Examination of minimum inhibitory concentration of amoxicillin and marbofloxacin against Streptococcus suis using standardised methods
D Sperling, H Karembe, M Zouharova, K Nedbalcova
The results of antimicrobial susceptibility testing of clinical isolates Streptococcus suis to amoxicillin and marbofloxacin obtained by agar dilution method and broth microdilution method with results obtained by commercially available E-test were compared. Comparisons between methods based on determination of minimal inhibitory concentration MIC of antimicrobials were assessed based on the degree and frequency of the categorical agreement (Agar dilution method as reference system) and the percentage of categorical agreement and error rate. Statistical evaluation was determined using the Bland-Atman method. Presented MIC values, determined for isolates in E-test, were slightly different from MIC values determined by dilutions tests, mainly due to the different defined testing concentrations. For E-test as test system and agar-dilution method as reference system no error of any class was detected (very major, major and minor error) and complete categorical agreement was obtained between the evaluated methods for amoxicillin. For amoxicilin, the regression and correlation analysis show linear relationships between the E-test and the two dilution methods with significant coefficients of determination (0.62 and 0.75). The slopes of the equality and regresion lines were not significantly different. However, the E-test tends to slightly overestimate the MIC values when compared to microdilution. The reverse is true when compared with agar dilution. There was a good agreement between E-test and the dilution methods with low bias (0.0013 and -0.0050), all the experimental data were within the computed limits of agreement. For marbofloxacin, the same trends were observed with lower coefficients of determination (0.42 and 0.73) and less favourable agreement. E-test constantly underestimated the MIC values when compared to the two dilution methods. No significant difference between microdilution and agar dilution was obtained.
antimicrobial; resistance; agar dilution; microdilution; E-test