Introduction to The Microbial World
© 2009 Kenneth Todar PhD Figure 1. The bacterium, Legionella. American Society for Microbiology.
The Microbial World
The microbial world is a realm of life made up of microorganisms and viruses. Microbiology is the branch of biological sciences concerned with the study of these microbes.
Microorganisms are unicellular organisms (capable of existence as single cells), too small to be seen with the naked eye. Among all forms of life on the earth, microorganisms predominate in numbers of species and in biomass, but their occurrence is generally underappreciated because of their small size and the need for a microscope to see individual cells. Although a light microscope is generally required to visualize a single microbial cell, microbial colonies and communities can readily be observed in nature. As discussed below, viruses are noncellular entities and cannot be considered microorganisms. Viruses and cellular microorganisms are considered microbes. Figure 2. Opalescent Pool in Yellowstone National Park, Wyoming USA. In these types of hot springs, the orange, yellow and brown colors are due to pigmented photosynthetic bacteria which make up the microbial mats. The mats are literally teeming with microbes. Other non-photosynthetic bacteria, as well as various archaea and algae are also residents of the hot spring community.
Figure 3. Cross section of a microbial mat showing the different layers of pigmented bacteria. Measurement is in centimeters.
Although most microorganisms are unicellular and do not differentiate or develop into multicellular forms composed of different types of cells, there are many exceptions, so that this criterion cannot be used alone to differentiate a microorganism from a macroorganism (multicellular organism). Figure 4. The cyanobacterium Anabaena. American Society for Microbiology. Two (not uncommon) exceptions that microorganisms are unicellular and