There may be some cost to each resistance gained, reducing the fitness of the bacteria

And there may be general resistance mechanisms that hit more than one chemical (like changes in membrane permeability and efflux pumps.) Over time, with more exposure, the costs can be expected to decline as resistance is optimized.

Ultimately resistance can evolve that kicks in only on exposure to the chemicals in question. Bacteria already do this with, say, the enzyme needed to metabolize lactose. The gene isn't expressed until lactose is present.

This is why we use “cocktails” on resistant cases now.

Kill it three different ways at once and it cannot adapt fast enough to leave dna fragments for the next microbe to pick up and continue the work.

Just as long as we don't start overusing the cocktail. Natural selection is a very powerful force, all it takes is one bacterium in a million that has resistance to all three methods to start an evolution.

Ideally you shelve several antibiotics for decades, with the goal of bacteria dropping resistance to it. Sounds like a game theory compliance nightmare though.