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Determination of penicillin MICs for Streptococcus pneumoniae by using a two- or three-disk diffusion procedure
M R Jacobs1, S Bajaksouzian, E L Palavecino-Fasola
1Department of Pathology, Case Western Reserve University, and University Hospitals of Cleveland, Ohio 44106, USA.
Journal of Clinical Microbiology
|February 12, 1998
Summary
The disk diffusion method accurately predicts penicillin minimum inhibitory concentrations (MICs) for Streptococcus pneumoniae using two or three antibiotic disks. This method offers reliable results comparable to standard MIC testing, enhancing clinical decision-making.
Area of Science:
- Clinical Microbiology
- Antimicrobial Susceptibility Testing
- Infectious Diseases
Background:
- Accurate determination of penicillin minimum inhibitory concentrations (MICs) is crucial for treating Streptococcus pneumoniae infections.
- Disk diffusion methods are widely used for antimicrobial susceptibility testing but require validation for specific antibiotic-bacteria combinations.
- Predicting penicillin MICs for S. pneumoniae using disk diffusion can optimize antibiotic selection and treatment outcomes.
Purpose of the Study:
- To evaluate the accuracy of the disk diffusion method in predicting penicillin MICs for Streptococcus pneumoniae.
- To compare the performance of two-disk (methicillin, penicillin) and three-disk (methicillin, oxacillin, penicillin) combination disk procedures against standard MIC methods.
- To assess the reliability of individual disk zone diameters for predicting penicillin susceptibility.
Main Methods:
- Testing 183 S. pneumoniae isolates using agar dilution, microdilution, and E-test to determine reference penicillin MICs.
- Performing regression analyses correlating MIC results with zone diameters from penicillin, methicillin, and oxacillin disks.
- Calculating MICs from regression models and comparing them to reference MICs, assessing essential agreement and error rates.
Main Results:
- High correlation (r = 0.97) between MICs predicted by two- and three-disk diffusion methods and reference MICs.
- Essential agreement rates of 98.9% (two-disk) and 98.4% (three-disk) with reference MICs, with no major errors.
- Individual disk accuracy was lower (r = 0.84-0.93), but methicillin and oxacillin disks effectively screened for penicillin susceptibility.
Conclusions:
- The combination disk diffusion method, using either two or three disks, provides accurate penicillin MIC predictions for S. pneumoniae.
- This method demonstrates comparable accuracy to E-test, agar dilution, and microdilution, offering a reliable alternative for susceptibility testing.
- The disk diffusion method is a valuable tool for guiding penicillin therapy in S. pneumoniae infections, improving clinical decision-making.