Metabolism and physiology of pathogenic bacterial obligate intracellular parasites

Bacterial obligate intracellular parasites (BOIPs) represent an exclusive group of bacterial pathogens that all depend on invasion of a eukaryotic host cell to reproduce. BOIPs are characterized by extensive adaptation to their respective replication niches, regardless of whether they replicate within the host cell cytoplasm or within specialized replication vacuoles. Genome reduction is also a hallmark of BOIPs that likely reflects streamlining of metabolic processes to reduce the need for de novo biosynthesis of energetically costly metabolic intermediates. Despite shared characteristics in lifestyle, BOIPs show considerable diversity in nutrient requirements, metabolic capabilities, and general physiology. In this review, we compare metabolic and physiological processes of prominent pathogenic BOIPs with special emphasis on carbon, energy, and amino acid metabolism. Recent advances are discussed in the context of historical views and opportunities for discovery.

Host learning selects for the coevolution of greater egg mimicry and narrower antiparasitic egg-rejection thresholds

Egg rejection is an effective and widespread antiparasitic defense to eliminate foreign eggs from the nests of hosts of brood parasitic birds. Several lines of observational and critical experimental evidence support a role for learning by hosts in the recognition of parasitic versus own eggs; specifically, individual hosts that have had prior or current experience with brood parasitism are more likely to reject foreign eggs. Here we confirm experimentally the role of prior experience in altering subsequent egg-rejection decisions in the American robin Turdus migratorius, a free-living host species of an obligate brood parasite, the brown-headed cowbird Molothrus ater. We then model the coevolutionary trajectory of both the extent of mimicry of host eggs by parasitic eggs and the host's egg rejection thresholds in response to an increasing role of learning in egg recognition. Critically, with more learning, we see the evolution of both narrower (more discriminating) rejection thresholds in hosts and greater egg mimicry in parasites. Increasing host clutch size (number of eggs/nest) and increasing parasite load (parasitism rate) also have narrowing effects on the egg-rejection threshold. Together, these results suggest that learning from prior experience with egg rejection may play an important role in the coevolution of egg-mimetic lineages of brood parasites and the refined egg rejection defenses of hosts.

Recent advances in genetic systems in obligate intracellular human-pathogenic bacteria

The ability to genetically manipulate a pathogen is fundamental to discovering factors governing host-pathogen interactions at the molecular level and is critical for devising treatment and prevention strategies. While the genetic "toolbox" for many important bacterial pathogens is extensive, approaches for modifying obligate intracellular bacterial pathogens were classically limited due in part to the uniqueness of their obligatory lifestyles. Many researchers have confronted these challenges over the past two and a half decades leading to the development of multiple approaches to construct plasmid-bearing recombinant strains and chromosomal gene inactivation and deletion mutants, along with gene-silencing methods enabling the study of essential genes. This review will highlight seminal genetic achievements and recent developments (past 5 years) for Anaplasma spp., Rickettsia spp., Chlamydia spp., and Coxiella burnetii including progress being made for the still intractable Orientia tsutsugamushi. Alongside commentary of the strengths and weaknesses of the various approaches, future research directions will be discussed to include methods for C. burnetii that should have utility in the other obligate intracellular bacteria. Collectively, the future appears bright for unraveling the molecular pathogenic mechanisms of these significant pathogens.

Metabolic Plasticity Aids Amphotropism of Coxiella burnetii

Coxiella burnetii, the causative agent of query (Q) fever in humans, is an obligate intracellular bacterium. C. burnetii can naturally infect a broad range of host organisms (e.g., mammals and arthropods) and cell types. This amphotropic nature of C. burnetii, in combination with its ability to utilize both glycolytic and gluconeogenic carbon sources, suggests that the pathogen relies on metabolic plasticity to replicate in nutritionally diverse intracellular environments. To test the significance of metabolic plasticity in C. burnetii host cell colonization, C. burnetii intracellular replication in seven distinct cell lines was compared between a metabolically competent parental strain and a mutant, CbΔpckA, unable to undergo gluconeogenesis. Both the parental strain and CbΔpckA mutant exhibited host cell-dependent infection phenotypes, which were influenced by alterations to host glycolytic or gluconeogenic substrate availability. Because the nutritional environment directly impacts host cell physiology, our analysis was extended to investigate the response of C. burnetii replication in mammalian host cells cultivated in a novel physiological medium based on the nutrient composition of mammalian interstitial fluid, interstitial fluid-modeled medium (IFmM). An infection model based on IFmM resulted in exacerbation of a replication defect exhibited by the CbΔpckA mutant in specific cell lines. The CbΔpckA mutant was also attenuated during infection of an animal host. Overall, the study underscores that gluconeogenic capacity aids C. burnetii amphotropism and that the amphotropic nature of C. burnetii should be considered when resolving virulence mechanisms in this pathogen.

Elucidating evolutionarily conserved mechanisms of diapause regulation using an in silico approach

Embryonic diapause is an enigmatic phenomenon that appears in diverse species. Although regulatory mechanisms have been established, there is much to be discovered. Herein, we have made the first comprehensive attempt to elucidate diapause regulatory mechanisms using a computational approach. We found transcription factors unique to promoters of genes in diapause species. From pathway analysis and STRING PPI networks, the signaling pathways regulated by these unique transcription factors were identified. The pathways were then consolidated into a model to combine various known mechanisms of diapause regulation. This work also highlighted certain transcription factors that may act as 'master transcription factors' to regulate the phenomenon. Promoter analysis further suggested evidence for independent evolution for some of regulatory elements involved in diapause.

Protein and DNA Biosynthesis Demonstrated in Host Cell-Free Phagosomes Containing Anaplasma phagocytophilum or Ehrlichia chaffeensis in Axenic Media

Rickettsiae belong to the Anaplasmataceae family, which includes mostly tick-transmitted pathogens causing human, canine, and ruminant diseases. Biochemical characterization of the pathogens remains a major challenge because of their obligate parasitism.

Rickettsiae belong to the Anaplasmataceae family, which includes mostly tick-transmitted pathogens causing human, canine, and ruminant diseases. Biochemical characterization of the pathogens remains a major challenge because of their obligate parasitism. We investigated the use of an axenic medium for growth of two important pathogens—Anaplasma phagocytophilum and Ehrlichia chaffeensis—in host cell-free phagosomes. We recently reported that the axenic medium promotes protein and DNA biosynthesis in host cell-free replicating form of E. chaffeensis, although the bacterial replication is limited. We now tested the hypothesis that growth on axenic medium can be improved if host cell-free rickettsia-containing phagosomes are used. Purification of phagosomes from A. phagocytophilum- and E. chaffeensis-infected host cells was accomplished by density gradient centrifugation combined with magnet-assisted cell sorting. Protein and DNA synthesis was observed for both organisms in cell-free phagosomes with glucose-6-phosphate and/or ATP. The levels of protein and DNA synthesis were the highest for a medium pH of 7. The data demonstrate bacterial DNA and protein synthesis for the first time in host cell-free phagosomes for two rickettsial pathogens. The host cell support-free axenic growth of obligate pathogenic rickettsiae will be critical in advancing research goals in many important tick-borne diseases impacting human and animal health.

Life in a bubble: the role of the labyrinth organ in determining territory, mating and aggressive behaviours in anabantoids

The anabantoids are a group of c. 137 species of air-breathing freshwater fishes found in Africa and southern Asia. All anabantoids have a pair of suprabranchial chambers that each house an air-breathing organ known as the labyrinth apparatus: a complex bony structure lined with thin, highly vascularised respiratory epithelium. The labyrinth apparatus allows anabantoids to extract oxygen from air and is a morpho-physiological innovation that has had a dramatic influence on the behaviour of these fishes. Air-breathing influences a wide range of anabantoid behaviours, including territorial displays, courtship and breeding and parental care and also equips these fishes to persist in hypoxic and polluted water. These traits also make anabantoids successful invaders of novel habitats, a global problem compounded by their popularity in the aquarium trade. By reviewing the functionality and evolution of air breathing in anabantoids, this review aims to examine the role of the labyrinth apparatus in modulating behaviour within this group. The anabantoids are a fascinating group and have often been cited as a model organism due to the stereotypical and easily identifiable behaviours that they adopt during social interactions. They also provide a unique opportunity to further our understanding about how fishes adapt their behaviour in response to an extreme environment, whilst limited by their own physiological constraints.

Halophilic Aspergillus penicillioides from athalassohaline, thalassohaline, and polyhaline environments

Aspergillus penicillioides is a true halophile, present in diverse econiches – from the hypersaline athalassohaline, and thalassohaline environments, to polyhaline systems, and in different geographical locations. Twenty seven isolates from these environments, were seen to be moderate halophiles, euryhaline in nature. They had an obligate need of a low a_w and were unable to grow on a regular defined medium such as Czapek Dox Agar, as well as on varied nutrient rich agar media such as Malt Extract, Potato Dextrose and Sabouraud Agar; however, growth was obtained on all these media when amended with 10% solar salt. In absence of added salt, the conidia either did not germinate, or when germinated, distortions and lysis were seen in the short mycelial forms; on media with salt, the mycelia and vesicles appeared normal.

Acid-base and ion balance in fishes with bimodal respiration

The evolution of air breathing during the Devonian provided early fishes with bimodal respiration with a stable O2 supply from air. This was, however, probably associated with challenges and trade-offs in terms of acid-base balance and ionoregulation due to reduced gill:water interaction and changes in gill morphology associated with air breathing. While many aspects of acid-base and ionoregulation in air-breathing fishes are similar to water breathers, the specific cellular and molecular mechanisms involved remain largely unstudied. In general, reduced ionic permeability appears to be an important adaptation in the few bimodal fishes investigated but it is not known if this is a general characteristic. The kidney appears to play an important role in minimizing ion loss to the freshwater environment in the few species investigated, and while ion uptake across the gut is probably important, it has been largely unexplored. In general, air breathing in facultative air-breathing fishes is associated with an acid-base disturbance, resulting in an increased partial pressure of arterial CO2 and a reduction in extracellular pH (pHE ); however, several fishes appear to be capable of tightly regulating tissue intracellular pH (pHI ), despite a large sustained reduction in pHE , a trait termed preferential pHI regulation. Further studies are needed to determine whether preferential pHI regulation is a general trait among bimodal fishes and if this confers reduced sensitivity to acid-base disturbances, including those induced by hypercarbia, exhaustive exercise and hypoxia or anoxia. Additionally, elucidating the cellular and molecular mechanisms may yield insight into whether preferential pHI regulation is a trait ultimately associated with the early evolution of air breathing in vertebrates.