This has to do with the chemical structure of penicilin and some other frugs. The modification that might be resistant will be the ability to produce an enzyme that breaks down the antibiotic. Remember that it happens with Methicillin that is a penicillin drug. Have in mind that Penicillins and cephalosporins share the same chemical structure and they tend to have the ability to produce that enzyme that can paralize the antibiotic
