Abstract
Avian pathogenic Escherichia coli (APEC) is the causal agent of colibacillosis, one of the most common bacterial infections in the poultry sector. Antimicrobial susceptibility testing (AST) is essential for rational and prudent antimicrobial therapy. Subsequently, uniformity in test results from the various testing methodologies used in diagnostic laboratories is pivotal. The aim of this study was therefore to evaluate the agreement between different AST methods in determining fluoroquinolone resistance in APEC. Twenty APEC isolates were selected and subjected to four different susceptibility tests: the quantitative microbroth dilution, agar dilution and gradient strip tests, and the qualitative disk diffusion method. The experiments were performed in triplicate. Categorical agreement, essential agreement and different errors were assessed. Moreover, agreement was also evaluated by calculating intraclass correlation coefficients (ICCs) for the quantitative tests and determining the Pearson correlation coefficients for the agreement between the disk diffusion method and the quantitative tests. Categorical agreement and essential agreement when compared with the microbroth technique ranged from 85–95% and 85–100%, respectively. No very major errors (false susceptible) and only one major error (false resistant) and minor errors (results involving an intermediary category) were detected. The calculated ICC values of the three quantitative tests fluctuated around 0.970 (range 0.940–0.988). There was a high negative correlation between the disk diffusion method and the other tests (correlation coefficients ranging from −0.979 to −0.940), indicating a clear inverse relationship between the minimum inhibitory concentration value and the zone diameter of growth inhibition. In conclusion, the overall agreement between the four different testing methodologies was very high. These results confirm the reliability of the disk diffusion and gradient strip test methods as substantiated alternatives, next to the gold standard agar and microbroth dilution, for fluoroquinolone susceptibility testing of APEC isolates.
Highlights
Colibacillosis is one of the major health threats in the poultry industry worldwide
We evaluated the agreement between the minimum inhibitory concentration (MIC)-gradient strip test and the more established microbroth and agar dilution tests together with the qualitative disk diffusion method for the evaluation of ENRO susceptibility or resistance for a collection of clinical Avian pathogenic Escherichia coli (APEC) isolates
In 2 experiments, the 20 strains were identified as 13 S, 2 I, and 5 R according to the gradient strip test, while in one experiment this was 12 S, 2 I, and 6 R
Summary
Colibacillosis is one of the major health threats in the poultry industry worldwide This disease refers to any localized or systemic infection that is caused by the heterogeneous avian pathogenic Escherichia coli (APEC) pathotype (Nolan et al, 2020). Vaccination strategies, only generating serotypeand strain-specific protection (Kariyawasam et al, 2004; Dziva and Stevens, 2008), are insufficient to control this disease. This illustrates the need for different management measures and appropriate antimicrobial treatment. The fluoroquinolone class of antimicrobial drugs are frequently employed for this indication (Li et al, 2007; Persoons et al, 2012; Joosten et al, 2019)
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