Antimicrobial susceptibility testing of pneumococci: 2. Determination of optimal disc diffusion test for detection of penicillin G resistance

Abstract

Detection of decreased penicillin susceptibility of pneumococci has become increasingly important in view of the emergence of significant resistance. This study investigated the value of various β-lactam discs in the detection of penicillin sensitive, intermediate resistant and resistant strains (minimum inhibitory concentrations of penicillin G of 1 mg/I respectively). Discs containing 0.018 ug of penicillin G were found to distinguish sensitive from intermediate and resistant strains. Discs containing 0.15 μg of penicillin G, 1 μg of oxacillin and 5 μg of methicillin were the most useful in separating the 3 groups. Cutoff zones between sensitive, intermediate resistant to resistant strains..