In English, adjectives almost always go before the noun. The 'red balloon', the 'big ball', the 'furious green ideas'.
There are some set phrases or more poetic sequences where it is allowed to have the adjective after the noun: 'attorney general', 'notary public', 'time immemorial'. The meaning would be preserved by having the adjective as normally in front of the noun, but for whatever cultural reasons the postpositive version is what is used.
(as an aside, since this is probably the motivation of the question: to contrast, in some languages, adjectives can go in front or after a noun, and for the same adjective have different meanings: but 'propre amour' means 'proper love' but 'amour propre' - means self-esteem ('self love') )
In the example you gave 'free bacteria', you are presumably and reasonably thinking that this corresponds in a similar way as in French to 'bacteria free'. But really this is not -in general- an allowable choice (of swapping before and after) but rather different grammatical process is going on.
is an adjective followed by a noun or
is a transformation of 'free of bacteria' ADJ PREP NOUN, so really even though it comes out as
it is really the noun modifying the adjective: what kind of free is it? it is the 'bacteria' kind of free. So it is not in free alternation between a pre- and post-position, rather a transformation that coincidentally looks like the alternation.