Structural features of lipid A of Rickettsia typhi

Pathogenic Rickettsia spp. as emerging models for bacterial biology

Our understanding of free-living bacterial models like Escherichia coli far outpaces that of obligate intracellular bacteria, which cannot be cultured axenically. All obligate intracellular bacteria are host-associated, and many cause serious human diseases. Their constant exposure to the distinct biochemical niche of the host has driven the evolution of numerous specialized bacteriological and genetic adaptations, as well as innovative molecular mechanisms of infection. Here, we review the history and use of pathogenic Rickettsia species, which cause an array of vector-borne vascular illnesses, as model systems to probe microbial biology. Although many challenges remain in our studies of these organisms, the rich pathogenic and biological diversity of Rickettsia spp. constitutes a unique backdrop to investigate how microbes survive and thrive in host and vector cells. We take a bacterial-focused perspective and highlight emerging insights that relate to new host-pathogen interactions, bacterial physiology, and evolution. The transformation of Rickettsia spp. from pathogens to models demonstrates how recalcitrant microbes may be leveraged in the lab to tap unmined bacterial diversity for new discoveries. Rickettsia spp. hold great promise as model systems not only to understand other obligate intracellular pathogens but also to discover new biology across and beyond bacteria.

Orientia and Rickettsia: different flowers from the same garden

Recent discoveries of basal extracellular Rickettsiales have illuminated divergent evolutionary paths to host dependency in later-evolving lineages. Family Rickettsiaceae, primarily comprised of numerous protist- and invertebrate-associated species, also includes human pathogens from two genera, Orientia and Rickettsia. Once considered sister taxa, these bacteria form distinct lineages with newly appreciated lifestyles and morphological traits. Contrasting other rickettsial human pathogens in Family Anaplasmataceae, Orientia and Rickettsia species do not reside in host-derived vacuoles and lack glycolytic potential. With only a few described mechanisms, strategies for commandeering host glycolysis to support cytosolic growth remain to be discovered. While regulatory systems for this unique mode of intracellular parasitism are unclear, conjugative transposons unique to Orientia and Rickettsia species provide insights that are critical for determining how these obligate intracellular pathogens overtake eukaryotic cytosol.

Why Does Piscirickettsia salmonis Break the Immunological Paradigm in Farmed Salmon? Biological Context to Understand the Relative Control of Piscirickettsiosis

Piscirickettsiosis (SRS) has been the most important infectious disease in Chilean salmon farming since the 1980s. It was one of the first to be described, and to date, it continues to be the main infectious cause of mortality. How can we better understand the epidemiological situation of SRS? The catch-all answer is that the Chilean salmon farming industry must fight year after year against a multifactorial disease, and apparently only the environment in Chile seems to favor the presence and persistence of Piscirickettsia salmonis. This is a fastidious, facultative intracellular bacterium that replicates in the host’s own immune cells and antigen-presenting cells and evades the adaptive cell-mediated immune response, which is why the existing vaccines are not effective in controlling it. Therefore, the Chilean salmon farming industry uses a lot of antibiotics—to control SRS—because otherwise, fish health and welfare would be significantly impaired, and a significantly higher volume of biomass would be lost per year. How can the ever-present risk of negative consequences of antibiotic use in salmon farming be balanced with the productive and economic viability of an animal production industry, as well as with the care of the aquatic environment and public health and with the sustainability of the industry? The answer that is easy, but no less true, is that we must know the enemy and how it interacts with its host. Much knowledge has been generated using this line of inquiry, however it remains insufficient. Considering the state-of-the-art summarized in this review, it can be stated that, from the point of view of fish immunology and vaccinology, we are quite far from reaching an effective and long-term solution for the control of SRS. For this reason, the aim of this critical review is to comprehensively discuss the current knowledge on the interaction between the bacteria and the host to promote the generation of more and better measures for the prevention and control of SRS.

Lipid A Structural Divergence in Rickettsia Pathogens

Spikes in rickettsioses occur as deforestation, urbanization, and homelessness increase human exposure to blood-feeding arthropods. Still, effective Rickettsia vaccines remain elusive.

Species of Rickettsia (Alphaproteobacteria: Rickettsiales) are obligate intracellular parasites of a wide range of eukaryotes, with recognized arthropod-borne human pathogens belonging to the transitional group (TRG), typhus group (TG), and spotted fever group (SFG) rickettsiae. Growing in the host cytosol, rickettsiae pilfer numerous metabolites to make a typical Gram-negative bacterial cell envelope. The O-antigen of rickettsial lipopolysaccharide (LPS) is immunogenic and has been shown to tether the S-layer to the rickettsial surface; however, little is known about the structure and immunogenicity of the Rickettsia lipid A moiety. The structure of lipid A, the membrane anchor of LPS, affects the ability of this molecule to interact with components of the host innate immune system, specifically the MD-2/TLR4 receptor complex. To dissect the host responses that can occur during Rickettsia in vitro and in vivo infection, structural analysis of Rickettsia lipid A is needed. Lipid A was extracted from four Rickettsia species and structurally analyzed. R. akari (TRG), R. typhi (TG), and R. montanensis (SFG) produced a similar structure, whereas R. rickettsii (SFG) altered the length of a secondary acyl group. While all structures have longer acyl chains than known highly inflammatory hexa-acylated lipid A structures, the R. rickettsii modification should differentially alter interactions with the hydrophobic internal pocket in MD2. The significance of these characteristics toward inflammatory potential as well as membrane dynamics between arthropod and vertebrate cellular environments warrants further investigation. Our work adds lipid A to the secretome and O-antigen as variable factors possibly correlating with phenotypically diverse rickettsioses.

IMPORTANCE Spikes in rickettsioses occur as deforestation, urbanization, and homelessness increase human exposure to blood-feeding arthropods. Still, effective Rickettsia vaccines remain elusive. Recent studies have determined that Rickettsia lipopolysaccharide anchors the protective S-layer to the bacterial surface and elicits bactericidal antibodies. Furthermore, growing immunological evidence suggests vertebrate sensors (MD-2/TLR4 and noncanonical inflammasome) typically triggered by the lipid A portion of lipopolysaccharide are activated during Rickettsia infection. However, the immunopotency of Rickettsia lipid A is unknown due to poor appreciation for its structure. We determined lipid A structures for four distinct rickettsiae, revealing longer acyl chains relative to highly inflammatory bacterial lipid A. Surprisingly, lipid A of the Rocky Mountain spotted fever agent deviates in structure from other rickettsiae. Thus, lipid A divergence may contribute to variable disease phenotypes, sounding an alarm for determining its immunopotency and possible utility (i.e., as an adjuvant or anti-inflammatory) for development of more prudent rickettsiacidal therapies.

Variation, Modification and Engineering of Lipid A in Endotoxin of Gram-Negative Bacteria

Lipid A of Gram-negative bacteria is known to represent a central role for the immunological activity of endotoxin. Chemical structure and biosynthetic pathways as well as specific receptors on phagocytic cells had been clarified by the beginning of the 21st century. Although the lipid A of enterobacteria including Escherichia coli share a common structure, other Gram-negative bacteria belonging to various classes of the phylum Proteobacteria and other taxonomical groups show wide variety of lipid A structure with relatively decreased endotoxic activity compared to that of E. coli. The structural diversity is produced from the difference of chain length of 3-hydroxy fatty acids and non-hydroxy fatty acids linked to their hydroxyl groups. In some bacteria, glucosamine in the backbone is substituted by another amino sugar, or phosphate groups bound to the backbone are modified. The variation of structure is also introduced by the enzymes that can modify electrostatic charges or acylation profiles of lipid A during or after its synthesis. Furthermore, lipid A structure can be artificially modified or engineered by the disruption and introduction of biosynthetic genes especially those of acyltransferases. These technologies may produce novel vaccine adjuvants or antagonistic drugs derived from endotoxin in the future.

Pattern Recognition Receptors in Innate Immunity to Obligate Intracellular Bacteria

Host pattern recognition receptors (PRRs) are crucial for sensing pathogenic microorganisms, launching innate responses, and shaping pathogen-specific adaptive immunity during infection. Rickettsia spp., Orientia tsutsugamushi, Anaplasma spp., Ehrlichia spp., and Coxiella burnetii are obligate intracellular bacteria, which can only replicate within host cells and must evade immune detection to successfully propagate. These five bacterial species are zoonotic pathogens of clinical or agricultural importance, yet, uncovering how immune recognition occurs has remained challenging. Recent evidence from in-vitro studies and animal models has offered new insights into the types and kinetics of PRR activation during infection with Rickettsia spp., A. phagocytophilum, E. chaffeensis, and C. burnetii, respectively. However, much less is known in these regards for O. tsutsugamushi infection, until the recent discovery for the role of the C-type lectin receptor Mincle during lethal infection in mice and in primary macrophage cultures. This review gives a brief summary for clinical and epidemiologic features of these five bacterial infections, focuses on fundamental biologic facets of infection, and recent advances in host recognition. In addition, we discuss knowledge gaps for innate recognition of these bacteria in the context of disease pathogenesis.

Activation of ASC Inflammasome Driven by Toll-Like Receptor 4 Contributes to Host Immunity against Rickettsial Infection

Rickettsiae are cytosolically replicating, obligately intracellular bacteria causing human infections worldwide with potentially fatal outcomes. We previously showed that Rickettsia australis activates ASC inflammasome in macrophages. In the present study, host susceptibility of ASC inflammasome-deficient mice to R. australis was significantly greater than that of C57BL/6 (B6) controls and was accompanied by increased rickettsial loads in various organs. Impaired host control of R. australis in vivo in ASC^-/- mice was associated with dramatically reduced levels of interleukin 1β (IL-1β), IL-18, and gamma interferon (IFN-γ) in sera. The intracellular concentrations of R. australis in bone marrow-derived macrophages (BMMs) of TLR4^-/- and ASC^-/- mice were significantly greater than those in BMMs of B6 controls, highlighting the important role of inflammasome and these molecules in controlling rickettsiae in macrophages. Compared to B6 BMMs, TLR4^-/- BMMs failed to secrete a significant level of IL-1β and had reduced expression levels of pro-IL-1β in response to infection with R. australis, suggesting that rickettsiae activate ASC inflammasome via a Toll-like receptor 4 (TLR4)-dependent mechanism. Further mechanistic studies suggest that the lipopolysaccharide (LPS) purified from R. australis together with ATP stimulation led to cleavage of pro-caspase-1 and pro-IL-1β, resulting in TLR4-dependent secretion of IL-1β. Taken together, these observations indicate that activation of ASC inflammasome, most likely driven by interaction of TLR4 with rickettsial LPS, contributes to host protective immunity against R. australis These findings provide key insights into defining the interactions of rickettsiae with the host innate immune system.

Rickettsia Lipid A Biosynthesis Utilizes the Late Acyltransferase LpxJ for Secondary Fatty Acid Addition

Members of the Rickettsia genus are obligate intracellular, Gram-negative coccobacilli that infect mammalian and arthropod hosts. Several rickettsial species are human pathogens and are transmitted by blood-feeding arthropods. In Gram-negative parasites, the outer membrane (OM) sits at the nexus of the host-pathogen interaction and is rich in lipopolysaccharide (LPS). The lipid A component of LPS anchors the molecule to the bacterial surface and is an endotoxic agonist of Toll-like receptor 4 (TLR4). Despite the apparent importance of lipid A in maintaining OM integrity, as well as its inflammatory potential during infection, this molecule is poorly characterized in Rickettsia pathogens. In this work, we have identified and characterized new members of the recently discovered LpxJ family of lipid A acyltransferases in both Rickettsia typhi and Rickettsia rickettsii, the etiological agents of murine typhus and Rocky Mountain spotted fever, respectively. Our results demonstrate that these enzymes catalyze the addition of a secondary acyl chain (C₁₄/C₁₆) to the 3'-linked primary acyl chain of the lipid A moiety in the final steps of the Raetz pathway of lipid A biosynthesis. Since lipid A architecture is fundamental to bacterial OM integrity, we believe that rickettsial LpxJ may be important in maintaining membrane dynamics to facilitate molecular interactions at the host-pathogen interface that are required for adhesion and invasion of mammalian cells. This work contributes to our understanding of rickettsial outer membrane physiology and sets a foundation for further exploration of the envelope and its role in pathogenesis.IMPORTANCE Lipopolysaccharide (LPS) triggers an inflammatory response through the TLR4-MD2 receptor complex and inflammatory caspases, a process mediated by the lipid A moiety of LPS. Species of Rickettsia directly engage both extracellular and intracellular immunosurveillance, yet little is known about rickettsial lipid A. Here, we demonstrate that the alternative lipid A acyltransferase, LpxJ, from Rickettsia typhi and R. rickettsii catalyzes the addition of C₁₆ fatty acid chains into the lipid A 3'-linked primary acyl chain, accounting for major structural differences relative to the highly inflammatory lipid A of Escherichia coli.

Wholly Rickettsia! Reconstructed Metabolic Profile of the Quintessential Bacterial Parasite of Eukaryotic Cells

Reductive genome evolution has purged many metabolic pathways from obligate intracellular Rickettsia (Alphaproteobacteria; Rickettsiaceae). While some aspects of host-dependent rickettsial metabolism have been characterized, the array of host-acquired metabolites and their cognate transporters remains unknown. This dearth of information has thwarted efforts to obtain an axenic Rickettsia culture, a major impediment to conventional genetic approaches. Using phylogenomics and computational pathway analysis, we reconstructed the Rickettsia metabolic and transport network, identifying 51 host-acquired metabolites (only 21 previously characterized) needed to compensate for degraded biosynthesis pathways. In the absence of glycolysis and the pentose phosphate pathway, cell envelope glycoconjugates are synthesized from three imported host sugars, with a range of additional host-acquired metabolites fueling the tricarboxylic acid cycle. Fatty acid and glycerophospholipid pathways also initiate from host precursors, and import of both isoprenes and terpenoids is required for the synthesis of ubiquinone and the lipid carrier of lipid I and O-antigen. Unlike metabolite-provisioning bacterial symbionts of arthropods, rickettsiae cannot synthesize B vitamins or most other cofactors, accentuating their parasitic nature. Six biosynthesis pathways contain holes (missing enzymes); similar patterns in taxonomically diverse bacteria suggest alternative enzymes that await discovery. A paucity of characterized and predicted transporters emphasizes the knowledge gap concerning how rickettsiae import host metabolites, some of which are large and not known to be transported by bacteria. Collectively, our reconstructed metabolic network offers clues to how rickettsiae hijack host metabolic pathways. This blueprint for growth determinants is an important step toward the design of axenic media to rescue rickettsiae from the eukaryotic cell.

A hallmark of obligate intracellular bacteria is the tradeoff of metabolic genes for the ability to acquire host metabolites. For species of Rickettsia, arthropod-borne parasites with the potential to cause serious human disease, the range of pilfered host metabolites is unknown. This information is critical for dissociating rickettsiae from eukaryotic cells to facilitate rickettsial genetic manipulation. In this study, we reconstructed the Rickettsia metabolic network and identified 51 host metabolites required to compensate patchwork Rickettsia biosynthesis pathways. Remarkably, some metabolites are not known to be transported by any bacteria, and overall, few cognate transporters were identified. Several pathways contain missing enzymes, yet similar pathways in unrelated bacteria indicate convergence and possible novel enzymes awaiting characterization. Our work illuminates the parasitic nature by which rickettsiae hijack host metabolism to counterbalance numerous disintegrated biosynthesis pathways that have arisen through evolution within the eukaryotic cell. This metabolic blueprint reveals what a Rickettsia axenic medium might entail.

Liver Necrosis and Lethal Systemic Inflammation in a Murine Model of Rickettsia typhi Infection: Role of Neutrophils, Macrophages and NK Cells

Rickettsia (R.) typhi is the causative agent of endemic typhus, an emerging febrile disease that is associated with complications such as pneumonia, encephalitis and liver dysfunction. To elucidate how innate immune mechanisms contribute to defense and pathology we here analyzed R. typhi infection of CB17 SCID mice that are congenic to BALB/c mice but lack adaptive immunity. CB17 SCID mice succumbed to R. typhi infection within 21 days and showed high bacterial load in spleen, brain, lung, and liver. Most evident pathological changes in R. typhi-infected CB17 SCID mice were massive liver necrosis and splenomegaly due to the disproportionate accumulation of neutrophils and macrophages (MΦ). Both neutrophils and MΦ infiltrated the liver and harbored R. typhi. Both cell populations expressed iNOS and produced reactive oxygen species (ROS) and, thus, exhibited an inflammatory and bactericidal phenotype. Surprisingly, depletion of neutrophils completely prevented liver necrosis but neither altered bacterial load nor protected CB17 SCID mice from death. Furthermore, the absence of neutrophils had no impact on the overwhelming systemic inflammatory response in these mice. This response was predominantly driven by activated MΦ and NK cells both of which expressed IFNγ and is considered as the reason of death. Finally, we observed that iNOS expression by MΦ and neutrophils did not correlate with R. typhi uptake in vivo. Moreover, we demonstrate that MΦ hardly respond to R. typhi in vitro. These findings indicate that R. typhi enters MΦ and also neutrophils unrecognized and that activation of these cells is mediated by other mechanisms in the context of tissue damage in vivo.