Pure Cultures

Infectious material or materials that contain pathogenic microorganisms can be located in pus, sputum, urine, feces, soil, water, and food. These infectious materials can contain several kinds if bacteria. If these materials are placed on a solid medium, colonies will form that are the exact copies of that same microorganism. A colony arises from a single spore, vegetative cell, or a group of the same organism that attaches to others like it into clumps or chains. Microbial colonies have distinct appearances that distinguish one microorganism from another.

The streak plate method is the most common way to get pure cultures of bacteria. A device called an inoculating loop is sterilized and dipped into a culture of a microorganism or microorganisms and then is "streaked" in a pattern over a nutrient medium. As the pattern is made, bacteria are rubbed off from the loop onto the nutrient medium. The last cells that are rubbed off the loop onto the medium are far enough apart to allow isolation of separate colonies of the original culture. (See Fig. 6-1.)

Fig. 6-1. Streak plate method used to isolate bacteria.

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