English: Diagram depicting one recognized way in which bacteria may be resistant to antibiotics: if the antibiotic functions by blocking the active site of the enzyme, the bacteria may evolve to produce an enzyme that will not allow the antibiotic to bind to its active site. (This diagram is modeled specifically on the way some strains of Staphylococcus aureus have evolved to resist the beta-lactam antibiotic methicillin by expressing the mecA gene.)

1- Both enzymes are structurally similar, but differ in the kind of substances they will allow in their active sites. (The differences between the two may actually be much more subtle than what is implied by the image.)

2- Both enzymes carry out normal functions in the bacterial cell. (In this case, the enzymes are depicted making cross-links in the bacterial cell wall, a function which is crucial to bacterial cell survival and replication.)

3- The beta-lactam antibiotic fits in the active site of the antibiotic-sensitive enzyme, but not in that of the resistant enzyme.

4- Once in the active site, the beta-lactam ring of the antibiotic springs open, permanently inactivating the sensitive enzyme. The resistant enzyme, however, is totally unaffected; it is free to continue carrying out its normal function in the bacteria.