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Kenneth Todar –AUTHOR
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 undifferentiated are seen in Anabaena. The organism lives as a multicellular filament or chain of cells. The predominant photosynthetic (bright yellow-green) cells conduct photosynthesis, while the obviously large "empty" cells occasionally seen along a filament are differentiated cells in which nitrogen fixation, but not photosynthesis, takes place. Cell Theory

To understand microbes it is necessary at the outset to review the basic concepts of biology regarding cells. A cell is considered to be the fundamental unit of life and an understanding of cell theory is the basis for the understanding of life, including microbial life. The main points of the cell theory are as follows: 1. All organisms are composed of basic membrane-enclosed units called cells. 2. All organisms are either unicellular (single cells) or multicellular (more than one cell). 3. All cells are fundamentally alike with regard to certain aspects of their chemistry, structure and metabolism. 4. Cells arise from previously existing cells by means of asexual or sexual reproduction. 5. Cells can mutate and evolve into new or different types of cells. Basically, a cell is a membrane-enclosed entity capable of self replication, mutation, and maintenance of genetic continuity. Figure 5. A model cell, the fundamental unit of life, in this case a procaryotic cell. A cell consists of a membrane that encloses the cell cytoplasm. The cytoplasm contains a nuclear region (DNA), which functions as the genetic center; ribosomes, which are required for the synthesis of proteins; enzymes and chemical precursors of cell material that are in solution in water. The membrane and enzymes carry out the functions of life as directed by the DNA. Most, but not all microbes have a cell wall, also present in plant cells but notably missing in animal cells.

Properties of living systems
Bios means life, and biology is the study of life, but biologists have a difficult time defining life. It is easier to describe life...
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