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Chapter 7 : The Care and Feeding of Microbes
At the time the agar-streaking procedure was introduced (and for the following 30 years), little was known about the specific nutrient requirements of different types of microbes. With the passage of time, it became evident that in many instances the chemical activities of bacteria and other microbes can be significantly affected by the presence or absence of particular nutrients in the growth medium. Since there is an extraordinary degree of metabolic versatility in the microbial world, thousands of culture media recipes have been proposed and used. For purposes of illustration, this chapter considers one important bacterium that is widely used in microbial research and happens to have relatively simple nutrient requirements, namely, Escherichia coli. Microbiologists in various countries were busy isolating pure cultures of microbes from soil, airborne dust particles, natural waters, and plant and animal surfaces. Cells of the standard (“type”) strain could be kept alive, but not growing, in the form of colonies or streaks on agar plates stored in the refrigerator at 4°C (39°F). An alternative procedure involves storage of cells at much lower temperatures. The American Type Culture Collection (ATCC) is the largest collection in the world and contains the most diverse assortment of known microbes. The scientific staff of the ATCC is constantly engaged in research aimed at improving cell preservation techniques and increasing knowledge of the properties of different species. A common technique now used for preserving microbial cells is based on freeze-drying (“lyophilization”).