I doubt you’ll win any arguments with this one; responses will tend to be:
“Those bacteria were there all along, or the people saying they are new, are lying to you. Nothing new has arisen, therefore evolution is false.”
“So what? - It’s only a micro change - not like a cat turning into a dog, therefore evolution is false.”
“It’s not a beneficial* change, therefore evolution is false.”
(*I mean, I know it’s an advantageous change for the mutant organism, but try explaining ‘beneficial’ to a creationist, in the context of a disease becoming more virulent. From their POV, God’s sick, broken world just broke a bit more and got sicker.)
I’m guessing that this is their strongest argument. They can submit that there are no new bacteria, just many more of the variation that was already there.
I agree that addressing a mutation as a preexisting variation verses a means of evolution is kind of turning your back to the obvious, but they manage to convince themselves anyway.
He was pretty clear about this, back in the day… any malady can be cured simply by having faith that God’s power will cure it, everything from illness to leprosy to blindness to demonic possession. We shouldn’t need antibiotics at all. He’s creating antibiotic-resistant diseases specifically so we’ll stop killing them, which will then cause us to stop using antibiotics and instead seek healing through His power.