Techniques for the Study of Bacteria

Microbiologists have defined the nutritional needs of a number of bacteria and developed culture media for growing them in the laboratory. Culture media typically contain a carbon source, essential elements such as nitrogen and phosphorus, certain vitamins, and other required ions and nutrients. Wild-type (prototrophic) bacteria can use these simple ingredients to synthesize all the compounds that they need for growth and reproduction. A medium that contains only the nutrients required by prototrophic bacteria is termed minimal medium. Mutant strains called auxotrophs lack one or more enzymes necessary for metabolizing nutrients or synthesizing essential molecules and will grow only on medium supplemented with one or more

B.2 Bacteria can be grown in liquid medium.

nutrients. For example, auxotrophic strains that are unable to synthesize the amino acid leucine will not grow on minimal medium but will grow on medium to which leucine has been added. Complete medium contains all the substances required by bacteria for growth and reproduction.

Cultures of bacteria are often grown in test tubes that contain sterile liquid medium ( FIGURE 8.2a). A few bacteria are added to the tube, and they grow and divide until all the nutrients are used up or—more commonly—until the concentration of their waste products becomes toxic. Bacteria are also grown in petri plates ( FIGURE 8.2b). Growth medium suspended in agar is poured into the bottom half of the petri plate, providing a solid, gel-like base for bacterial growth. The chief advantage of this method is that it allows one to isolate and count bacteria, which individually are too small to see without a microscope. In a process called plating, a dilute solution of bacteria is spread over the surface of an agar-filled petri plate. As each bacterium grows and divides, it gives rise to a visible clump of genetically identical cells (a colony). Genetically pure strains of the bacteria can be isolated by collecting bacteria from a single colony and transferring them to a new test tube or petri plate.

Because individual bacteria are too small to be seen directly, it is often easier to study phenotypes that affect the appearance of the colony ( FIGURE 8.3) or can be de-

Undulate Bacterial Colony

I 8.3 Bacteria can be grown on solid media and show a variety of pheno-types. (a) Smooth, circular raised surface.

(b) Granular, circular raised surface.

(c) Elevated folds on a flat colony with irregular edges. (d) Irregular elevations on a raised colony with an undulating edge. (Parts a and d, Biophoto Associates/Photo Researchers; part b, Dr. E. Bottone/Peter Arnold; part c, Larry Jensen/Visuals Unlimited.)

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