bacteria

The regular Latin plural: New Latin bacteria, plural of bacterium, from Greek baktḗrion, "little rod." As a plural it takes plural agreement (the bacteria are, these bacteria), and a single organism is a bacterium. Using the same form for one organism is the proscribed singular, so keeping the two apart is the simplest way to write it safely.

Here the word counts kinds rather than cells, and that is what earns it a plural of its own: bacterias. A. C. Panchdhari's Water Supply and Sanitary Installations (2002) sets anaerobic against aerobic bacteria and then speaks of "both these bacterias." The usage is marked US. Writers who want to avoid it can say what the sense means outright: a species or strain of bacterium.

Treating the plural as if it named one organism (a bacteria, one bacteria) is marked as proscribed, and it is the kind of usage careful editors correct. The safe form for a single cell is still bacterium. Unlike the "kind of bacterium" use, which counts types, this one counts individual cells, so the singular bacterium can always replace it.

An insult that turns a word for microbes on a person: a lowlife, a slob. It is slang and derogatory, so it belongs to arguments and banter, never to anything formal. Grammatically it floats. It can mean one person (a bacteria) or a whole group, because the singular-or-plural question raised by the proscribed singular hardly matters in an insult.