Microbiology, the field of science that studies microorganisms-viruses, archaea, bacteria, fungi, algae, and protozoa, is a major scientific discipline that is at the forefront of science leading into the twenty-first century. Although the microbial world is highly ubiquitous and microorganisms were the first living inhabitants of Earth, it was not until the seventeenth century that microorganisms were first observed through primitive microscopes. At first there was little notion of the importance of microorganisms and their relationships to other living organisms. Gradually methods were developed that permitted the study of microorganisms. By posing and testing hypotheses using scientific methodologies, including the use of controlled experiments, nineteenth century microbiologists such as Louis Pasteur and Robert Koch extended the breadth of science to the examination of the microbial world. These pioneering studies revealed the enormous diversity of microorganisms and their importance for human health and environmental quality, as well as their importance for industrial biotechnological applications. Today's microbiologists use microorganisms as research tools to unravel molecular mysteries and to solve practical problems in diverse fields from medicine to waste management.

Microorganisms are very small life forms that generally require magnification to be observed; they are the unifying focus of microbiology (the science that deals with microorganisms). For the most part, they-represent an unseen, invisible world, but their small size belies their importance. Some microorganisms cause diseases, with which all of us have at some time been confronted. These represent only a minor fraction of all microorganisms. The majority of microorganisms are beneficial. Many carry out the metabolic processes that are responsible for the chemical transformations that maintain the ecological balance necessary for life on Earth. Without these microorganisms life on Earth would not exist.

Major Microbial Group 

• Viruses 

• Archaea 

• Bacteria 

• Fungi 

• Algae 

• Protozoa