because bacteria divid rapidly, many mutations occur.
most mutations confer no advantage or are even fatal, but occasionally a mutation gives an advantage to a line of bacteria that other bacteria's dont have.. such as a resistance to an antibiotic.
how do antibiotic tx's strengthen resistant mutants:
the antibiotic kills the competetion
the surviving bacteria are thus resistant to the antibiotic (aquired resistance)
resistance can be transmitted to other bacteria.