Microbiology is the study of microbes. With only rare exceptions, individual microbes can be observed only with the use of various types of microscopes.
The two major categories of microbes are acellular microbes (also called infectious particles) and cellular microbes (also called microorganisms).
Acellular microbes include viruses and prions.
Cellular microbes include all bacteria, all archaea, all protozoa, some algae, and some fungi. Because viruses are acellular (not composed of cells), they should be referred to as “acellular microbes” or “infectious particles” rather than microorganisms.
Microbes are ubiquitous, meaning that they are found virtually everywhere.
Microbes that live on and in various parts of the human body are referred to as our indigenous microbiota.
Only a small percentage of known microbes cause disease. Those that do are called pathogens or infectious agents.
Pathogens cause two types of diseases: infectious diseases and microbial intoxications. Microbes that do not cause disease are called nonpathogens. Opportunistic pathogens do not cause disease under ordinary conditions, but have the potential to cause disease should the opportunity present itself (e.g., if they gain access to the “wrong place” at the “wrong time”).
Many microbes are involved in the decomposition of dead organisms and the waste products of living organisms. Collectively, they are referred to as decomposers or saprophytes. A saprophyte is defined as an organism that lives on dead and/or decaying organic matter.
The use of microbes to clean up toxic wastes and other industrial waste products is known as bioremediation.
Many microbes play essential roles in various elemental cycles, such as the carbon, nitrogen, oxygen, sulfur, and phosphorous cycles. Photosynthetic algae and bacteria (such as cyanobacteria) produce the majority of the oxygen in our atmosphere.
Many microbes are used in various industries, such as food, beverage, chemical, and antibiotic industries. The use of living organisms or their derivatives to make or modify useful products or processes is called biotechnology.
Three of the early microbiologists who made significant contributions to our present understanding of microbes were Anton van Leeuwenhoek, Louis Pasteur, and Robert Koch.
Robert Koch and his colleagues established an experimental procedure to prove that a specific microbe is the cause of a specific infectious disease. This scientific procedure became known as Koch’s Postulates.
Infectious diseases that are caused by the combined effects of two or more different microbes are referred to as synergistic infections or polymicrobiasl infections.