Antimicrobial susceptibility testing of pneumococci

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 are 7 and 20 mm for 0.15 μg penicillin G and 1 μg oxacillin discs and 16 and 25 mm for 5 μg methicillin discs. Routine testing of penicillin G susceptibility on all pneumococcal isolates is now essential to monitor changes in penicillin G susceptibility.