Learn English – What does the word “that” refer to in this sentence

pronouns

Consider the following sentence:

Certain gram positive bacteria can be associated with oil glands that play a role in acne and skin disease.

I'm wondering what does the word that refer to. Is it 'oil glands' or 'certain gram positive bacteria'?

It is 'oil glands'.

The word order makes it quite clear.

[...] the glands that [...]

If it were the other way around the sentence would have to be:

Certain bacteria that play a role in acne and skin disease are associated with oily glands.