Antimicrobial susceptibility testing of Streptococcus pneumoniae by micro-broth dilution

Thirty-three clinical isolates of Streptococcus pneumoniae were tested for susceptibility to penicillin and ampicillin by a standard agar dilution method. Results were compared to those obtained using a micro-broth dilution technique in which Mueller-Hinton broth was supplemented with 5% difibrinated whole sheep blood. Among the 33 strains, 2 were resistant (minimal inhibitory concentration, 8 micrograms/ml), 10 were relatively resistant (minimal inhibitory concentration, 0.12 to 0.5 micrograms/ml), and 20 were susceptible (minimal inhibitory concentration, less than or equal to 0.06 micrograms/ml) to penicillin by both methods. Only one stain showed a two-dilutional-step difference by micro-broth and agar dilution testing resulting in categorization as relatively resistant by the former method but susceptible by the latter. A 1-microgram oxacillin disk correctly identified 11 of the 12 resistant strains. The micro-broth dilution technique is a reliable, simple method for penicillin or ampicillin susceptibility testing of pneumococci and economically feasibile to perform manually or with a semiautomated system.