Antimicrobial susceptibility testing of pneumococci

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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.