The world of microbes

The World of Microbes

Microbes - bacteria, fungi, algae, protozoa and viruses - affect every aspect of life on earth. They have an amazing diversity of form and can exist in a wide range of habitats from hot springs to the icy wastes of Antarctica and inside the bodies of animals and plants. Microbes cause diseases like 'flu or malaria, but most are completely harmless. They are essential to the cycling of nutrients in the ecosystems of the planet. Microbial activity is exploited for the benefit of humankind in many ways, such as the production of medicines, food and enzymes, in the clean-up of sewage and other wastes and in the exciting advances resulting from developments in molecular biology techniques.

Classification of Microbes
Until the middle of the 19th century all living organisms were classified into two groups, animals and plants. There were problems with this simplistic system, for example fungi look like plants but they do not photosynthesise. Over the years, a number of classification systems were put forward; the most well known is the Five Kingdom system which groups all living things by cell type, level of organization and nutrition as follows:
1. Animals
2. Plants
3. Fungi
4. Protoctista (algae and protozoa)
5. Monera (bacteria)
The first four groups represent eukaryotes (cells have a nucleus and membrane-bound organelles). The Monera includes all prokaryotes (cells lack a nucleus and membrane-bound organelles).

In the light of recent advances in molecular biology, which allow the comparison of the sequencing of ribosomal RNA of organisms, a new classification system is preferred by scientists. It is based on three lines of descent from a common ancestor. Each group is called a Domain:
1. Bacteria (true bacteria) - prokaryotes
2. Archaea (archaebacteria) - prokaryotes
3. Eukarya - eukaryotes

The archaebacteria are prokaryotic in general structure and share many bacterial characteristics. However they share with eukaryotes a some ribosomal sequences that are not found in bacteria.

Viruses are not usually included in classification systems as they are non-cellular and they are dependent on a host cell for their replication and metabolic processes.

Within their domains, identification of microbes begins with their physical appearance, followed by biochemical and genetic tests.