Antagonism between netilmicin and β-lactam antibiotics for S. aureus KU5801 in the double-disk diffusion test. M, methicillin; Z, cefazolin; T, cefotaxime; I, imipenem; A, aztreonam; N, netilmicin. Each of the susceptibility test disks contains 50 μg for the β-lactam antibiotics and 125 μg for netilmicin.