Acellular organisms

Not every infectious biological entity fits inside a cell, and this section covers the acellular organisms — viruses, viroids and prions — none of which are part of Whittaker's Five Kingdom classification.

a. Viruses — The name "virus" (from the Latin for venom or poison) was given by Louis Pasteur. The specific term "virus" for these obligate parasites was proposed by M. J. Beijerinck, after he observed that they were able to migrate through an agar gel; because the infectious agent behaved like a soluble fluid, he called the filtrate "contagium vivum fluidum." It was the scientist Wendell Stanley who later demonstrated that viruses are inert outside a host cell and can even be crystallised — showing that they are made up largely of protein.

Viruses lack their own cellular machinery. They consist of a protein coat, the capsid — built from smaller protein units called capsomeres, arranged in either helical or polyhedral form — wrapped around a strand of nucleic acid. This is why they are considered acellular. Outside a host cell, viruses are inactive; but once they enter a specific host cell, they hijack that cell's machinery and use it to duplicate themselves, which is why viruses are often described as infectious nucleoprotein particles.

A virus's genetic material is usually either single-stranded RNA (very rarely double-stranded RNA) or double-stranded DNA (rarely single-stranded DNA) — but a virus never carries both DNA and RNA together at the same time. Viruses that specifically infect bacterial cells are called bacteriophages, and these normally carry double-stranded DNA (Fig 2.14 shows a bacteriophage's head, core, collar, contractile sheath, basal plate and tail fibres, which it uses to attach to and inject its genetic material into a host bacterium). Based on their genetic material, viruses are broadly grouped as DNA viruses or RNA viruses.

Viruses are responsible for a wide range of diseases. In plants they cause disorders such as leaf curling, yellowing and mosaic formation (the Tobacco Mosaic Virus, Fig 2.13, is a classic example). In animals they cause diseases such as foot-and-mouth disease and swine flu. In humans, viruses cause smallpox, mumps, herpes, the common cold, and AIDS, among many other diseases. …