Comparative Colistin Susceptibility Testing Methods for
Escherichia coli and Klebsiella pneumoniae
Darat Ruangkriengsin MD1, Naratchaphan Pati BSc1, Naruemon Maknakhon MSc1, Rachanis Tan-arsuwongkul BSc2,
Chakornpat Jumderm BSc2, Surapee Tiengrim MSc2, Visanu Thamlikitkul MD1
Affiliation :
1 Division of Infectious Diseases and Tropical Medicine, Department of Medicine, Faculty of Medicine Siriraj Hospital, Mahidol University,
Bangkok, Thailand
2 Department of Clinical Microbiology and Applied Technology, Faculty of Medical Technology, Mahidol University, Bangkok, Thailand
Objective : To determine the accuracy and utility of colistin susceptibility testing from the agar dilution (AD), agar gradient diffusion
(AG), and disk diffusion (DD) methods compared to broth microdilution (BMD), which is considered the reference method for
colistin susceptibility testing of E. coli and K. pneumoniae clinical isolates.
Materials and Methods : E. coli and K. pneumoniae isolates were evaluated for colistin susceptibility by the AD, AG, DD, and BMD
methods, and the AD, AG, and DD results were compared with the minimum inhibitory concentration (MIC) of colistin from BMD.
The reference value breakpoints for colistin susceptibility from BMD were 1 or less and 2 mg/L or less.
Results : Three hundred twenty-six non-duplicate clinical isolates of E. coli and K. pneumoniae were included. Of those, 16 and 214
were carbapenem-resistant E. coli and K. pneumoniae, respectively. Colistin MIC by AD was in 100% agreement with BMD. Overall
agreement of AG and DD with BMD was moderate, but some values of AG and DD were found to be useful.
Conclusion : Colistin MIC measured by AG (2 or less and 4 or more mg/L), and inhibition zone diameters of colistin disk by DD (14
or more and 11 or less mm) are useful for determining colistin susceptibility in carbapenem-resistant E. coli and K. pneumoniae in
settings where colistin MIC measured by BMD is unavailable.
Keywords : Colistin, Antibiotic susceptibility test, Broth microdilution, Agar dilution, Agar gradient diffusion, Disk diffusion, E. coli,
K. pneumoniae
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