A VIRAL RETROSPECTIVE
Were these mysterious infectious agents exclusive to plants? No! Around the same time that TMV was being investigated, Friedrich Loeffler and Paul Frosch (two of Koch’s former students) were working hard to get at the root cause of Foot-and-mouth disease (FMD), an illness that affects cloven-hoof animals like cattle. By applying Koch’s postulates, they knew that the disease was infectious. However, when they filtered their infected samples, FMD seemed to pass through the smallest of pores where bacteria could not. This was the first proof of a viral animal infection.
At this point, viruses were considered the black holes of biology. There was a considerable body of indirect evidence that pointed to the existence of this novel infectious agent. Much like an unseen (until last year) black hole that impacts the orbits of nearby stars and produces gravitational waves, viruses infect and spread waves of disease without an (at the time) identifiable trace. It wasn’t until the invention of the high-resolution electron microscope in the 1930s that virus particles were first imaged. Thirty years later this was followed by the development of the first antiviral drugs.
So, what is a virus? Well, like Beijerinck thought1, it is certainly a distinct kind of microorganism. First, they are tiny! Viruses are roughly 8 to 40 times smaller than bacteria (depending on the virus). If we enlarged a virus to the size of a nickel (a US five cent coin), the bacterium would be the size of a dinner plate. Their minute size also means that their overall structure, although still intricate, is less complex than that of a bacterium as they are basically comprised of three main parts: genetic material (in DNA or RNA form); a protein coat (capsid) to surround and protect the genetic information; and a lipid envelope containing receptors required for cellular infection. Second, although they have a genome, viruses are not able to replicate their genes on their own and can only grow and multiply within a host cell. This obligate parasitic lifestyle, wherein the virus infects, hijacks the host’s molecular machinery, replicates, and kills the cell to spread itself, is precisely why they are considered to not be alive2.
Remarkably, these pseudo-living nanoscopic organisms that once seemed elusive and rare are in fact ubiquitous and diverse! Although they require live host cells to replicate, the scope of hosts that viruses target spans everything from humans, pigs, fish, and birds to plants, starfish, and even bacteria! Each virus has a particular host that it can infect. These unique host-pathogen interactions are established at the molecular level via specific receptor binding events. However, some viruses can broaden the scope of their host targets, thus expanding their breadth of infectivity. For such a small particle of a being, this is quite impressive.
full article in PLASMA 6 (coming August 2020)