Examination of the minimum inhibitory concentration of amoxicillin and marbofloxacin against Streptococcus suis using standardised methods

The results of the antimicrobial susceptibility testing of clinical isolates Streptococcus suis to amoxicillin and marbofloxacin obtained by the agar dilution method and broth microdilution method with the results obtained by the commercially available E-test were compared. Comparisons between the methods based on the determination of the minimal inhibitory concentration (MIC) of the antimicrobials were assessed based on the degree and frequency of the categorical agreement ( Agar dilution method as a reference system) and the percentage of the categorical agreement and error rate. A statistical evaluation was determined using the Bland-Atman method. The presented MIC values, determined for the isolates in the E-test, were slightly different from the MIC values determined by the dilution tests, mainly due to the different defined testing concentrations. For the E-test as the test system and agar-dilution method as the reference system, no error of any class was detected (very major, major and minor error) and a complete categorical agreement was obtained between the evaluated methods for amoxicillin. For amoxicillin, 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 regression lines were not significantly different. However, the E-test tends to slightly overestimate the MIC values when compared to the microdilution. The reverse is true when compared with the agar dilution. There was good agreement between the E-test and the dilution methods with a 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 a less favourable agreement. The E-test constantly underestimated the MIC values when compared to the two dilution methods. No significant difference between the microdilution and agar dilution was obtained.