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Microbial parasitoids : giant viruses and tiny bacteria Deeg, Christoph Michael

Abstract

Microbial parasitoids that exploit other microbes are abundant but remain a poorly explored frontier in microbiology. To study such pathogens, a high throughput screen was developed using ultrafiltration and flow cytometry, resulting in the isolation of five giant viruses and one bacterial pathogen infecting heterotrophic flagellates, as well as a bacterial predator of prokaryotes. Bodo saltans virus (BsV) is the first characterized representative of the most abundant group of giant viruses in oceans, so far only known from metagenomic data. Its 1.39 Mb genome encodes 1227 predicted ORFs; yet, much of its translational apparatus has been lost, including all tRNAs. Essential genes are invaded by homing endonuclease-encoding self-splicing introns that may defend against competing viruses. Ankyrin-repeat proteins that are putative anti-host factors show extensive gene duplication via a genomic accordion, indicating an ongoing evolutionary arms race and highlighting the rapid evolution and genomic plasticity leading to genome gigantism in giant viruses. Chromulinavorax destructans is an isolate from the TM6/Dependentiae phylum that infects and lyses the abundant heterotrophic flagellate Spumella elongata. Chromulinavorax destructans is characterized by a high degree of reduction and specialization. Its 1.2 Mb genome shows no metabolic potential, relying on an extensive transporter system to import nutrients and energy in the form of ATP from the host. It replicates by extensively reorganizing and expanding the host mitochondrion. Almost half of the inferred proteins contain signal sequences for secretion, which include many proteins of unknown function as well as 98 copies of ankyrin-repeat proteins, suggesting the presence of an extensive host-manipulation apparatus. Bdellovibrio salishius was found to exploit a beta-proteobacterium in an epibiotic manner. Despite this, B. salishius encodes a complex genomic complement more similar to periplasmic species as well as several biosynthesis pathways not previously found in epibiotic species. Bdellovibrio salishius is a representative of a widely distributed basal cluster within the genus Bdellovibrio, suggesting that epibiotic feeding might be a common predation type in nature and ancestral feature in the genus. The microorganisms described here broaden our understanding of microbial diversity and the unusual genomic functions associated with a parasitoid lifestyle amongst microbes.

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