts and potentially assembling partial to complete bacterial genomes from the eukaryotic reads. A robust phylogeny based on whole-genome sequences supports the current taxonomic delineations within the Anaplasmataceae and Rickettsiaceae, with monophyly of each of the six genera strongly supported. The differences between RefSeq and Pathosystems Resource Integration Center (PATRIC) annotation undoubtedly result in discrepancies in protein family clustering between this work and previous studies. Two mitochondrial small-subunit rRNA gene sequences were included in the phylogeny estimation, and this lineage branched after the Holosporaceae, but basal to the derived Rickettsiales. The diversity of the Rickettsiales highlighted in this chapter presents an exciting challenge for rickettsiology.
The majority of species belonging to the genus Rickettsia are divided into two groups, the spotted fever group rickettsiae (SFGR) and typhus group rickettsiae (TGR), based on the diseases that they cause, the presence of major surface antigens, and the ability to promote intracellular actin-based motility. This chapter focuses on the cell biology involved in the internalization of SFGR into nonphagocytic mammalian cells using Rickettsia conorii as a model organism. It highlights the current knowledge regarding the bacterial proteins and cognate host cell receptors involved in initiating this process. Early studies on the mechanism(s) utilized by rickettsiae to invade nonphagocytic mammalian cells identified cellular actin dynamics as playing an important role. In some invasive pathogens, depletion of membrane cholesterol using methyl-β-cyclodextrin disrupts the composition of lipid rafts and inhibits R.conorii invasion of nonphagocytic cells, suggesting that the presence of Ku70 within these microdomains is important for efficient bacterial entry. Mechanistic similarities to the invasion pathways utilized by zippering pathogens suggested that R.conorii also usurps these types of signaling events. Bioinformatic analyses of sequenced rickettsial genomes revealed the presence of a gene family. Four genes in this family, namely sca0, sca1, sca2, and sca5, are present as intact open reading frames in the genomes of the majority of SFGR. The signals involved in rickettsial outer membrane protein B (rompB)-dependent invasion closely resemble those observed during the invasion of R.conorii into Vero cells and include the activation of actin and microtubule dynamics and the stimulation of protein tyrosine kinase and PI-kinase activities.
Much progress has been made in one's understanding of the mechanism of rickettsial actin-based motility, although the process of cell-to-cell spread remains poorly understood. This chapter examines historical and recent developments in our understanding of how rickettsiae establish intracellular infection. The focus is on the three major stages that include: (i) escape from the phagosome, (ii) intracellular growth, and (iii) actinbased motility. Several bacterial activities that may function in membrane disruption have since been discovered, including phospholipase A2 (PLA2), phospholipase D (PLD), and hemolysins (TlyA and TlyC). Each of these activities and its potential role in phagosome escape is discussed in the chapter. Interestingly, the growth kinetics for the spotted fever group rickettsiae (SFGR) species R. rickettsii did not follow the simple kinetics observed for the typhus group rickettsiae (TGR) species R.prowazekii. The function of intracellular movement is to promote cell-to-cell spread, a process that is discussed in the chapter.
Studies of representative members, primarily Anaplasma phagocytophilum and Ehrlichia chaffeensis, have shed much light on the exquisite mechanisms by which Anaplasmataceae pathogens manipulate their host cells, and are discussed following overviews of the diseases that they cause, their notable genomic features, and their infection and developmental cycles. The secretion of type IV secretion system (T4SS) effectors into host cells is critical for survival of facultative and obligate intracellular bacterial pathogens. Apoptosis is initiated by enzymatic caspases, which are inactive until they are activated by apoptotic signaling pathways. The A. phagocytophilum-occupied vacuole (ApV) excludes fusion with secretory vesicles and specific granules harboring NADPH oxidase and proteolytic enzymes. Preferentially recruiting Rab GTPases that are predominantly found on slow recycling endosomes potentially provides A. phagocytophilum with four intracellular survival advantages. First, A. phagocytophilum is auxotrophic for 16 amino acids. Second, the mechanism by which A. phagocytophilum obtains LDL endocytic pathway-derived cholesterol for incorporation into its cell wall is unknown. Third, continual delivery of recycling endosomes to the ApV would conceivably provide an unlimited supply of host membrane material to allow for expansion of the AVM, which would be necessary to accommodate growing intravacuolar bacterial populations. Fourth, by coating the AVM with recycling endosome- associated Rab GTPases, the ApV camouflages itself as a recycling endosome, which is likely a means by which it protects itself from fusing with lysosomes. Finally, much of what authors know regarding Anaplasmataceae pathogen manipulation of host cell functions is derived from studies of A. phagocytophilum and E. chaffeensis.
One of the major emphases of the author's research program is to understand how obligate intracytoplasmic growth has affected the physiology of Rickettsia prowazekii. This chapter discusses metabolism and reductive evolution from the pathogenic rickettsia's point of view. Rapid advances in sequencing technologies have contributed to the ever-expanding availability of genome sequence information. This has significantly augmented our understanding of the factors that influence virulence and shape pathogen evolution at the genome level. The chapter summarizes the studies describing rickettsial physiology and metabolism before 1998, when the first rickettsial genome sequence became available. It provides insight into some of the key experiments that guided the field during a productive period in rickettsial research. The R.prowazekii adenosine triphosphate (ATP)/adenosine diphosphate (ADP) translocase is the best-characterized rickettsial transport system. It is well established that the ATP/ADP translocase functions via an obligate exchange antiport mechanism and thus requires the presence of substrate on both sides of the membrane to catalyze transport. Studies examining the physiology of rickettsiae that are growing intracellularly have contributed much to the understanding of rickettsia-host interactions. The chapter discusses how obligate intracellular growth has affected the rickettsia's capacity for gene regulation. As a final facet of rickettsial gene regulation, transcriptional termination is also explained in the chapter.
Although spotted fever group rickettsiae (SFGR) and typhus group rickettsiae (TGR) represent two major antigenically defined and historically well-known subdivisions of pathogenic Rickettsia species, recent in-depth characterization by neighbor-joining phylodendrogramic analysis has distributed rickettsiae into ancestral, spotted fever, typhus, and transitional subgroups. Importantly, well-established and widely accepted in vivo models of infection closely mimicking the pathogenesis of Rocky Mountain spotted fever (RMSF) and epidemic typhus in humans employ infection of susceptible mouse strains with Rickettsia conorii and R. typhi, respectively. The endothelial cell responses to R. prowazekii and signaling mechanisms that determine the interplay between the host and the unique rickettsial pathogen, which is capable of escaping immune surveillance to cause recrudescent infections, remain critically important but neglected areas of scientific inquiry. Thus, one of the major critical gaps in the understanding of rickettsial pathogenesis is the definition of the biological basis of rickettsial affinity and consequent interactions with vascular endothelium. R. rickettsii infection induces a biphasic pattern of NF-ΚB activation in cultured human endothelial cells that is characterized by an early transient activation phase and a late sustained phase. There is increasing recognition that apoptosis, a tightly regulated process of altruistic suicide, plays a central role in complex interactions between an invading pathogen and host cell defense. First recognized for their ability to impede viral replication, interferons (IFNs) play a critical role in determining host survival in response to viral infection. During bacterial infections, IFN signaling defends the host by integrating early innate immune responses with later events.
The family Anaplasmataceae includes several species of obligate intracellular gram-negative bacteria. The early immune responses to Ehrlichia and Anaplasma infections are likely to be important factors in determining the outcome of infection and clinical course of disease. This chapter discusses recent studies on the kinetics and quality of early immune responses to Ehrlichia and briefly to Anaplasma, and the implications for development of successful preventative immune-based therapeutic approaches. Five Anaplasmataceae members can infect humans, but only two of them—Ehrlichia chaffeensis, the causative agent of human monocytic ehrlichiosis (HME); and Anaplasma phagocytophilum, the causative agent of human granulocytic anaplasmosis (HGA)—have been thoroughly investigated. There is only limited information on the mechanisms of pathogenesis of Ehrlichia and Anaplasma and the role of host innate immune responses in contributing to disease virulence in humans; therefore, animal models are necessary to identify bacterial and host characteristics of disease. Murine models have been developed to study differences in host immunity and disease susceptibility. E.muris-infected DCs from MyD88— / — mice are able to stimulate gamma interferon (IFN-ү); production by innate invariant natural killer T (iNKT) lymphocytes. Such responses were dependent on direct recognition of unidentified ehrlichial ligands by CD1d, but were not dependent on TLR signals. The author has investigated the contribution of NK cells to protective immunity against Ehrlichia. Studies of the pathogenesis of severe and fatal ehrlichiosis and anaplasmosis, or protective immunity against Ehrlichia and Anaplasma infection, have largely focused on the analysis of effector CD4+ T cells and their control by CD8+ T cells.
Genera belonging to the family Rickettsiaceae, Rickettsia and Orientia, include human pathogens. The diseases they cause are referred to as rickettsioses in this chapter. They share three critical factors with implications in pathogenesis and immunity: (i) they are transmitted by arthropod vectors; (ii) they are obligate intracellular bacteria that inhabit the cell cytoplasm; and (iii) the predominant target is the endothelium. The only exception is Rickettsia akari, the agent of rickettsial-pox, which predominantly infects monocytes and macrophages. The chapter emphasizes on some findings and suggests a framework for a modern conceptualization of the field of rickettsiology at the interface with immunology. The development of the adaptive immune response is conditioned by the innate immune mechanisms activated during early events of the infection. The study of the endothelium in the context of true endothelium-target infections offers new opportunities to explore the role of the endothelium in orchestrating or modifying immune responses. The study of the response to two of the most successful human vaccines in history, the yellow fever vaccine and the smallpox vaccine, is likely to yield relevant paradigms that we could use as guideposts in rickettsiology. Development of a modern vaccine against Rickettsiaor Orientia should aim at mimicking a physiological immune response in the sense that all branches of adaptive immunity should be stimulated. This implies the identification of a combination of antigens that together can stimulate protective responses mediated by CD8+ T cells, CD4+ T cells, and B cells (antibodies).
The critical interface between the host immune system and the pathogen in large part determines the outcome of an infection. In this chapter, the author presents a discussion on immunity to enable the readers understand how a pathogen is able to establish infection and how the immune system is able to control or clear infection. The adaptive immune response is of particular interest because under the appropriate conditions it can lead to long-lasting, pathogen-specific immunity, which is the basis for vaccine development. One of the challenges when considering Anaplasma, Ehrlichia, Neorickettsia, and Wolbachia is the extent and variety of ways in which many of these organisms modulate the host immune response. A. phagocytophilum and A. marginale modulate the host immune response, though in a somewhat different fashion. Importantly, most animals that survive acute disease are able to control but not clear the infection. In a study conducted, both immunized and infected animals had comparable antibody repertoires to major surface protein 2 (MSP2) in terms of breadth of response and titer. Among the immunized animals, there was no association between either breadth or magnitude of the anti-MSP2 response and either complete protection from infection or control of bacteremia. Together, these data argue that protection afforded with the outer membrane vaccine is due to immune responses directed at outer membrane proteins (OMPs) other than the immunodominant and antigenically variable MSP2. Virulence factors are targets for some of the most effective vaccines currently in use.
This chapter focuses on the genera Anaplasma and Ehrlichia. Typically, antigenically variable proteins are immunodominant and thus allow evasion of the predominant immune responses. The donor allele repertoire remains unchanged during antigenic variation, while the expression-site variant is lost. This method is employed by some rickettsiae in the family Anaplasmataceae and allows for lifelong persistence in the host with donor allele repertoires 10- to 100-fold smaller than those found in African trypanosomes. Earlier work has demonstrated that major surface proteins (MSP)2 and MSP3 are immunodominant, antigenically variable proteins that are instrumental in evading the host immune response. Anaplasma ovis, a small ruminant pathogen, has been shown to establish persistent infections in goats. Antigenic variation in A. phagocytophilum is effected through the homolog of A. marginale msp2. The antibody response to the MSP2 HVR is variant specific and diminishes rapidly, consistent with the idea that antigenic variation of MSP2 is responsible for persistence. Research shows that the implications of donor allele repertoires go beyond antigenic variation in the individual hosts, and that they also play critical roles in the epidemiology of pathogen strain structure and possibly host tropism.
Bacteria within the order Rickettsiales would have little impact on human and veterinary medicine in the absence of the arthropod vector. Interestingly, the influence of primary infections with one Rickettsia sp. can influence the success of transovarial transmission of a second. This chapter details some fascinating trends observed regarding vertical and horizontal transmission. Biotic and abiotic factors determine the stability of any sylvatic or zoonotic transmission cycle. The chapter centers on a discussion of the attributes of successful pathogen transmission in the context of the vector's ability to modulate (i) the mammalian host's response during acquisition and transmission and (ii) microbial growth within the vector during the maintenance phase. The discussion in these two sections essentially defines the environment and competency of both the vector and mammalian host as determinants of transmission and transmission efficiency. The chapter ends with a survey of fluctuating ecological trends that can enhance or diminish the potency of vector-borne rickettsial zoonotic cycles. Even though acquisition rates were similar for each transmission experiment, intergenera transmission required cofeeding of multiple infected mites with uninfected mites. Rickettsial diseases have the potential to change the outcomes of war and prey on the unfortunate circumstances that arise from disaster.
This chapter reviews the research that has led to the first successes in genetic transformation of arthropod-borne bacteria belonging to the order Rickettsiales in the Alphaproteobacteria subdivision. Seminal results provided the very first evidence that direct genetic manipulation of rickettsiae was achievable, and that the bacteria were able to maintain and express foreign genetic sequences inserted via allelic exchange/homologous recombination under control of rickettsial or Escherichia coli promoters. This work set the stage for all subsequent research efforts aspiring to manipulate and analyze rickettsiae in a manner that is nearly commonplace in extracellular bacteria. The Himar1 transposase allele A7 has been successfully used for mutagenesis of A.phagocytophilum and fever groups of rickettsial tick symbionts have been spotted using fluorescent markers and antibiotic resistance. Selection of mutants using growth inhibitors is an indispensible strategy to recover mutants from the background of nontransformed bacteria. The most common strategy is incorporation of an antibiotic resistance gene in the transformation cassette. When working with human or animal pathogens, this is a sensitive issue, as introduction of resistance to antibiotics used to treat disease induced by the pathogen to be transformed is not encouraged. This poses a dilemma, as the most effective selection is likely achieved by using the clinically most effective antibiotics. The authors suggests that shuttle vectors would be useful for testing the function of genes that are naturally defective in certain Rickettsia spp., in complementation assays, overexpression of native or foreign genes, testing gene regulation, and other applications carried out in E.coli.
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The Quarterly Review of Biology
The order Rickettsiales comprises an enormous diversity of bacteria that live and grow inside eukaryotic cells, including a number of important human pathogens that are vectored by blood-feeding arthropods, including ticks, lice, fleas, and mites. Within the family Rickettsiaceae, human pathogens include typhus group Rickettsia (e.g., R. prowazekii and R. typhi, the causal agents of epidemic typhus and murine typhus), spotted fever Rickettsia (e.g., R. rickettsii, the causal agent of Rocky Mountain spotted fever), and Orientia tsutsugamushi, the causal agent of scrub typhus. Within the family Anaplasmataceae, human pathogens include Ehrlichia chaffeensis and Anaplasma phagocytophilum, causal agents of human ehrlichiosis. As the lineage that gave rise to mitochondria is also closely allied with this order, the study of Rickettsiales may yield important insights into basic functions and evolution of the eukaryotic cell.
This book focuses on human pathogenic Rickettsiales and will be especially useful for microbiologists, immunologists, physicians, epidemiologists, and public health workers, as well as medical students and graduate students in microbiology and immunology. There are 14 chapters, all by different authors. The bulk of the volume focuses on how these pathogens invade mammalian cells, how they establish intracellular infections, and on innate and adaptive immune responses. This consists of paired chapters, alternating in focus between Rickettsiaceae and Anaplasmataceae, as pathogens from these two families exhibit major differences with important immunological consequences, with the former quickly escaping the host cell vacuole and the latter persisting within it. Also, the cell walls of pathogens in the Anaplasmataceae appear to lack lipopolysaccharides and peptidoglycan. Two chapters will be especially useful for physicians and public health workers—one on clinical aspects of infection, including diagnosis and treatment, and one on public health and epidemiology, primarily in the U.S. but touching on other parts of the world as well.
This book is timely as it discusses a number of important recent developments in the field. These include the fact that the incidence of infections has increased significantly. Also, a number of strains have only recently been shown to cause disease. Environmental surveys have uncovered a great diversity of novel strains in a wide range of eukaryotes. Genome sequencing has transformed the field, guiding searches for novel virulence factors and potential vaccine candidates. Rickettsia genomes are models in the study of reductive genome evolution. The recent discovery of plasmids in Rickettsia has also opened up new possibilities for genetic transformation.
The volume specifically avoids treatment of Wolbachia, a widespread vertically transmitted symbiont of terrestrial arthropods. There is little discussion of nonpathogenic Rickettsiales, except for an interesting chapter on phylogeny and diversity, including poorly studied lineages that infect protists. Other topics that receive less attention are effects of infection and immune responses in the arthropod vector, and the biology of infection in native nonhuman hosts.
The book is nicely produced with clear, useful figures and tables. In summary, this is an excellent reference and survey of human-pathogenic Rickettsiales.
The Quarterly Review of Biology
Volume 89, Number 3, Pages 271-272
Reviewer: Steve Perlman, Biology, University of Victoria, Victoria, British Columbia, Canada
Review Date: September 2014
This book reviews the recent advances in rickettsiology (Anaplasma, Ehrlichia, Neorickettsia, Orientia and Rickettsia) over recent years and as such provides a wealth of relevant, state-of-the-art thinking on these microbes. Rather than giving a complete review of the subject, this book presents new data, methods and current thinking regarding this group of fascinating micro- organisms. The book provides, by way of introduction, chapters on the clinical challenges, public health issues and comparative genomics before a series of chapters focused upon the pathogenesis of the group. This is rounded off with a thought-provoking chapter on perspectives for future directions of rickettsial research mediated through genetic manipulation. These have been written by active researchers in the field and thus give valuable insights into the recent findings that have changed our understanding of the Rickettsiales, thus providing a hugely valuable resource to other researchers in the field. By way of criticism, the book presents a very American view (particularly evident during the introductory chapters), and would benefit from a more global perspective, though this might also reflect funding support in this area. Furthermore, chapters are written as isolated units and are prone to contain some duplication when read back-to-back.
This book provides a wonderful resource to those young researchers coming into the field of rickettsiology and would also be valuable to clinical or veterinary researchers, but perhaps not quite so accessible to the non-specialist. The price is high and might be prohibitive to those who might be considering personal copies of this book.
Society for General Microbiology: Microbiology Today
Reviewer: Sally Cutler, University of East London
Review Date: February 2013