It’s like whipping cream. All of a sudden it goes stiff and you can turn the bowl upside down without it falling out. In the same way, a common mistake in speech solidifies and becomes firmly attached to the language.
I don’t think bacteria has quite been whipped into a singular shape yet, even though one is always reading thing like ‘bacteria’s ability to evolve its way around antibiotics’.
Such mistakes often occur in newspapers, where rush preserves erroneous forms that in oral speech bubble up and burst, to be lost to any record.
A word just on the turn is media. The first example of it found by the Oxford English Dictionary, in the sense of ‘means of communication’, was used, or misused, as a singular entity. ‘Mass media represents the most economical way of getting the story over,’ wrote an advertising man in 1923.
In becoming a singular, media was helped by not showing itself a plural by ending in an s. It might be as singular as magma to anyone not knowing Latin. But I think a more important element is semantic: media has followed the fortunes of means. Means has for centuries meant a mediator or intermediate instrument.
For 400 years, means, despite ending in an s, has been used in a singular sense, and today we speak of a means to an end, and would not dream of saying a mean to an end.
There is also a feeling that media has became a collective noun, like the mob or crowd, which could take a singular verb as easily as a plural. Media now usually forgoes the specific adjective mass, which indicated its nature, but is often limited by the adjective social, which, as with social diseases, reflects a cooperative origin.
Bacteria has a strong collective identity (one seldom encounters just the one bacterium, and bacterium is mostly applied to a class of micro-organisms). It possesses an alien (Latin) marker of plurality that, as with phenomena, trips up speakers unused to marshalling semi-learned words. Similar uncertainty is heard in producing vertebra as the singular form of vertebrae or ovum as the singular of ova. But bacteria is proving more strongly resistant to corrective measures.