Rediscovering the genus Lyticum, multiflagellated symbionts of the order Rickettsiales

Among the bacterial symbionts harbored by the model organism Paramecium, many still lack a recent investigation that includes a molecular characterization. The genus Lyticum consists of two species of large-sized bacteria displaying numerous flagella, despite their inability to move inside their hosts’ cytoplasm. We present a multidisciplinary redescription of both species, using the deposited type strains as well as newly collected material. On the basis of 16S rRNA gene sequences, we assigned Lyticum to the order Rickettsiales, that is intensely studied because of its pathogenic representatives and its position as the extant group most closely related to the mitochondrial ancestor. We provide conclusive proofs that at least some Rickettsiales possess actual flagella, a feature that has been recently predicted from genomic data but never confirmed. We give support to the hypothesis that the mitochondrial ancestor could have been flagellated, and provide the basis for further studies on these ciliate endosymbionts

Flagellar Movement in Two Bacteria of the Family <i>Rickettsiaceae</i>: A Re-Evaluation of Motility in an Evolutionary Perspective

<div><p>Bacteria of the family <i>Rickettsiaceae</i> have always been largely studied not only for their importance in the medical field, but also as model systems in evolutionary biology. In fact, they share a recent common ancestor with mitochondria. The most studied species, belonging to genera <i>Rickettsia</i> and <i>Orientia</i>, are hosted by terrestrial arthropods and include many human pathogens. Nevertheless, recent findings show that a large part of <i>Rickettsiaceae</i> biodiversity actually resides outside the group of well-known pathogenic bacteria. Collecting data on these recently described non-conventional members of the family is crucial in order to gain information on ancestral features of the whole group. Although bacteria of the family <i>Rickettsiaceae</i>, and of the whole order <i>Rickettsiales</i>, are formally described as non-flagellated prokaryotes, some recent findings renewed the debate about this feature. In this paper we report the first finding of members of the family displaying numerous flagella and active movement inside their host cells. These two new taxa are hosted in aquatic environments by protist ciliates and are described here by means of ultrastructural and molecular characterization. Data here reported suggest that the ancestor of <i>Rickettsiales</i> displayed flagellar movement and re-evaluate the hypothesis that motility played a key-role in the origin of mitochondria. Moreover, our study highlights that the aquatic environment represents a well exploited habitat for bacteria of the family <i>Rickettsiaceae</i>. Our results encourage a deep re-consideration of ecological and morphological traits of the family and of the whole order.</p></div

FISH experiments performed on “<i>Candidatus</i> Trichorickettsia mobilis” and “<i>Candidatus</i> Gigarickettsia flagellata”.

<p>Symbionts inside <i>P. nephridiatum</i> PAR (A) and inside <i>P. multimicronucleatum</i> LSA (C) with probe RickFla_430 (Cy3, red signal) together with eubacterial probe EUB338 (fluoresceine, green signal). Food bacteria, labeled only in green, are visible inside food vacuoles in LSA (C). FISH experiment performed on “<i>Candidatus</i> Gigarickettsia flagellata” with probe GigaRick_436 (green signal) is shown in B. Bars: 20 µm.</p

Bayesian Inference tree of the family <i>Rickettsiaceae</i>.

<p>The tree was built on the unmodified character matrix (see text) employing the GTR+I+G (8 gamma categories) model. Numbers associated to nodes represent Maximum Likelihood bootstraps and Posterior Probabilities, respectively (values below 70|0.85 are omitted). Taxa that received a formal or provisional binomial name are in bold. The bar stands for an inferred sequence divergence of 5%. “<i>Ca.</i>”, “<i>Candidatus</i>”; “bac.”, “bacterium associated to”.</p