Brilliant takedown of a grammar jerk in the comments at Ars Technica. User Kikjou writes: "Bacteria is plural of bacterium. Please use is correctly. The same goes for media and medium, which is not in this article but is often misused in scientific writing." To which Ars Centurion Okton responds:
In Latin maybe. And the phrase you are nitpicking is actually "from a bacteria". So if you were anything but pedantic, you would exclaim "that requires the ablative of source! In the singular." Following your logic, the article should read "...from a bacterio". But wait, Latin has no indefinite article, so whether it is "a bacterium" or "a bacterio", the noun phrase is redundant since indefiniteness is presupposed in simple noun forms. But "from bacterio" is neither grammatical English nor comprehensible Latin. And the ablative of source usually employs a preposition, so "from a bacteria" should read "ab bacterio" to be exact.
Problem is: This is not FRICKING Latin. This is a word of Latin origin that has entered into English. Therefore our rules apply. Because if you demand a Latin singular, I demand the proper Latin case, pronunciation, etc. We took the plural form for obvious reasons. Because of the physical size of bacteria, the word became a mass noun and functions as both plural and singular. Same reason for taking "data": it is collective. There are rarely "bacterias". And certainly no "datas". One sheep. Two sheep. Ten sheep. One form, all numbers. It is legal in English - accept it.
I can only assume you are one of those people who thinks the plural of "octopus" is "octopi" as well. Except there is no such word as "octopus" in Latin. The word is "polypus", "Octopus" is from the Greek ὀκτάπους, and the plural of that is ὀκτάποδες. And even if people knew "octopodes" was the true plural, they would say it wrong since the epsilon is not silent. Why? Because it is adapted for usage in the new language. It has no obligations to its old morphology and phonology.
By your inanity, if you ever say the word "cherry" for a single unit, I have every right to chastise you. That word never existed in French. "Cheris(e)" is the singular form that was introduced into English. How dare you impose English conventions of depluralization on it! You will say "Shair-eez" for one piece of fruit, you'll do it in a beret and you'll like it. You doctrinaire dope.
via Ed Yong