Evaluation of colistin susceptibility by four phenotypic methods compared to broth microdilution in multidrug-resistant Klebsiella pneumoniae
Abstract Background Colistin, one of the last-line treatment options for carbapenem-resistant microorganisms, presents challenges in determining its antimicrobial susceptibility. This study aims to compare colistin broth disk elution (CBDE), modified CBDE (mCBDE), the colistin agar test (CAT), and r...
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| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
BMC
2025-07-01
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| Series: | BMC Microbiology |
| Subjects: | |
| Online Access: | https://doi.org/10.1186/s12866-025-04121-1 |
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| Summary: | Abstract Background Colistin, one of the last-line treatment options for carbapenem-resistant microorganisms, presents challenges in determining its antimicrobial susceptibility. This study aims to compare colistin broth disk elution (CBDE), modified CBDE (mCBDE), the colistin agar test (CAT), and rapid polymyxin NP (RPNP) tests with the reference broth microdilution (BMD) in determining colistin susceptibility of 116 multidrug-resistant (MDR) Klebsiella pneumoniae isolates. Isolates were collected from blood and respiratory tract samples of different patients between 2022 and 2023 in Türkiye. For each isolate, a single 0.5 McFarland inoculum was prepared to inoculate CBDE, mCBDE, and CAT in parallel, then the suspension was adjusted to a density of 3-3.5 for RPNP. All tests were conducted simultaneously under the same incubation conditions. In the comparative analysis of the tests, bias, essential agreement (EA), and sensitivity were assessed according to ISO 20776-2:2021. Additionally, EA, categorical agreement (CA), major error (ME), and very major error (VME) were evaluated by the CLSI guidelines. Results The minimum inhibitory concentration (MIC) values for MIC50 and MIC90 were 2 µg/ml and 16 µg/ml, respectively. All methods achieved the CA above 90%, with no VMEs (0%). The ME rates for CBDE, mCBDE, CAT, and RPNP were 3.4%, 8.6%, 12%, and 10.3%, respectively. The EA rates of CBDE and CAT were 95.6% and 91.3%, respectively, while bias rates were 10.8% and 24.7%. The sensitivity of both mCBDE and RPNP was 100%. Consistent with other studies in this field, a comprehensive evaluation of all methods revealed that 58.3% (7/12) of the isolates responsible for MEs had an MIC value of 1 µg/ml, 25% (3/12) had an MIC of 2 µg/ml, and 16.6% (2/12) had an MIC of 0.25 µg/ml. Most errors occurred in isolates with MIC values near the breakpoint. The CBDE demonstrated the best performance in all criteria assessed. Conclusion This study highlights the benefits of alternative phenotypic methods compared to reference BMD for assessing colistin susceptibility, including ease of use, cost-effectiveness, and rapid results. Our findings emphasize the need for more comprehensive multicenter studies to validate and improve these methods, especially those with more isolates with MIC values near the breakpoint. |
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| ISSN: | 1471-2180 |