TASS: A novel genus of intracellular bacterial parasite found in ciliates
Biologists have discovered and described a novel intracellular parasite of the ciliate Paramecium. According to the scientists, this bacterium has the smallest genome among similar microorganisms. The research findings are published in the scientific journal Environmental Microbiology Reports and reported by the Press Office of the Russian Science Foundation.
Mitochondria that act as the power plants of the cell in eukaryotes – these are organisms whose cells contain a nucleus – evolved from alphaproteobacteria. Most likely, the ancestors of mitochondria once inhabited host cells, and later became symbionts. The newly-described alphaproteobacterium also inhabits unicellular eukaryotes.
‘In the article, scientists from St Petersburg University and the Zoological Institute of the Russian Academy of Sciences, together with foreign colleagues, described a novel bacterium from the genus Holosporales, which they named Candidatus Gromoviella agglomerans. This bacterium inhabits the host cytoplasm – the semifluid substance of the host cell. Candidatus Gromoviella agglomerans frequently forms quite large aggregates. Possibly due to such aggregates, host cells sometimes display lethal division defects,’ the scientists reported.
Most likely, the genome of this bacterium was highly reduced in the process of evolution. Similar changes could be observed among fast-evolving Holosporaceae family, with some lineage-specific traits. Pronounced genome reduction can be linked to parasitism, as these prokaryotes display a limited predicted metabolism. This implies a heavy dependence on Paramecium in the synthesis of the substances they need. Possibly, the ancestors of mitochondria also followed a similar path at one time.
Alexey Potekhin, Professor in the Department of Microbiology at St Petersburg University
As the authors of the article pointed out, in prokaryote nomenclature, uncultured bacteria (i.e. bacteria that cannot be grown in the laboratory) are named using the word ‘Candidatus’ followed by a genus name with a specific epithet. Bacteria of the order Holosporales that became obligatorily associated with eukaryotes are included in the category of uncultured bacterial phyla. They can only survive inside the host cell. Candidatus Gromoviella agglomerans presents a very small genome (<600 kbp). So far, it is the smallest among the known genomes of intracellular symbionts of ciliates. Due to pronounced genome reduction, the microorganism has a limited metabolism. It is therefore heavily dependent on the host cell. Candidatus Gromoviella agglomerans has to obtain many substances from the cytoplasm of ciliates.