Cultures were grown overnight at 37° C in LB media containing 2% w/v glucose, 50 μg/ml spectinomycin, and 1 mM IPTG. The overnight cultures were then diluted to an OD of 1 × 10−5 before 50 μl of diluted culture was added to a plate and spread with an L-shaped spreader. Plates were made with 20 ml of LB-agar containing 2% w/v glucose, 50 μg/ml spectinomycin, and 1 mM IPTG. Differing levels of antibiotic were added to each plate depending on the MIC for the wild-type protein. For CAT-I, plates were made with 0, 16, 32, 64, 128, 256, 512, and 1024 μg/ml chloramphenicol. For NDM-1, plates were made with 0, 32, 64, 128, 256, 512, 1024, 2048, and 4096 μg/ml ampicillin. For AadB, plates were made with 0, 8, 16, 32, 64, 128, 256, and 512 μg/ml kanamycin. Plates were incubated for 16 h at 37° C before colonies were counted. We identified the MIC as the antibiotic concentration at which there were fewer than 5% of the colony forming units (CFU) relative to a plate without antibiotics.