Site author Richard Steane
The BioTopics website gives access to interactive resource material, developed to support the learning and teaching of Biology at a variety of levels.


The word micro-organism is a general term for a (very) small organism, so small that the use of a microscope is required to see details of its structure. The study of micro-organisms (also known as microbes) is called microbiology, and it is increasingly relevant in Biology.

Specialists divide microbiology according to their interests, as follows :
Fill in the names in the table below.

Main groups of micro-organisms Study called Specialist called
(not really living organisms like the rest)
virology virologist
bacteria bacteriology bacteriologist
fungi mycology mycologist
possibly less important
protozoology protozoologist
algaealgology (phycology)algologist (phycologist)

Each of these groups includes organisms which can be seen to be useful to man (especially in the context of biotechnology), and others which are harmful (mainly because of diseases - of animals and plants, or spoilage - of stored products, especially food). There are broad similarities in the way that these micro-organisms grow, but there are distinct differences in detail which must be appreciated.

From a classification point of view, these micro-organisms are now thought to merit separating from other more familiar living organisms (Plants and Animals), so they have been given Kingdoms of their own:

Group of micro-organism Kingdom
Bacteria and Blue-green "algae" Monera
Protozoa and Algae Protoctists (Protists)
(as a separate Kingdom, not a subdivision of the plants)

Viruses are categorised according to a different system, because they are so unlike the others.


If a micro-organism has an adverse effect on another organism, e.g. causing a disease in Man, perhaps by getting inside and damaging its cells, or affecting it with a chemical substance which it produces, it is said to be pathogenic (adjective) or a pathogen (noun). It may also be described as the causative organism of that disease.

Pathogens can be said to be parasitic because they live at the expense of the other organism - their "host".

All viruses are pathogenic because they enter cells and cause adverse effects. Some of the worst diseases of Man are viral, i.e. caused by a virus.

Some bacteria (ones which cause headlines!) are pathogenic, and a few cause quite serious diseases. Other bacteria seem to be useful to Man in their correct contexts, whilst other apparently fairly harmless bacteria may cause diseases in certain circumstances, e.g. old or young people. In addition, there may be different varieties or strains (especially of bacteria and viruses) which show different characteristics from the normal type or species.

Similarly, there are examples of protozoans (protoctists) and fungi which are pathogenic.

Why are micro-organisms such powerful pathogens?

Size and distribution: Being so small, many micro-organisms can fit into a small space, or spread out (thinly!) over a large area, although most are not able to move of their own accord.

Reproductive potential: Because they can multiply rapidly, it only takes a few bacteria or viruses to cause an infection. Usually, micro-organisms reproduce asexually, so they can produce millions in a few hours.

Variability: With such large potential populations, new varieties can arise due to mutation, and characteristics like resistance to antibiotics can spread easily, even between unrelated species!

Insidiousness: They cannot be seen directly, and whilst reproducing and preparing to spread they can cause great damage to cells and internal working parts of organisms.

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