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 plate was exposed to airborne contaminants.
b) The culture in the tube was contaminated.
c) The plate wasn't incubated in an inverted position.
d) The loop wasn't flamed between quadrant streaks.

Answer 1

Unexpected results on an agar plate after performing a streak plate could be due to contamination of the culture tube, exposure to airborne contaminants, not flaming the loop between quadrant streaks, or incorrect incubation with the plate not in an inverted position.

Step-by-step explanation:

The unexpected plate result after incubation when a student performs a streak plate using inoculum from a broth culture tube labeled "Escherichia coli" could be due to several reasons. If we assume that the plate showed a mix of different bacterial colonies instead of one, this could be due to:

• Contamination of the culture tube: The original culture tube may have been contaminated with another bacterial species.
• Airborne contaminants: The plate or culture medium could have been exposed to the air and picked up contaminants, leading to the growth of multiple types of bacteria.
• Poor technique: Not flaming the loop between quadrant streaks could result in the transfer of the same bacteria across all quadrants, leading to no reduction in colony number as expected with proper streaking technique.
• Incorrect incubation: If the plate was not incubated in an inverted position, condensation could drop onto the agar surface and spread bacteria, thus not isolating single colonies.

Each of these scenarios would result in a plate that shows multiple types of bacterial colonies, which indicates that the culture is not pure as originally intended.