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Comparison of E-test with agar dilution for determining susceptibility of Streptococcus pneumoniae to penicillin
Abstract:
Minimum inhibitory concentrations (MICs) of penicillin for Streptococcus pneumoniae were determined by the E-test and the agar dilution method. Ninety Streptococcus pneumoniae strains were tested, of which 16 were resistant, 33 intermediate, and 41 susceptible by agar dilution. By the E-test, 80 (88.9%) strains agreed with these determinations within one log2 dilution step, and no strains disagreed by more than two dilution steps. Sixty-eight of the 70 strains with discrepant MICs read lower in the E-test, resulting in 15 strains being placed in different susceptibility categories when classified by this test. Exact MICs rather than classification groups should be used to determine appropriate antibiotic therapy, since small differences in MICs determined by different methods can lead to a significant degree of misclassification.
Insights
Comparing penicillin minimum inhibitory concentrations (MICs) for Streptococcus pneumoniae, the E-test and agar dilution methods showed agreement. However, discrepancies led to misclassification of susceptibility, highlighting the need for exact MICs in antibiotic therapy decisions.
Area of Science:
- Microbiology
- Clinical Pharmacology
- Infectious Diseases
Background:
- Penicillin is a critical antibiotic for treating Streptococcus pneumoniae infections.
- Accurate determination of penicillin susceptibility in Streptococcus pneumoniae is essential for effective treatment.
- Discrepancies between antimicrobial susceptibility testing methods can impact clinical decisions.
Purpose of the Study:
- To compare the minimum inhibitory concentrations (MICs) of penicillin for Streptococcus pneumoniae using the E-test and agar dilution methods.
- To evaluate the agreement and discrepancies between the two methods in classifying bacterial susceptibility.
- To assess the clinical implications of method-dependent variations in MIC determination.
Main Methods:
- Ninety clinical isolates of Streptococcus pneumoniae were tested.
- Minimum inhibitory concentrations (MICs) were determined using both the E-test and the agar dilution method.
- Susceptibility categories (resistant, intermediate, susceptible) were assigned based on established breakpoints.
Main Results:
- Agar dilution classified 16 strains as resistant, 33 as intermediate, and 41 as susceptible.
- E-test results agreed within one log2 dilution step for 88.9% of strains.
- Sixty-eight of 70 discrepant strains showed lower MICs with the E-test, leading to 15 strains being reclassified into different susceptibility categories.
Conclusions:
- While the E-test and agar dilution methods show good overall agreement for penicillin MICs in Streptococcus pneumoniae, significant discrepancies can occur.
- The tendency for the E-test to yield lower MICs can result in misclassification of susceptibility.
- Exact MIC values, rather than categorical interpretations, are crucial for guiding appropriate antibiotic therapy to avoid treatment failure.