[wkrsz]

I meant that if you have two separate substances, we can reasonably assume they will also be used individually, giving bacteria chance to develop resistance. Then strains with different resistances meet in one organism and swap genes.

Although that's extremely simplified. I recall reading that the usual mechanism is somewhat different. When you take one antibiotic to fight one pathogen, also attacks other bacteria in your gut microbiome (also those benign and even useful). Those bacterial also develop resistance. Unfortunately they can later share their resistance genes with harmful bacteria.

[fluoridation]

Okay, but unless the older single action antibiotics are completely taken out of the market, it doesn't really matter, does it?