From the Guidelines
Colistin resistance in gram-negative bacilli can be detected through several established methods, with broth microdilution (BMD) being the gold standard, as it provides the most accurate susceptibility information and directly influences clinicians’ choice of antimicrobial therapy 1.
Detection Methods
The following methods can be used to detect colistin resistance:
- Broth microdilution (BMD): determines the minimum inhibitory concentration (MIC) of colistin against bacterial isolates
- Commercial systems like Vitek 2, Phoenix, and MicroScan: can detect colistin resistance but may have limitations in accuracy
- Molecular methods: include PCR and whole genome sequencing to detect resistance genes like mcr-1 through mcr-10, which confer plasmid-mediated colistin resistance
- Rapid tests: such as the Rapid Polymyxin NP test detect bacterial growth in the presence of colistin through pH indicators, providing results within 2-4 hours
- Agar-based methods: include colistin agar spot test, colistin broth disk elution, and gradient diffusion tests (E-test), though these are less reliable than BMD
- MALDI-TOF MS: can also be used to detect colistin resistance by analyzing bacterial protein profiles
Considerations
When implementing these tests, laboratories should consider that BMD remains the reference method, and confirmatory testing is recommended for isolates showing resistance by other methods 1. Accurate detection is crucial as colistin is often a last-resort antibiotic for multidrug-resistant gram-negative infections. The detection of colistin heteroresistant A. baumannii in clinical isolates provides a strong warning that, if colistin is used inappropriately, there may be substantial potential for the rapid development of resistance and therapeutic failure 1.
From the Research
Detection Methods of Colistin Resistant Gram-Negative Bacilli
- The broth microdilution (BMD) method is recommended for the detection of colistin resistance, but it is labor-intensive, time-consuming, and difficult to apply in routine laboratories 2.
- Various methods have been developed for the detection of colistin resistance, including disk elution, commercial microdilution, and rapid polymyxin-NP tests 2.
- A study compared the performance characteristics of different methods for testing susceptibility to colistin, including broth microdilution, broth macrodilution, agar dilution, and disc-diffusion, and found that broth macrodilution was very reliable for colistin MIC determination 3.
- Another study compared seven methods of colistin susceptibility testing, including disk diffusion, agar dilution, Etest, broth microdilution, and VITEK 2, and found that the colistin Etest, agar dilution, and the VITEK 2 showed a high level of agreement with the broth microdilution reference method 4.
- A commercial broth microdilution panel (Sensititre) was evaluated for colistin susceptibility testing of clinical isolates of Escherichia coli and Klebsiella pneumoniae, and showed high categorical agreement but low essential agreement with the reference BMD method 5.
- Three commercialized BMD panels (Sensititre, UMIC, and MicroScan) were evaluated to determine colistin susceptibility, and the Sensititre system was found to be the most reliable marketed BMD panel with a categorical agreement of 97.8% 6.
Comparison of Detection Methods
- The following methods have been compared for the detection of colistin resistance:
- Broth microdilution (BMD) 2, 3, 4, 5, 6
- Broth macrodilution (BMAD) 2, 3
- Agar dilution (AD) 3, 4
- Disc-diffusion (DD) 3, 4
- Etest 4
- VITEK 2 4
- Commercial microdilution 2
- Rapid polymyxin-NP tests (RPNP) 2
- Sensititre 5, 6
- UMIC 6
- MicroScan 6
Advantages and Disadvantages of Detection Methods
- Advantages of BMD: recommended method, high accuracy 2, 3, 4, 5, 6
- Disadvantages of BMD: labor-intensive, time-consuming, difficult to apply in routine laboratories 2
- Advantages of disk elution: efficient, easy, low-cost, good performance 2
- Disadvantages of disk elution: not recommended as a standalone method 2
- Advantages of commercial microdilution: easy to use, high categorical agreement 5, 6
- Disadvantages of commercial microdilution: low essential agreement, high very major error rate 5, 6