The Genus Caedibacter Comprises Endosymbionts of Paramecium spp. Related to the Rickettsiales ( Alphaproteobacteria ) and to Francisella tularensis ( Gammaproteobacteria )

Abstract

Obligate bacterial endosymbionts of paramecia able to form refractile inclusion bodies (R bodies), thereby conferring a killer trait upon their ciliate hosts, have traditionally been grouped into the genus Caedibacter. Of the six species described to date, only the Paramecium caudatum symbiont Caedibacter caryophilus has been phylogenetically characterized by its 16S rRNA gene sequence, and it was found to be a member of the Alphaproteobacteria related to the Rickettsiales. In this study, the Caedibacter taeniospiralis type strain, an R-body-producing cytoplasmatic symbiont of Paramecium tetraurelia strain 51k, was investigated by comparative 16S rRNA sequence analysis and fluorescence in situ hybridization with specific oligonucleotide probes. C. taeniospiralis is not closely related to C. caryophilus (80% 16S rRNA sequence similarity) but forms a novel evolutionary lineage within the Gammaproteobacteria with the family Francisellaceae as a sister group (87% 16S rRNA sequence similarity). These findings demonstrate that the genus Caedibacter is polyphyletic and comprises at least two phylogenetically different bacterial species belonging to two different classes of the Proteobacteria. Comparative phylogenetic analysis of C. caryophilus, five closely related Acanthamoeba endosymbionts (including one previously uncharacterized amoebal symbiont identified in this study), and their hosts suggests that the progenitor of the alphaproteobacterial C. caryophilus lived within acanthamoebae prior to the infection of paramecia.The ability of certain paramecia to kill other paramecia was first described in 1938 by Sonneborn, who observed that sensitive paramecia exhibit distinct morphological symptoms upon ingestion of toxic "particles" released by the killer strain and ultimately die (52). In 1958, electron microscopy studies revealed that these heritable cytoplasmic particles are gramnegative rod-shaped prokaryotes (16) which eluded cultivation using standard laboratory media (30,32) and that the toxic effect is associated with proteinaceous, refractile inclusion bodies (R bodies) found inside the bacterial endosymbionts (30,32). Subsequently, all R-body-producing obligate intracellular symbionts of paramecia were combined into the genus Caedibacter and classified according to morphological, functional, and phenotypic properties (17,32,35). Within the genus Caedibacter, the six species Caedibacter caryophilus, Caedibacter varicaedens, Caedibacter taeniospiralis, Caedibacter pseudomutans, Caedibacter paraconjugatus, and Caedibacter macronucleorum (11, 38, 47) have been recognized. C. caryophilus, the only member of the genus Caedibacter that has been characterized by its 16S rRNA gene sequence, was found to be affiliated with the Alphaproteobacteria (57). Within the Alphaproteobacteria, C. caryophilus clusters together with obligate endosymbionts of acanthamoebae (6, 20) and the paramecium endosymbionts Holospora obtusa and Holospora elegans (3,14).In this study, the phylogenetic affiliation of the Paramecium tetraurelia ...

Microorganisms Resistant to Free-Living Amoebae

“…Interestingly, the branching of host and endosymbiont phylogenetic trees was congruent (22,114,117). This suggested that the ancestor of the C. caryophilus-related endosymbionts lived within an amoebal progenitor and coevolved with their hosts during the diversification of the different Acanthamoeba sublineages (22). If this is true, C. caryophilus will have only recently transferred to the ciliate Paramecium caudatum.…”

“…A recent phylogenetic analysis of the 16S rRNA genes of all these symbionts of Acanthamoeba, of the ciliate Paramecium caudatum (C. caryophilus, H. obtusa, and H. elegans) and of the shrimp Penaeus vannamei (NHP bacterium) showed that Candidatus O. thessalonicensis was phylogenetically close to Candidatus P. acanthamoebae (22). Interestingly, the branching of host and endosymbiont phylogenetic trees was congruent (22,114,117). This suggested that the ancestor of the C. caryophilus-related endosymbionts lived within an amoebal progenitor and coevolved with their hosts during the diversification of the different Acanthamoeba sublineages (22).…”

“Candidatus Sonnebornia yantaiensis”, a member of candidate division OD1, as intracellular bacteria of the ciliated protist Paramecium bursaria (Ciliophora, Oligohymenophorea)

“…More recently, our knowledge of the diversity of bacteria associated with ciliates has increased significantly, primarily as a result of extensive molecular investigations of some commonly occurring species of the genera Paramecium and Euplotes. These studies have demonstrated that most bacterial symbionts belong to one of four classes of the phylum Proteobacteria, namely Alphaproteobacteria (including pathogenic Rickettsia) [2,8,9,18,19,21,22,51,59,60,62,64,67,69], Betaproteobacteria [58,63,66,68], Gammaproteobacteria [4,10,48,53] and Deltaproteobacteria [15]. Associations between ciliates and bacteria of other phyla seem to be infrequent, for example, Verrucomicrobia [42] and Firmicutes [54].…”

Euplotes magnicirratus (Ciliophora, Hypotrichia) depends on its bacterial endosymbiont 'Candidatus Devosia euplotis' for food digestion

“…The phylogenetic affiliation has been determined for only a few of these, i.e. Caedibacter caryophilus (Springer et al 1993), C. teniospiralis (Beier et al 2002), Holospora obtusa (Amann et al 1991), Polynucleobacter necessarius (Springer et al 1996), and 'Candidatus Devosia euplotis' (Vannini et al in press). Second, the significance of the symbiotic relationships and the possibility that the symbionts provide their host with selective advantages have been investigated.…”