Here's why:

* Collective nouns refer to a group of individuals considered as a single unit.

* While bacteria are single-celled organisms, we often talk about them as a collective, like a group or a population.

* For example, we might say "The bacteria in the soil are essential for plant growth."

It's important to note that "bacteria" can also be used as a plural noun when referring to multiple individual bacteria. For example, "There were bacteria on the countertop."