While it's common for a single cell to develop into a visible colony on an agar plate, it's not a universal rule. Here's why:

* Cell aggregation: Some bacteria, like Streptococcus, naturally form chains or clusters. If a clump of cells lands on the agar, it might develop into a single colony, even though it started from multiple cells.

* Micro-colonies: Occasionally, a single cell might divide very slowly or form a small, barely visible micro-colony. When these micro-colonies are close together, they can appear as a single, larger colony even though they originated from multiple cells.

* Contamination: If an agar plate is contaminated with more than one type of bacteria, you could have multiple species growing into discrete colonies, even if they didn't originate from a single cell each.

However, the general principle holds:

* A single, isolated colony on an agar plate is likely to have originated from a single bacterial cell.

* This is because the agar provides a controlled environment where individual cells can grow and reproduce without interference from other organisms.

To confirm the origin of a colony, you can:

* Streak plate technique: This method aims to isolate single cells by repeatedly streaking the bacteria across the agar plate. This can help to ensure that each colony is derived from a single cell.

* Microscopic examination: Examining the colony under a microscope can help determine if it's made up of a single species or multiple species.

In conclusion, while it's a common assumption, not every discrete colony on an agar plate represents the growth of one cell. However, it's a good starting point for understanding bacterial growth and is often a valid assumption in many cases.