Minute constituents of our environment which remain indistinguishable or lack detail under a light microscope, can be visualized by electron microscopy. Microorganisms and allergens are excellent examples. Multicellular organisms developed from single-celled bacteria and also play many important roles in recycling organic materials. Only a small part of microorganisms is pathogenic but some of them are very dangerous. In contrast, other microorganisms have been used to produce antibiotics to control the ill effects of pathogenic microorganisms.
Bacteria are ubiquitous.
They also reside on our skin and inside our bodies where they protect us from alien pathogens. Bacteria which reside in nodules on the roots of legumes convert molecular nitrogen from the air into forms which the plants can utilize. Sinorhizobium meliloti is produced commercially to improve the yield of alfalfa. Bacillus subtilis may act as a natural fungicide and protect flowers, vegetables, and various crops. Friendly' bacteria protect us from pathogens , that means microorganisms which cause diseases.