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False susceptible penicillin G MICs for Streptococcus pneumoniae with a commercial microdilution system
Abstract:
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.
Insights
Commercial MIC panels may inaccurately show Streptococcus pneumoniae as susceptible to penicillin G. This can lead to misdiagnosis, as standard methods reveal actual penicillin resistance in pneumococci.
Area of Science:
- Clinical Microbiology
- Antimicrobial Resistance
Background:
- Mueller-Hinton broth is standard for antimicrobial susceptibility testing.
- Some streptococci require specialized media for growth.
- Commercial MIC panels may use undisclosed broth formulations.
Purpose of the Study:
- To evaluate the reliability of a commercial MIC panel for penicillin G susceptibility testing in Streptococcus pneumoniae.
- To identify potential discrepancies between commercial MIC panels and standard methods for detecting penicillin resistance.
Main Methods:
- Oxacillin disk diffusion testing was used to identify penicillin-resistant Streptococcus pneumoniae isolates.
- Susceptibility testing was performed using commercial MIC panels, Mueller-Hinton agar dilution, and Mueller-Hinton broth microdilution.
- Seven confirmed penicillin-resistant isolates were retested using multiple methods.
Main Results:
- Commercial MIC panels yielded false-susceptible results for penicillin G in several Streptococcus pneumoniae isolates.
- Five isolates showed growth in the commercial panel's control well but no growth in penicillin G wells.
- Standard methods (agar dilution, broth microdilution) consistently identified resistance (R) or intermediate (I) susceptibility to penicillin G.
Conclusions:
- Frozen commercial MIC panels from the tested source are unreliable for detecting penicillin G resistance in Streptococcus pneumoniae.
- Results from these specific commercial MIC panels should be confirmed using alternative, validated methods.
- Accurate antimicrobial susceptibility testing is crucial for effective treatment of pneumococcal infections.