DEFINITIONS

A 'microorganism' is generally interpreted as 'an organism of microscopic size', but is incapable of precise scientific definition. This section of the syllabus is concerned with pathogenic microorganisms, that is, those which can cause infectious disease. Such organisms may be protozoa, fungi, bacteria or viruses.

The term 'protozoan' is used in the syllabus specifically in connection with the malarial parasite. Although no longer having official status in defining a taxonomic group, the term is still widely used for convenience and is relatively easy for students to understand in terms of 'heterotrophic organisms, consisting of a single cell which lacks a cell wall;
most can move around independently'. The terms 'protozoan' and 'protozoa' should therefore be introduced to students on that basis; either term could appear in examination questions.

Students should be made aware of the importance of using correct terminology when referring to microorganisms. Precise terms such as bacteria, viruses, fungi or microorganisms are required in examination answers, rather than germs.

Students should understand the terms pathogen and parasite. They should also realise that pathogenicity is a direct consequence of the parasitic mode of life. Parasitic microorganisms may inflict mechanical, chemical or metabolic damage on their host.

The inclusion of viruses as pathogenic microorganisms (implying that they have the status of living organisms, as well as of being parasites) may be debated. This might form a homework assignment for more able students; first, however, the work on viruses which follows later in this section would need completion.

Background

The syllabus does not include any of the large number of microscopic multi-cellular animals, such as parasitic nematodes or platyhelminthes, which cause an immense amount of human disease.

At the other end of the range, none of the infectious particles smaller than viruses is considered. In this category come prions, the subviral particles causing diseases such as scrapie and bovine spongiform encephalopathy (BSE).