The Experts below are selected from a list of 288 Experts worldwide ranked by ideXlab platform
Andreas Leclerque - One of the best experts on this subject based on the ideXlab platform.
-
Host group adaptation of `Candidatus Rickettsiella isopodorum´, a lineage of intracellular gamma-proteobacterial pathogens carried by woodlice
2015Co-Authors: Andreas Leclerque, Regina KleespiesAbstract:The taxonomic genus Rickettsiella (Gammaproteobacteria; Legionellales) comprises intracellular bacteria associated with a wide range of arthropods including insects, arachnids and crustaceans that are currently under intensive evaluation as possible sources of new biocontrol agents. A multilocus sequence analysis approach has been employed to independently evaluate the previously established 16S rRNA based phylogeny for two Rickettsiella strains associated with isopods from California and Germany. Phylogenetic reconstruction from three protein-encoding marker genes, namely ftsY, gidA, and sucB, confirms the very tight phylogenetic relationship of Rickettsiella bacteria from closely related hosts, but distant geographic origins. These findings are generally indicative of stable host adaptation and, therefore, challenge the currently adopted view of rather unspecific host – pathogen relationships within the Rickettsiella. Reference: Kleespies, R.G., Federici, B.A. & Leclerque A. 2014: Ultrastructural characterization and multilocus sequence analysis (MLSA) of `Candidatus Rickettsiella isopodorum´, a new lineage of intracellular bacteria infecting woodlice (Crustacea: Isopoda). Systematic and Applied Microbiology 37(5): 351–359.
-
Ultrastructural characterization and multilocus sequence analysis (MLSA) of 'Candidatus Rickettsiella isopodorum', a new lineage of intracellular bacteria infecting woodlice (Crustacea: Isopoda).
Systematic and applied microbiology, 2014Co-Authors: Regina Kleespies, Brian A. Federici, Andreas LeclerqueAbstract:The taxonomic genus Rickettsiella (Gammaproteobacteria; Legionellales) comprises intracellular bacteria associated with a wide range of arthropods including insects, arachnids and crustaceans. The present study provides ultrastructural together with genetic evidence for a Rickettsiella bacterium in the common rough woodlouse, Porcellio scaber (Isopoda, Porcellionidae), occurring in Germany, and shows that this bacterium is very closely related to one of the same genus occurring in California that infects the pill bug, Armadillidium vulgare (Isopoda, Armadillidiidae). Both bacterial isolates displayed the ultrastructural features described previously for crustacean-associated bacteria of the genus Rickettsiella, including the absence of well-defined associated protein crystals; occurrence of the latter is a typical characteristic of infection by this type of bacteria in insects, but has not been reported in crustaceans. A molecular systematic approach combining multilocus sequence analysis (MLSA) with likelihood-based significance testing demonstrated that despite their distant geographic origins, both bacteria form a tight sub-clade within the genus Rickettsiella. In the 16S rRNA gene trees, this sub-clade includes other bacterial sequences from woodlice. Moreover, the bacterial specimens from P. scaber and A. vulgare are found genetically or morphologically different from each of the four currently recognized Rickettsiella species. Therefore, the designation ‘Candidatus Rickettsiella isopodorum’ is introduced for this new lineage of isopod-associated Rickettsiella bacteria.
-
Electron microscope and genetic analysis of an intracellular bacterium associated with the common rough woodlouse, Porcellio scaber (Isopoda, Porcellionidae)
2013Co-Authors: Regina Kleespies, Andreas LeclerqueAbstract:The common rough woodlouse, Porcellio scaber, is a common and widespread isopod species of Western and Northern Europe. A previously unknown intracellular bacterium has been identified in a diseased Porcellio larva. Microscopic studies revealed the subcellular structures characteristic of infection by Rickettsiella-like bacteria. Molecular phylogenetic analysis based on the 16S ribosomal RNA encoding rrs gene demonstrated that the woodlouse pathogen belongs to the taxonomic genus Rickettsiella (Gammaproteobacteria; Legionellales). Moreover, genetic analysis makes it likely that this new pathotype should be considered a member of the “Rickettsiella armadillidii complex”, i.e. a group of Rickettsiella bacteria found mainly in terrestrial isopods. R. armadillidii is currently placed in synonymy with the nomenclatural type species, Rickettsiella popilliae. The present study does not lend support to this synonymization.
-
Natural infection of wireworm, Agriotes sp. (Coleoptera; Elateridae), with Rickettsiella bacteria
2011Co-Authors: Christina Schuster, Regina Kleespies, Claudia Ritter, Simon Feiertag, Andreas LeclerqueAbstract:Wireworms, the polyphagous larvae of click beetles belonging to the genus Agriotes (Coleoptera: Elateridae), are severe and widespread agricultural pests affecting numerous crops worldwide. A previously unknown intracellular bacterium has been identified in a diseased Agriotes larva. Microscopic studies revealed the subcellular structures characteristic of rickettsiosis. Molecular phylogenetic analysis based on 16S ribosomal RNA encoding rrs gene demonstrated that the wireworm pathogen belongs to the taxonomic genus Rickettsiella (Gammaproteobacteria; Legionellales). The pathotype designation `Rickettsiella agriotidis´ has been proposed to refer to this organism. Moreover, genetic analysis makes it likely that this new pathotype should be considered a synonym of the nomenclatural type species, Rickettsiella popilliae.
-
Whole genome-based assessment of the taxonomic position of the arthropod pathogenic bacterium Rickettsiella grylli.
FEMS microbiology letters, 2008Co-Authors: Andreas LeclerqueAbstract:Rickettsiella grylli is an intracellular bacterial pathogen of aquatic and terrestrial arthropods. Previous determination of its 16S rRNA-encoding sequence has led to the taxonomic classification of the genus Rickettsiella in the class Gammaproteobacteria, order Legionellales, family Coxiellaceae, i.e. in close vicinity to vertebrate pathogenic bacteria of the genera Coxiella and Legionella. Here we use the additional information available from the recently published first whole genome sequence from this genus to evaluate critically the taxonomic classification of R. grylli beyond the 16S rRNA gene level. Using phylogenetic reconstruction, together with significance testing on a data basis defined by a core set of 211 previously identified families of protein-encoding genes, together with a reanalysis of 16S rRNA gene data, the present study firmly corroborates the assignment of this species to both the class Gammaproteobacteria and the order Legionellales. However, the results obtained from concatenated and single protein, single protein-encoding gene, and 16S rRNA gene data demonstrate a similar phylogenetic distance of R. grylli to both the Coxiellaceae and the Legionellaceae and are, therefore, inconsistent with its current family-level classification. Consequently, a respective reorganization of the order Legionellales is proposed.
Didier Raoult - One of the best experts on this subject based on the ideXlab platform.
-
A Novel Obligate Intracellular Gamma-Proteobacterium Associated with Ixodid Ticks, Diplorickettsia massiliensis,
2013Co-Authors: Oleg Mediannikov, Zuzana Sekeyová, Marie-laure Birg, Didier RaoultAbstract:Background: Obligate intracellular bacteria of arthropods often exhibit a significant role in either human health or arthropod ecology. Methodology/Principal Findings: An obligate intracellular gamma-proteobacterium was isolated from the actively questing hard tick Ixodes ricinus using mammalian and amphibian cell lines. Transmission electron microscopy revealed a unique morphology of the bacterium, including intravacuolar localization of bacteria grouped predominantly in pairs and internal structures composed of electron-dense crystal-like structures and regular multilayer sheath-like structures. The isolate 20B was characterized to determine its taxonomic position using a polyphasic approach. Comparative 16S rRNA gene sequence analysis showed that this strain belongs to the family Coxiellaceae, order Legionellales of Gammaproteobacteria, and the closest relatives are different Rickettsiella spp. The level of 16S rRNA gene sequence similarity between strain 20B and other recognized species of the family was below 94.5%. Partial sequences of the rpoB, parC and ftsY genes confirmed the phylogenetic position of the new isolate. The G+C content estimated on the basis of whole genome analysis of strain 20B was 37.88%. On the basis of its phenotypic and genotypic properties, together with phylogenetic distinctiveness, we propose that strain 20B to be classified in the new genus Diplorickettsia as the type strain of a novel species named Diplorickettsia massiliensis sp. nov
-
RESEARCH ARTICLE Open Access Insight into cross-talk between intra-amoebal pathogens
2013Co-Authors: Gregory Gimenez, Claire Bertelli, Claire Moliner, Catherine Robert, Didier Raoult, Pierre-edouard FournierAbstract:Background: Amoebae are phagocytic protists where genetic exchanges might take place between amoebaresistant bacteria. These amoebal pathogens are able to escape the phagocytic behaviour of their host. They belong to different bacterial phyla and often show a larger genome size than human-infecting pathogens. This characteristic is proposed to be the result of frequent gene exchanges with other bacteria that share a sympatric lifestyle and contrasts with the genome reduction observed among strict human pathogens. Results: We sequenced the genome of a new amoebal pathogen, Legionella drancourtii, and compared its gene content to that of a Chlamydia-related bacterium, Parachlamydia acanthamoebae. Phylogenetic reconstructions identified seven potential horizontal gene transfers (HGTs) between the two amoeba-resistant bacteria, including a complete operon of four genes that encodes an ABC-type transporter. These comparisons pinpointed potential cases of gene exchange between P. acanthamoebae and Legionella pneumophila, as well as gene exchanges between other members of the Legionellales and Chlamydiales orders. Moreover, nine cases represent possible HGTs between representatives from the Legionellales or Chlamydiales and members of the Rickettsiales order. Conclusions: This study identifies numerous gene exchanges between intracellular Legionellales and Chlamydiales bacteria, which could preferentially occur within common inclusions in their amoebal hosts. Therefore i
-
Genome Sequence of Diplorickettsia massiliensis, an Emerging Ixodes ricinus-Associated Human Pathogen
Journal of bacteriology, 2012Co-Authors: Mano J. Mathew, Catherine Robert, Pierre-edouard Fournier, Geetha Subramanian, Thi-tien Nguyen, Oleg Mediannikov, Didier RaoultAbstract:Diplorickettsia massiliensis is a gammaproteobacterium in the order Legionellales and an agent of tick-borne infection. We sequenced the genome from strain 20B, isolated from an Ixodes ricinus tick. The genome consists of a 1,727,973-bp chromosome but no plasmid and includes 2,269 protein-coding genes and 42 RNA genes, including 3 rRNA genes.
-
Insight into cross-talk between intra-amoebal pathogens
BMC Genomics, 2011Co-Authors: Gregory Gimenez, Claire Bertelli, Claire Moliner, Catherine Robert, Didier Raoult, Pierre-edouard Fournier, Gilbert GreubAbstract:Background Amoebae are phagocytic protists where genetic exchanges might take place between amoeba-resistant bacteria. These amoebal pathogens are able to escape the phagocytic behaviour of their host. They belong to different bacterial phyla and often show a larger genome size than human-infecting pathogens. This characteristic is proposed to be the result of frequent gene exchanges with other bacteria that share a sympatric lifestyle and contrasts with the genome reduction observed among strict human pathogens. Results We sequenced the genome of a new amoebal pathogen, Legionella drancourtii , and compared its gene content to that of a Chlamydia- related bacterium, Parachlamydia acanthamoebae . Phylogenetic reconstructions identified seven potential horizontal gene transfers (HGTs) between the two amoeba-resistant bacteria, including a complete operon of four genes that encodes an ABC-type transporter. These comparisons pinpointed potential cases of gene exchange between P. acanthamoebae and Legionella pneumophila , as well as gene exchanges between other members of the Legionellales and Chlamydiales orders. Moreover, nine cases represent possible HGTs between representatives from the Legionellales or Chlamydiales and members of the Rickettsiales order. Conclusions This study identifies numerous gene exchanges between intracellular Legionellales and Chlamydiales bacteria, which could preferentially occur within common inclusions in their amoebal hosts. Therefore it contributes to improve our knowledge on the intra-amoebal gene properties associated to their specific lifestyle.
-
A Novel Obligate Intracellular Gamma-Proteobacterium Associated with Ixodid Ticks, Diplorickettsia massiliensis, Gen. Nov., Sp. Nov
PloS one, 2010Co-Authors: Oleg Mediannikov, Zuzana Sekeyová, Marie-laure Birg, Didier RaoultAbstract:Background Obligate intracellular bacteria of arthropods often exhibit a significant role in either human health or arthropod ecology. Methodology/Principal Findings An obligate intracellular gamma-proteobacterium was isolated from the actively questing hard tick Ixodes ricinus using mammalian and amphibian cell lines. Transmission electron microscopy revealed a unique morphology of the bacterium, including intravacuolar localization of bacteria grouped predominantly in pairs and internal structures composed of electron-dense crystal-like structures and regular multilayer sheath-like structures. The isolate 20B was characterized to determine its taxonomic position using a polyphasic approach. Comparative 16S rRNA gene sequence analysis showed that this strain belongs to the family Coxiellaceae, order Legionellales of Gamma-proteobacteria, and the closest relatives are different Rickettsiella spp. The level of 16S rRNA gene sequence similarity between strain 20B and other recognized species of the family was below 94.5%. Partial sequences of the rpoB, parC and ftsY genes confirmed the phylogenetic position of the new isolate. The G+C content estimated on the basis of whole genome analysis of strain 20B was 37.88%. On the basis of its phenotypic and genotypic properties, together with phylogenetic distinctiveness, we propose that strain 20B to be classified in the new genus Diplorickettsia as the type strain of a novel species named Diplorickettsia massiliensis sp. nov. Conclusions/Significance Considering the source of its isolation (hard tick, often biting humans) the role of this bacterium in the pathology of humans, animals and ticks should be further investigated.
Regina Kleespies - One of the best experts on this subject based on the ideXlab platform.
-
Host group adaptation of `Candidatus Rickettsiella isopodorum´, a lineage of intracellular gamma-proteobacterial pathogens carried by woodlice
2015Co-Authors: Andreas Leclerque, Regina KleespiesAbstract:The taxonomic genus Rickettsiella (Gammaproteobacteria; Legionellales) comprises intracellular bacteria associated with a wide range of arthropods including insects, arachnids and crustaceans that are currently under intensive evaluation as possible sources of new biocontrol agents. A multilocus sequence analysis approach has been employed to independently evaluate the previously established 16S rRNA based phylogeny for two Rickettsiella strains associated with isopods from California and Germany. Phylogenetic reconstruction from three protein-encoding marker genes, namely ftsY, gidA, and sucB, confirms the very tight phylogenetic relationship of Rickettsiella bacteria from closely related hosts, but distant geographic origins. These findings are generally indicative of stable host adaptation and, therefore, challenge the currently adopted view of rather unspecific host – pathogen relationships within the Rickettsiella. Reference: Kleespies, R.G., Federici, B.A. & Leclerque A. 2014: Ultrastructural characterization and multilocus sequence analysis (MLSA) of `Candidatus Rickettsiella isopodorum´, a new lineage of intracellular bacteria infecting woodlice (Crustacea: Isopoda). Systematic and Applied Microbiology 37(5): 351–359.
-
Ultrastructural characterization and multilocus sequence analysis (MLSA) of 'Candidatus Rickettsiella isopodorum', a new lineage of intracellular bacteria infecting woodlice (Crustacea: Isopoda).
Systematic and applied microbiology, 2014Co-Authors: Regina Kleespies, Brian A. Federici, Andreas LeclerqueAbstract:The taxonomic genus Rickettsiella (Gammaproteobacteria; Legionellales) comprises intracellular bacteria associated with a wide range of arthropods including insects, arachnids and crustaceans. The present study provides ultrastructural together with genetic evidence for a Rickettsiella bacterium in the common rough woodlouse, Porcellio scaber (Isopoda, Porcellionidae), occurring in Germany, and shows that this bacterium is very closely related to one of the same genus occurring in California that infects the pill bug, Armadillidium vulgare (Isopoda, Armadillidiidae). Both bacterial isolates displayed the ultrastructural features described previously for crustacean-associated bacteria of the genus Rickettsiella, including the absence of well-defined associated protein crystals; occurrence of the latter is a typical characteristic of infection by this type of bacteria in insects, but has not been reported in crustaceans. A molecular systematic approach combining multilocus sequence analysis (MLSA) with likelihood-based significance testing demonstrated that despite their distant geographic origins, both bacteria form a tight sub-clade within the genus Rickettsiella. In the 16S rRNA gene trees, this sub-clade includes other bacterial sequences from woodlice. Moreover, the bacterial specimens from P. scaber and A. vulgare are found genetically or morphologically different from each of the four currently recognized Rickettsiella species. Therefore, the designation ‘Candidatus Rickettsiella isopodorum’ is introduced for this new lineage of isopod-associated Rickettsiella bacteria.
-
Electron microscope and genetic analysis of an intracellular bacterium associated with the common rough woodlouse, Porcellio scaber (Isopoda, Porcellionidae)
2013Co-Authors: Regina Kleespies, Andreas LeclerqueAbstract:The common rough woodlouse, Porcellio scaber, is a common and widespread isopod species of Western and Northern Europe. A previously unknown intracellular bacterium has been identified in a diseased Porcellio larva. Microscopic studies revealed the subcellular structures characteristic of infection by Rickettsiella-like bacteria. Molecular phylogenetic analysis based on the 16S ribosomal RNA encoding rrs gene demonstrated that the woodlouse pathogen belongs to the taxonomic genus Rickettsiella (Gammaproteobacteria; Legionellales). Moreover, genetic analysis makes it likely that this new pathotype should be considered a member of the “Rickettsiella armadillidii complex”, i.e. a group of Rickettsiella bacteria found mainly in terrestrial isopods. R. armadillidii is currently placed in synonymy with the nomenclatural type species, Rickettsiella popilliae. The present study does not lend support to this synonymization.
-
Natural infection of wireworm, Agriotes sp. (Coleoptera; Elateridae), with Rickettsiella bacteria
2011Co-Authors: Christina Schuster, Regina Kleespies, Claudia Ritter, Simon Feiertag, Andreas LeclerqueAbstract:Wireworms, the polyphagous larvae of click beetles belonging to the genus Agriotes (Coleoptera: Elateridae), are severe and widespread agricultural pests affecting numerous crops worldwide. A previously unknown intracellular bacterium has been identified in a diseased Agriotes larva. Microscopic studies revealed the subcellular structures characteristic of rickettsiosis. Molecular phylogenetic analysis based on 16S ribosomal RNA encoding rrs gene demonstrated that the wireworm pathogen belongs to the taxonomic genus Rickettsiella (Gammaproteobacteria; Legionellales). The pathotype designation `Rickettsiella agriotidis´ has been proposed to refer to this organism. Moreover, genetic analysis makes it likely that this new pathotype should be considered a synonym of the nomenclatural type species, Rickettsiella popilliae.
-
Genetic and electron-microscopic characterization of Rickettsiella tipulae, an intracellular bacterial pathogen of the crane fly, Tipula paludosa.
Journal of invertebrate pathology, 2008Co-Authors: Andreas Leclerque, Regina KleespiesAbstract:Rickettsiella tipulae is an intracellular bacterial pathogen of larvae of the crane fly, Tipula paludosa (Diptera: Tipulidae) and has previously been claimed to represent an independent species within the genus Rickettsiella. Recently, this taxon has been reorganized and transferred as a whole from the alpha-proteobacterial order Rickettsiales to the gamma-proteobacterial order Legionellales. Here we present the electron-microscopic identification of this rickettsial pathogen together with the first DNA sequence information for R. tipulae. The results of our 16S rDNA-based phylogenetic analysis demonstrate that the transfer to the order Legionellales is justified for R. tipulae. However, there is no phylogenetic basis to consider R. tipulae an independent species, but instead conclusive evidence substantiating its species level co-assignment with Rickettsiella melolonthae. Furthermore, implications of our results for a possible reorganization of the internal structure of the genus Rickettsiella are discussed.
Lionel Guy - One of the best experts on this subject based on the ideXlab platform.
-
Host-adaptation in Legionellales is 2.4 Gya, coincident with eukaryogenesis
2019Co-Authors: Eric Hugoson, Tea Ammunet, Lionel GuyAbstract:Events of host-adaptation involving bacteria is at the origin of the most salient events in the evolution of Eukaryotes: the seminal fusion with an archaeon, and the emergence of both the mitochondrion and the chloroplast. Understanding how bacteria contributed to these events is difficult, due to the scarcity of ancient clades containing host-adapted bacteria only. One such clade is the deep-branching gammaproteobacterial order Legionellales - containing among others Coxiella and Legionella - of which all known members grow inside eukaryotic cells. Here, by analyzing 35 novel Legionellales genomes mainly acquired through metagenomics, we show that this order is much more diverse than previously thought, and that key host-adaptation events took place very early in its evolution. Crucial virulence factors like the Type IVB secretion (Dot/Icm) system and two shared effector proteins were gained in the last Legionellales common ancestor (LLCA), while many metabolic gene families where lost in LLCA and its immediate descendants. We estimate that the LLCA is circa 2.4 billion years old, predating the last eukaryotic common ancestor by at least 0.5 Ga. This strongly indicates that host-adaptation occurred only once, and early, in the whole order and suggests that Legionellales started exploiting and manipulating eukaryotic cells with advanced molecular machines during early eukaryogenesis.
-
The all-intracellular order Legionellales is unexpectedly diverse, globally distributed and lowly abundant
FEMS microbiology ecology, 2018Co-Authors: Tiscar Graells, Helena Ishak, Madeleine Larsson, Lionel GuyAbstract:Legionellales is an order of the Gammaproteobacteria, only composed of host-adapted, intracellular bacteria, including the accidental human pathogens Legionella pneumophila and Coxiella burnetii. Although the diversity in terms of lifestyle is large across the order, only a few genera have been sequenced, owing to the difficulty to grow intracellular bacteria in pure culture. In particular, we know little about their global distribution and abundance. Here, we analyze 16/18S rDNA amplicons both from tens of thousands of published studies and from two separate sampling campaigns in and around ponds and in a silver mine. We demonstrate that the diversity of the order is much larger than previously thought, with over 450 uncultured genera. We show that Legionellales are found in about half of the samples from freshwater, soil and marine environments and quasi-ubiquitous in man-made environments. Their abundance is low, typically 0.1%, with few samples up to 1%. Most Legionellales OTUs are globally distributed, while many do not belong to a previously identified species. This study sheds a new light on the ubiquity and diversity of one major group of host-adapted bacteria. It also emphasizes the need to use metagenomics to better understand the role of host-adapted bacteria in all environments.
-
Withdrawn as Duplicate: The all-intracellular order Legionellales is unexpectedly diverse, globally distributed and lowly abundant.
FEMS microbiology ecology, 2018Co-Authors: Tiscar Graells, Helena Ishak, Madeleine Larsson, Lionel GuyAbstract:Legionellales is an order of the Gammaproteobacteria, only composed of host-adapted, intracellular bacteria, including the accidental human pathogens Legionella pneumophila and Coxiella burnetii. Although the diversity in terms of lifestyle is large across the order, only a few genera have been sequenced, owing to the difficulty to grow intracellular bacteria in pure culture. In particular, we know little about their global distribution and abundance.Here, we analyze 16/18S rDNA amplicons both from tens of thousands of published studies and from two separate sampling campaigns in and around ponds and in a silver mine. We demonstrate that the diversity of the order is much larger than previously thought, with over 450 uncultured genera. We show that Legionellales are found in about half of the samples from freshwater, soil and marine environments, and quasi-ubiquitous in man-made environments. Their abundance is low, typically 0.1%, with few samples up to 1%. Most Legionellales OTUs are globally distributed, while many do not belong to a previously identified species.This study sheds a new light on the ubiquity and diversity of one major group of host-adapted bacteria. It also emphasizes the need to use metagenomics to better understand the role of host-adapted bacteria in all environments.
Matthias Horn - One of the best experts on this subject based on the ideXlab platform.
-
‘Candidatus Cochliophilus cryoturris’ (Coxiellaceae), a symbiont of the testate amoeba Cochliopodium minus
Scientific Reports, 2017Co-Authors: Han-fei Tsao, Ute Scheikl, Jean-marie Volland, Martina Köhsler, Monika Bright, Julia Walochnik, Matthias HornAbstract:Free-living amoebae are well known for their role in controlling microbial community composition through grazing, but some groups, namely Acanthamoeba species, also frequently serve as hosts for bacterial symbionts. Here we report the first identification of a bacterial symbiont in the testate amoeba Cochliopodium . The amoeba was isolated from a cooling tower water sample and identified as C. minus . Fluorescence in situ hybridization and transmission electron microscopy revealed intracellular symbionts located in vacuoles. 16S rRNA-based phylogenetic analysis identified the endosymbiont as member of a monophyletic group within the family Coxiellaceae (Gammaprotebacteria; Legionellales), only moderately related to known amoeba symbionts. We propose to tentatively classify these bacteria as ‘ Candidatus Cochliophilus cryoturris’. Our findings add both, a novel group of amoeba and a novel group of symbionts, to the growing list of bacteria-amoeba relationships.