The accuracy of four commercial broth microdilution tests in the determination of the minimum inhibitory concentration o
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Annals of Clinical Microbiology and Antimicrobials Open Access
RESEARCH
The accuracy of four commercial broth microdilution tests in the determination of the minimum inhibitory concentration of colistin Erlangga Yusuf* , Mireille van Westreenen, Wil Goessens and Peter Croughs
Abstract Colistin is considered as one of the last-resort antibiotics and reliable antimicrobial susceptibility testing is therefore crucial. The reference standard for AST according to EUCAST and CLSI is broth microdilution (BMD). However, BMD is labor intensive to perform. Commercial antimicrobial susceptibility tests derived from BMD method are available. We investigated the performance of four different commercial tests: Sensititre™, SensiTest™ Colistin, Micronaut MIC Strip Colistin and UMIC Colistin using 70 clinical isolates (half of them was deemed by VITEK2 as resistant), including isolates from cystic fibrosis patients and mcr-1 bearing isolates. We used two reference standards: BMD and composite MIC as determined by all four tests. Sensititre™ had essential agreement (EA, defined as minimum inhibitory concentration within ± 1 dilution) of 87% and 89% compared to BMD and composite reference standard, respectively. For SensiTest™, the EA’s were 93% and 90%. For UMIC, 87% and 90%, and for Micronaut, 83% and 84%. All four tests demonstrated categorical agreement (CA) above 90%. CA for SensiTest™ and Micronaut was both 96%, UMIC 94%, and Sensititre™ 93%. All tests were reproducible as tested in two quality control isolates. In conclusion, in clinical isolates from a large referral center, the four commercial tests for determination of colistin minimum inhibitory concentrations showed acceptable performance. Keywords: Colistin, Broth microdilution, Minimum inhibitory concentration, mcr-1, Cystic fibrosis Introduction Colistin is a polypeptide antibiotic that targets the outer membrane of Gram-negative bacteria [1]. It is considered as a last-resort antibiotic due to the increasing number of extensively drug-resistant bacteria and limited availability of new antimicrobial agents [2–5]. It is used in particular for treatment of infections caused by extensively drug-resistant (XDR) Klebsiella pneumoniae, Pseudomonas aeruginosa and Acinetobacter baumanii [5]. The proportion of microorganisms resistant to colistin *Correspondence: [email protected] Department of Medical Microbiology and Infectious Diseases, Erasmus MC University Medical Center, Doctor Molewaterplein 40, 3015 GD Rotterdam, The Netherlands
is still relatively low at this moment but there is concern that it may increase in the future [5]. In a multicenter study in the US, the proportion of carbapenem resistant K. pneumoniae that were also colistin-resistant was 17% [6]. Reliable antimicrobial susceptibility testing (AST) for colistin is thus important, in order to prevent the patients receiving this nephrotoxic agent when it is unlikely to be effective. Routine AST’s commonly used by microbiology laboratories include automated systems such as VITEK2. Yet, this system is n
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