Widespread resistance to particular antibiotics is also the product of an advanced industrial society where new variants of pathogens spread easily amongst bill

Widespread resistance to particular antibiotics is also the product of an advanced industrial society where new variants of pathogens spread easily amongst billions of internationally-travelling city dwellers and widespread prophylactic use of the antibiotic creates strong selection pressures. Its a bit different after an apocalypse. In theory, an isolated post-apocalyptic community could roll a 1 and get bacteria that is resistant to locally available natural and stockpiled antibiotics in their community, but that's quite low down their list of concerns.