What this medication does is (to inhibit / inhibit / inhibits) the reproduction of the certain kinds of bacteria.
To inhibit is the one I would choose, similar like it is to prevent incidents. "is" is one of Be verbs, there are be+doing/adjective, without "is" it should be "inhibits" as "What this medication does" equals "it". Then verb can't follow by another verb unless it is connected by "to" like "I am going to camp at the forest". Actually, I have joust found something that can explain what I had written in a professional and technical wayFunctions of the to-infinitivewhich is shared by Davyd Diniz
The sentence can be reordered as:
[ (to inhibit / inhibit / inhibits) the reproduction of the certain kinds of bacteria ] is what this medication does
In this view, the fragment must stand on its own as a noun phrase (the subject), and to inhibit has this property. However, after "is", "inhibit" also works. You can find examples on the web by searching "what this * do is" in google books.
Then "inhibit" or "to inhibit" work.