The prefix that changes the meaning of bacterial from "relating to or caused by bacteria" to "preventing bacteria" is anti-
The prefix anti means “against,” “opposite of,” “antiparticle of,” used in the formation of compound words ( anticline ); used freely in combination with elements of any origin ( antibody; antifreeze; antiknock; antilepton ).
Prefixes are letters which added to the beginning of a word in order to make a new word with a different meaning. Prefixes can, for instance, create a new word opposite in meaning to the word the prefix is attached to.