It's 'bacteria', not 'bacterium'

Bacteria comes from 'bacteria' in greek, which means 'rod' or 'stick' (walking stick). The fact that some people neolatinized and fabricated a 'bacterium' means absolutely nothing.

If you want to be pedantic, say 'bacterion' and 'bacteria', but then again you would probably write it 'bakterion'.

Wikipedia is not the definitive source for a lot of things, least of all this.

All living languages are subject to continual change anyhow, that's the rule that should stop people from dictating how we use the language. Language is just there for us to pass on information, in most cases of nitpicking that gets overlooked.

I had to look this up and it's far from authoritative but Wikipedia doesn't seem to think that's entirely true for data, at least (that is, you may say "a bacteria" but you do not say "a data"): http://en.wikipedia.org/wiki/Data

In English, the word datum is still used in the general sense of "an item given". [..] Any measurement or result is a datum, but data point is more usual [..] Even when a very small quantity of data is referenced (one number, for example) the phrase piece of data is often used, as opposed to datum. The debate over appropriate usage is ongoing.

[1]http://en.wiktionary.org/wiki/stadium#Noun [2]http://en.wiktionary.org/wiki/forum#Noun