A student streaks a 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 the unexpected plate result shown here after incubation?
The culture in the tube was contaminated.E. coli colonies should all have the same color, size, shape, and margins. However, there are colonies on this plate that have different color, size, and shapes. This means that another bacterial species is present—a contaminant. Notice, too, that both colonies are growing along the streak lines. This suggests that both species came from the broth tube. Airborne contaminants introduced during streaking would, in contrast, be more randomly scattered about the plate.