Optochin Revisited: Defining the Optimal Type of Blood Agar for Presumptive Identification of Streptococcus pneumoniae

To determine the optimal media for optochin susceptibility testing of Streptococcus pneumoniae, we measured inhibition zones for 72 S. pneumoniae and 22 Streptococcus viridans isolates on three blood-containing media. Because 15.3, 0, and 22.2% of S. pneumoniae organisms were misidentified on Columbia agar, Trypticase soy agar (TSA), and Mueller-Hinton agar, respectively, each containing sheep blood, we recommend that TSA-sheep blood agar be used.