An Interesting Plural Quirkiness

Is “bacteria” singular or plural? Well, it’s plural. The singular is “bacterium.” But here’s an example of using the singular to refer to a whole bunch of the little critters. Twice:

A bacterium named Moorella thermoacetica won’t work for free. But UC Berkeley researchers have figured out it has an appetite for gold. And in exchange for this special treat, the bacterium has revealed a more efficient path to producing solar fuels through artificial photosynthesis.

The writer isn’t referring to a single bacterial cell, but the whole species. 

We do this a lot in American English, refer to a collective noun in the singular. We say “the team is,” for example.