Question

1. What might be the problem if you incubated the streak plate technique in the upright position?

2. Is it possible to tell if you have a pure culture in a broth? on a slant? on a plate streaked using the quadrant method? if so, explain how. If not, explain why not.

Answer 1

Answer 1. Streak plate technique is used to culture bacteria, especially for isolating pure colonies from a mixed culture. A small inoculum of bacterial sample is taken in an inoculation loop and streaked on agar plate.

If we incubate a streak plate in an upright position, moisture may develop on the top lid of the petri plate, which condenses and drop down to the culture plate, leading to mix of the colonies. This is why, bacterial agar plates are always incubated in an upside down or inverted manner.

Answer 2: Yes, we can determine whether a broth contains pure culture or not by visualising it under the microscope. Minute amount of broth can be taken and streaked on a slide, air-dried and can be stained to visualise under a microscope.

We can also determine pure culture from agar slants. Agar slants are prepared by pouring hot melted agar in a test tube, and then incubate it in a slanted position. This is a type of solid media, so pure cultured=s can be found in individual colonies.

I have shown one diagram of a quadrant streaked plate. As we move clockwise with the inoculation loop, bacterial load decreases and isolation of discrete colonies are found.