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Disk prediffusion is a reliable method for testing colistin susceptibility in porcine E. coli strains
F Boyen1, F Vangroenweghe, P Butaye
1Department of Pathology, Bacteriology and Avian Diseases, Faculty of Veterinary Medicine, Ghent University, Salisburylaan 133, 9820 Merelbeke, Belgium. filip.boyen@UGent.be
Abstract:
During the last few years, acquired resistance to colistin in Escherichia coli, but also in other bacterial species, has been reported. It has been shown that the disk diffusion test is not a reliable method for the detection of this resistance. Therefore, there is a need for a reliable and cheap test to determine colistin susceptibility of pathogenic E. coli strains. In the current research, the colistin susceptibility of E. coli isolated during the period 2005-2006 from pigs was determined. Results obtained with the Kirby Bauer disk diffusion test (Neosensitabs, Rosco), the disk prediffusion test (Neosensitabs, Rosco) and the E-test (AB Biodisk) were compared with the results of the reference agar dilution assay. The MIC values or inhibition zones showed a bimodal distribution for the results obtained by all test methods, except the disk diffusion assay, suggesting acquired resistance in 15 strains (9.6%). The E-test and disk prediffusion assay generated results within acceptable levels compared to the reference agar dilution assay. The categorical agreement with the results obtained by the agar dilution method were good to very good for all tests, except the disk diffusion assay. In conclusion, current results suggest that, in addition to the E-test, the disk prediffusion test is a reliable, alternative agar-based colistin susceptibility method for testing colistin susceptibility of E. coli isolates in diagnostic bacteriology.
Insights
Acquired resistance to colistin in Escherichia coli is a growing concern. The disk prediffusion test, alongside the E-test, offers a reliable method for detecting colistin resistance in E. coli, unlike the disk diffusion test.
Area of Science:
- Microbiology
- Antimicrobial Resistance
- Veterinary Medicine
Background:
- Acquired resistance to colistin in Escherichia coli (E. coli) has been increasingly reported.
- The disk diffusion test is unreliable for detecting colistin resistance in E. coli.
- A need exists for a dependable and cost-effective method to assess colistin susceptibility in pathogenic E. coli.
Purpose of the Study:
- To evaluate the reliability of different laboratory methods for determining colistin susceptibility in E. coli.
- To compare the E-test and disk prediffusion test against the reference agar dilution assay.
- To identify a suitable alternative to the disk diffusion test for colistin resistance detection.
Main Methods:
- Colistin susceptibility of E. coli strains isolated from pigs (2005-2006) was determined.
- Kirby Bauer disk diffusion, disk prediffusion, and E-test methods were compared to the agar dilution assay.
- Minimum Inhibitory Concentration (MIC) values and inhibition zones were analyzed.
Main Results:
- A bimodal distribution in MIC values/inhibition zones, indicative of acquired resistance, was observed in 15 strains (9.6%) across most tested methods.
- The E-test and disk prediffusion assay demonstrated acceptable agreement with the reference agar dilution method.
- The disk diffusion assay showed poor categorical agreement compared to the reference method.
Conclusions:
- The E-test and disk prediffusion test are reliable agar-based methods for assessing colistin susceptibility in E. coli.
- These methods are suitable alternatives for diagnostic bacteriology, especially given the limitations of the disk diffusion test.
- Reliable detection of colistin resistance is crucial for managing E. coli infections.
