Lab Exercise 5: Pure culture techniques OBJECTIVES 1. Perform a streak-plate to separate the cells of a mixed culture so that discrete colonies can be isolated. 2. Perform a pour-plate (loop) dilution to separate cells of a mixed culture and compare growth characteristics beneath and on the surface of the agar. INTRODUCTION As you learned in Lab 4, microbes exist everywhere, and very rarely do they occur as a single species. Robert Koch, known as the father of medical microbiology, was one of the first to recognize that isolating a microbe (in his case, bacterium) away from other microbes was crucial for his own argument that microbes cause disease, as well as understanding characteristics of the microbe itself. His studies on Bacillus anthracis contributed to many of the laboratory techniques we still use today, including the method for isolating pure cultures of bacteria. The most commonly used method in the laboratory for isolating microbes is the streak plate , and to a lesser extent, the pour plate . Both methods rely on dilution of bacterial cells in a sample to the point at which a single cell can divide giving rise to a single pure colony . The pure colony is essentially a clone of cells that all are identical copies of the original cell and can be used for further study. Lab Exercises I. Streak plate method Table supplies Individual supplies 1 mixed culture of Serratia marcescens, Escherichia coli, and Chromobacterium violaceum 1 environmental body broth (from Lab Exercise 4) 2 nutrient agar plates Inoculating loop
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