CLASSIFICATION OF MICRO ORGANISMS

VIRUSES

A virus (Latin for “poison”) is an obligate intracellular parasite that can only replicate inside a living host cell. Once inside the living host cell, a virus becomes integrated in the metabolism of its host, making a virus difficult to control by chemical means. You cannot kill a virus with antibiotics. Drugs that destroy the host’s ability to be used by a virus for replication tend to also be highly toxic and have a negatively and sometimes deadly effect on the host cell.

Before a virus enters a cell, it is a free virus particle called a virion. A virion cannot grow or carry out any biosynthetic or biochemical functions because it is metabolically inert. Viruses are not cells. They vary in size from 20 nanometres (polio virus) to 300 nanometres (smallpox virus) and cannot be seen under a light microscope.

In 1933, microbiologist Wendell Stanley of the Rockefeller Institute for Medical Research showed that viruses could be regarded as chemical matter rather than as living organisms. All viruses contain a nucleic acid core, a protein coat called capsid and may contain a membrane called envelope.

Examples of viruses are:

DNA VIRUSES such as Hepadnaviruses (Hepatitis B virus), Herpesviridae (Herpes virus, pox virus, cytomegalovirus, Epstein Barr virus) and HPV.

RNA VIRUSES such as Dengue virus, Hepatitis A, C, D, E, F and G virus, rhinovirus, polio virus, HIV, measles virus, rabies virus and haemorrhagic viruses such as Lassa, Ebola, yellow fever and chikunguya virus.

FUNGI

The fungi are non-photosynthetic protitis growing as a mass of branching, interlacing filaments ("hyphae") known as a mycelium. Although the hyphae exhibit cross-walls, the cross-walls are perforated and allow free passage of nuclei and cytoplasm. The entire organism is thus a coenocyte (a multinucleated mass of continuous cytoplasm) confined within a series of branching tubes. These tubes, made of polysaccharides such as chitin, are homologous with cell walls. The mycelial forms are called moulds; a few types, yeasts, do not form a mycelium but are easily recognized as fungi by the nature of their sexual reproductive processes and by the presence of transitional forms.

Examples of fungi are Rhizopus nigricans (black bread mould), basidiomyces (mushrooms), Penicillium notatum, which produces penicillin and Candida albicans, which causes vaginal yeast infections in humans.

BACTERIA

Bacteria are unicellular prokaryotic cells varying in size, shape and morphology. They are found everywhere in nature. There are approximately more than ten times the number of bacteria in our body than the number of cells we have. The highest number being in the gut followed by skin, some bacteria living in the body are harmless; these are called commensals or normal flora. Some bacteria on the other hand are pathogenic and cause diseases such as typhoid fever, tuberculosis, gonorrhoea and syphilis.

RICKETTSIAS AND CHLAMYDIAS

Rickettsias and Chlamydias are intracellular parasites that need a host in order to reproduce and therefore enter the cell of a host. These bacteria were once thought to be viruses that invaded cells. They are classified as bacteria because they have bacterial cell walls and contain DNA and RNA, which is not the case with a virus. Rickettsias and Chlamydias have no means of mobility because they lack flagella. They are also gram-negative.

MYCOPLASMA

Mycoplasmas are very small facultatively anaerobic bacteria (some are obligately anaerobic) that have taken on many shapes (pleomorphic) and were once thought to be viruses because they lack a cell wall. However, they have a cell membrane, DNA and RNA, which distinguishes them from viruses.

Mycoplasmas can also resemble fungi because some Mycoplasmas produce filaments that are commonly seen in fungi. It is these filaments that led scientists to name it Mycoplasma. Myco means “fungus.”