Abstract
Prior to the discovery of the mimivirus in 2003, viruses were thought to be physically small and genetically simple. Mimivirus, with its ~750-nm particle size and its ~1.2-Mbp genome, shattered these notions and changed what it meant to be a virus. Since this discovery, the isolation and characterization of giant viruses has exploded. One of the more recently discovered giant viruses, Samba virus, is a Mimivirus that was isolated from the Rio Negro in the Brazilian Amazon. Initial characterization of Samba has revealed some structural information, although the preparation techniques used are prone to the generation of structural artifacts. To generate more native-like structural information for Samba, we analyzed the virus through cryo-electron microscopy, cryo-electron tomography, scanning electron microscopy, and fluorescence microscopy. These microscopy techniques demonstrated that Samba particles have a capsid diameter of ~527 nm and a fiber length of ~155 nm, making Samba the largest Mimivirus yet characterized. We also compared Samba to a fiberless mimivirus variant. Samba particles, unlike those of mimivirus, do not appear to be rigid, and quasi-icosahedral, although the two viruses share many common features, including a multi-layered capsid and an asymmetric nucleocapsid, which may be common amongst the Mimiviruses.
Highlights
Following the discovery that the causative agent of tobacco mosaic disease could pass through sterile (0.22 μm) filters [1], viruses were thought to be small and simple; containing only a few genes [2]
The thickness of the ice layer detracted from the contrast of Samba virus (SMBV) cryo-electron microscopy (cryo-EM) images, especially while using a 200-keV
SMBV particles were imaged using a 300-keV transmission electron microscopy (TEM) (JEOL 3200, data not shown), but these images displayed no appreciable difference in quality from the micrographs collected at
Summary
Following the discovery that the causative agent of tobacco mosaic disease could pass through sterile (0.22 μm) filters [1], viruses were thought to be small and simple; containing only a few genes [2]. Isolated in 1993 from a water cooling tower in Bradford, UK following a pneumonia outbreak, the so-called “Bradford coccus” was initially classified as a bacteria. These supposed cocci were visible under the light microscope and appeared to stain Gram positive [3]. Viruses 2017, 9, 30; doi:10.3390/v9020030 www.mdpi.com/journal/viruses (~700-nm particle diameter). It was not until 2003 that the inability to culture the Bradford coccus, and its lack of a 16S ribosomal RNA sequence, lead to the re-classification of this organism as the microbe-mimicking (Mimi) virus [3]
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