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

Abstract

Bacteria of the family Rickettsiaceae 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 Rickettsia and Orientia, are hosted by terrestrial arthropods and include many human pathogens. Nevertheless, recent findings show that a large part of Rickettsiaceae 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 Rickettsiaceae, and of the whole order Rickettsiales, 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 Rickettsiales 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 Rickettsiaceae. Our results encourage a deep re-consideration of ecological and morphological traits of the family and of the whole order.

Extensive genetic diversity of Rickettsiales bacteria in multiple mosquito species

“…The sequences WHCUTA-121 and WHCUTA-130 exhibited a close evolutionary relationship with those of the Rickettsia symbiont of Nephotettix cincticeps 34. Interestingly, the sequence of WHCUTL-65 was distinct, but most closely related to Candidatus Trichorickettsia mobilis isolated from Paramecium nephridiatum (97.0% identity)35, while the sequence WHANSA-146 was distinct from all known Rickettsial bacteria. Hence, our data suggest the presence of two novel Rickettsia species (or symbionts/endosybionts) in mosquitoes, designated as Candidatus Rickettsia sp.…”

A Rickettsiales symbiont of amoebae with ancient features

“…Lost in most Rickettsiales lineages, the flagellar genes in Jidaibacter can thus be considered an ancient feature inherited from free‐living bacteria. Although we did not observe flagellar structures or motility for Jidaibacter , flagellar motility was just recently detected in two novel Rickettsiaceae symbionts of ciliates, and flagellar structures were found in Lyticum species, symbionts of paramecia and new members of the Midichloriaceae (Boscaro et al ., 2013a,b; Vannini et al ., ).…”

Disentangling the Taxonomy of Rickettsiales and Description of Two Novel Symbionts (“Candidatus Bealeia paramacronuclearis” and “Candidatus Fokinia cryptica”) Sharing the Cytoplasm of the Ciliate Protist Paramecium biaurelia

“…Euplotes spp. (Spirotrichea) infected by Polynucleobacter necessarius and other bacteria have been reported as well (16, 24, 62); the Euplotes aediculatus isolate In from New Delhi (India) bore three different cytoplasmic endosymbionts (18, 29, 63) beyond P. necessarius (64).…”