Question

A student performs a streak plate using inoculum from a broth culture tube. The culture tube was labeled "Escherichia coli," indicating that the culture was composed of a single bacterial species. Which of the following best explains this unexpected plate result seen after incubation?

a) The loop wasn't flamed between quadrant streaks.
b) The culture in the tube was contaminated.
c) The plate was contaminated during streaking.
d) The plate wasn't properly rotated between streaks.

Answer 1

c) The plate was contaminated during streaking. The plate was contaminated during streaking.

Step-by-step explanation:

The unexpected plate result seen after incubation is most likely due to c) The plate was contaminated during streaking.

In streak plating, the goal is to isolate a single type of bacteria by diluting and spreading them over the surface of an agar plate. If the plate was contaminated during streaking, it means that other bacteria or contaminants were introduced during the process, leading to the growth of multiple bacterial species instead of a pure culture of Escherichia coli.

To prevent contamination during streaking, it is important to maintain sterile techniques, such as properly flaming the loop between quadrant streaks and ensuring that the plate is not touched by any external surfaces or contaminants.