Lab Exercise 5:
Pure culture techniques
Perform a streak-plate to separate the cells of a mixed culture so that discrete
colonies can be isolated.
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.
As you learned in Lab 4, microbes exist everywhere, and
very rarely do they occur as a single species.
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
His studies on
many of the laboratory techniques we still use today,
including the method for isolating
The most commonly used method in the laboratory for isolating microbes is the
, and to a lesser extent, the
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
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.
Streak plate method
1 mixed culture of
1 environmental body broth (from Lab
2 nutrient agar plates