Microbes 101: Viruses
Viruses represent the most microscopic microbes. They are typically less than a micron, meaning hundreds or even thousands could fit into a single bacterial cell. Viruses are parasitic packages of proteins and genes that completely depend on a host cell to reproduce; they use the cell’s machinery to produce copies of its genome and proteins, which are then assembled into progeny viruses. Viruses are everywhere; they are the most numerous biological entities on the planet. Not a single life form in any habitat has been found to be free of viruses, including all other types of microbes. The term phage refers to a virus that infects bacteria or archaea. Viruses come in many forms. Some have beautiful geometric shells, others mask themselves in a layer of lipids stolen from the host cell; some have a genome of RNA, others of DNA. Overall they represent a vast amount of genetic, structural, and replication strategy diversity.
Since viruses cannot reproduce on their own, there has been a longstanding debate of whether or not they are “living” organisms. But in the grand scheme of things, what organism cannot live without another that it depends on? After all, humans could not survive without the microbial contributions to our atmosphere, our environment, and our own bodies. Parasites too require a host for their survival. Even more significantly, scientists have recently discovered “giant” viruses, which dwarf most other viruses in both physical size and that of their genomes. These behemouths exhibit capabilities we normally associate with cells — from curating their own genomes to chemical communication. As a result, giant viruses have reinvigorated the debate over the life form status of viruses. While such discoveries blur the lines between viruses and their “living” microbial peers, they are currently not included in the Tree of Life.
I Contain Multitudes is a multi-part video series dedicated to exploring the wonderful, hidden world of the microbiome. The series is hosted by science writer Ed Yong and produced by HHMI Tangled Bank Studios in association with Room 608.
