Vannini, Claudia


Publications (8)

At the threshold of symbiosis: the genome of obligately endosymbiotic ‘Candidatus Nebulobacter yamunensis’ is almost indistinguishable from that of a cultivable strain

Citation
Giannotti et al. (2022). Microbial Genomics 8 (12)
Names
Ca. Nebulobacter yamunensis Ca. Nebulobacter
Subjects
General Medicine
Abstract
Comparing obligate endosymbionts with their free-living relatives is a powerful approach to investigate the evolution of symbioses, and it has led to the identification of several genomic traits consistently associated with the establishment of symbiosis. ‘Candidatus Nebulobacter yamunensis’ is an obligate bacterial endosymbiont of the ciliate Euplotes that seemingly depends on its host for survival. A subsequently characterized bacterial strain with an identical 16S rRNA gene sequence, named Fastidiosibacter lacustris , can instead be maintained in pure culture. We analysed the genomes of ‘Candidatus Nebulobacter’ and Fastidiosibacter seeking to identify key differences between their functional traits and genomic structure that might shed light on a recent transition to obligate endosymbiosis. Surprisingly, we found almost no such differences: the two genomes share a high level of sequence identity, the same overall structure, and largely overlapping sets of genes. The similarities between the genomes of the two strains are at odds with their different ecological niches, confirmed here with a parallel growth experiment. Although other pairs of closely related symbiotic/free-living bacteria have been compared in the past, ‘Candidatus Nebulobacter’ and Fastidiosibacter represent an extreme example proving that a small number of (unknown) factors might play a pivotal role in the earliest stages of obligate endosymbiosis establishment.

The “Other” <i>Rickettsiales</i> : an Overview of the Family “ <i>Candidatus</i> Midichloriaceae”

Citation
Giannotti et al. (2022). Applied and Environmental Microbiology 88 (6)
Names
Ca. Midichloriaceae
Subjects
Applied Microbiology and Biotechnology Biotechnology Ecology Food Science
Abstract
Among endosymbiotic bacterial lineages, few are as intensely studied as Rickettsiales , which include the causative agents of spotted fever, typhus, and anaplasmosis. However, an important subgroup called “ Candidatus Midichloriaceae” receives little attention despite accounting for a third of the diversity of Rickettsiales and harboring a wide range of bacteria with unique features, like the ability to infect mitochondria.

“Candidatus Midichloriaceae” fam. nov. (Rickettsiales), an Ecologically Widespread Clade of Intracellular Alphaproteobacteria

Citation
Montagna et al. (2013). Applied and Environmental Microbiology 79 (10)
Names
Ca. Midichloriaceae Ca. Midichloria mitochondrii
Subjects
Applied Microbiology and Biotechnology Biotechnology Ecology Food Science
Abstract
ABSTRACT “ Candidatus Midichloria mitochondrii” is an intramitochondrial bacterium of the order Rickettsiales associated with the sheep tick Ixodes ricinus . Bacteria phylogenetically related to “ Ca . Midichloria mitochondrii” (midichloria and like organisms [MALOs]) have been shown to be associated with a wide range of hosts, from amoebae to a variety of animals, including humans. Despite numerous studies focused on specific members of the MALO group, no comprehensive phylogenetic and statistical analyses have so far been performed on the group as a whole. Here, we present a multidisciplinary investigation based on 16S rRNA gene sequences using both phylogenetic and statistical methods, thereby analyzing MALOs in the overall framework of the Rickettsiales . This study revealed that (i) MALOs form a monophyletic group; (ii) the MALO group is structured into distinct subgroups, verifying current genera as significant evolutionary units and identifying several subclades that could represent novel genera; (iii) the MALO group ranks at the level of described Rickettsiales families, leading to the proposal of the novel family “ Candidatus Midichloriaceae.” In addition, based on the phylogenetic trees generated, we present an evolutionary scenario to interpret the distribution and life history transitions of these microorganisms associated with highly divergent eukaryotic hosts: we suggest that aquatic/environmental protista have acted as evolutionary reservoirs for members of this novel family, from which one or more lineages with the capacity of infecting metazoa have evolved.

“ Candidatus Anadelfobacter veles” and “ Candidatus Cyrtobacter comes,” Two New Rickettsiales Species Hosted by the Protist Ciliate Euplotes harpa (Ciliophora, Spirotrichea)

Citation
Vannini et al. (2010). Applied and Environmental Microbiology 76 (12)
Names
Ca. Cyrtobacter comes Ca. Anadelfobacter veles
Subjects
Applied Microbiology and Biotechnology Biotechnology Ecology Food Science
Abstract
ABSTRACT The order Rickettsiales ( Alphaproteobacteria ) is a well-known group containing obligate endocellular prokaryotes. The order encompasses three families ( Rickettsiaceae , Anaplasmataceae , and Holosporaceae ) and a fourth, family-level cluster, which includes only one candidate species, “ Candidatus Midichloria mitochondrii,” as well as several unnamed bacterial symbionts. The broad host range exhibited by the members of the “ Candidatus Midichloria” clade suggests their eventual relevance for a better understanding of the evolution of symbiosis and host specificity of Rickettsiales . In this paper, two new bacteria belonging to the “ Candidatus Midichloria” clade, hosted by two different strains of the ciliate protist Euplotes harpa , are described on the basis of ultrastructural observations, comparative 16S rRNA gene sequence analysis, and an estimation of the percentage of infection. Ultrastructure of these bacteria shows some unusual features: one has an electron-dense cytoplasm, and the other one lacks a symbiosomal membrane. The latter was up to now considered an exclusive feature of bacteria belonging to the family Rickettsiaceae . 16S rRNA gene phylogenetic analysis unambiguously places the new bacteria in the “ Candidatus Midichloria” clade, although their phylogenetic relationships with other members of the clade are not clearly resolved. This is the first report of a ciliate-borne bacterium belonging to the “ Candidatus Midichloria” clade. On the basis of the data obtained, the two bacteria are proposed as two new candidate genera and species, “ Candidatus Anadelfobacter veles” and “ Candidatus Cyrtobacter comes.”