(i) Pathogenic organisms – Pathogenic organisms are those, which produce disease.
(ii) Nonpathogenic organisms – Non-pathogenic organisms are those, which break down the refusal and sewage, and those, which help in making curds, cheese, bread and alcohol.
There are four main groups of microorganisms.
- Bacteria include cocci, bacilli, spirella and spirochaetes
- Fungi are plant like organisms and include yeast. Yeast are useful for making bread and wine. Penicillin also is produced from the yeast. Some fungi are pathogenic and cause diseases such as ringworm and oral thrush.
- Protozoa are animal type of organisms,. They are much bigger than bacteria. Examples: malaria parasite causes the malaria fever and the entamoeba hystelytica causes dysentery.
- Viruses are smaller than bacteria. It can pass through the finest filters. They cannot be seen through an ordinary microscope, but only through an electron microscope. Common viral infections are common cold, measles, chickenpox, smallpox, rabies and poliomyelitis.