The mechanisms of antibiotic resistance can be broadly classified into four categories. First, there is enzymatic modification of the antibiotic includes beta-lactamases, which cleave the beta-lactam ring in penicillins and cephalosporins, and other (e.g. AmpC) enzymes, which hydrolyze most beta-lactams except.
The second mechanism of antibiotic resistance is alteration of the target site on the bacterial cell wall. This involves the production of altered penicillin-binding proteins (PBP), which decrease the affinity of the antibiotic for its target.
Third, bacteria may use efflux pumps, which prevent a variety of antibiotics from accumulating in the bacteria cell, by pumping them out of the bacterial cell. Finally, since some antibiotics (e.g. sulfonamides) work by inhibiting the synthesis of molecules that are vital to the bacteria survival like folic acid, bacteria may just stop producing these molecules, and scavenge for folic acid from the environment instead.