Well, aside from the generic defense mechanism of pumping unwanted chemicals out of the bacterial cell, or an enzyme for degrading the antibiotic. Multiple mut

Well, aside from the generic defense mechanism of pumping unwanted chemicals out of the bacterial cell, or an enzyme for degrading the antibiotic. Multiple mutations could also occur if the cells are being exposed to sublethal amounts of the antibiotic, say in sewers (which would also be an environment for evolution of enzymes to degrade the chemical.)