False susceptible penicillin G MICs for Streptococcus pneumoniae with a commercial microdilution system

A commercial minimal inhibitory concentration (MIC) panel that contains a broth of undisclosed formulation intended to support growth of streptococci unable to grow in Mueller-Hinton broth was found to give false susceptible MIC results for pneumococci. The authors encountered several Streptococcus pneumoniae isolates that showed resistance by the oxacillin disk diffusion test, but showed a susceptible penicillin G MIC when tested in an unsupplemented MIC panel. Seven isolates that showed penicillin G resistance by oxacillin disk diffusion testing were retested for penicillin G susceptibility with oxacillin disk diffusion, commercial MIC panels, Mueller-Hinton with blood agar dilution, and Mueller-Hinton with blood broth microdilution. Five of these isolates produced a button of growth in the commercial MIC panel growth control well, but no growth in the penicillin G-containing wells (0.06-4.0 micrograms/mL), suggesting a valid test system and susceptibility to penicillin G. When tested by the three standard methods, these seven isolates showed resistance (R) or relative resistance (I) to penicillin G. These data indicate that frozen MIC panels from this commercial source (American Micro Scan, Campbell, CA) are not reliable for detection of resistance to penicillin G in S. pneumoniae, and if these panels are used, results must be confirmed by another method.