Mimivirus, Marseillevirus and Megavirales

At the beginning of the 21st century, a microorganism which was visible under the microscope, could be Gram-stained and had been considered a bacterium for several years was identified as Mimivirus, the largest virus known at the time. This opened up the world of giant viruses which are today considered to represent a fourth domain of microbes. In this special issue of Intervirology, recent findings on two new families of giant viruses, Marseilleviridae and Mimiviridae, are presented. The data confirm that both groups of viruses may be found in the environment (soil, water, insects) and in humans (blood, stool). New techniques to culture, purify and isolate these viruses are also reported. These new findings show that the world of giant viruses is expanding, and challenge the traditional classification of microbes into three domains (bacteria, archaea and eukaryotes) based on size and ribosomal genes. A new classification of microbes into four branches – giant viruses (Megavirales), bacteria, archaea and eukaryotes – is postulated which reflects more realistically our current knowledge of the microbial world. Anyone interested in this revolution in virology should read this publication which may contribute to the discovery of further giant viruses that are as yet unknown.