Colistin Susceptibility Testing Using the MicroScan® Colistin Well

Abstract

BackgroundAntimicrobial resistance is a threat to public health. As carbapenem-resistant Enterobacteriaceae, multidrug-resistant Pseudomonas aeruginosa, and multidrug-resistant Acinetobacter baumannii have increased in prevalence, interest has increased in using colistin as a therapeutic option. However, testing for colistin susceptibility is problematic for most clinical microbiology laboratories. Also, there is a paucity of surveillance data on the prevalence of colistin resistance in the United States. MicroScan® Gram-negative panels include a colistin well (4 μg/mL) to aid in the identification of bacteria, but it is not known whether this well can be used to assess the prevalence of colistin resistance.MethodsAll Escherichia coli, Klebsiella pneumoniae, K. oxytoca, Enterobacter cloacae, E. aerogenes, P. aeruginosa, or A. baumannii identified at the Emory University clinical microbiology laboratory between January 1, 2016 and December 31, 2016 were included in the study. Routine bacterial identification and antimicrobial susceptibility testing were performed using the MicroScan WalkAway 96 plus® and the Neg Breakpoint Combo Panel Type 41 or 44 (transition from 41 to 44 was made in March 2016). When these isolates were susceptible to three or fewer charted drugs, or upon provider request, a colistin ETEST® was performed.ResultsThere were 288 out of 9296 isolates (3.1%) that had growth in the colistin MicroScan® well, suggesting colistin resistance. This included 79 E. coli (1.5%), 90 Klebsiella spp. (4.2%), 91 Enterobacter spp. (15.1%), 24 P. aeruginosa (1.9%), and 4 A. baumannii (3.7%). ETEST® was performed on 40 of the 288 isolates (13.9%). The MicroScan® colistin well result was confirmed (defined as an ETEST® MIC>2) in 27 out of 40 cases (67.5%).ConclusionResistance to colistin, at this single academic medical center, may be higher than was previously appreciated. This is a concerning finding. Further investigation is needed to determine whether the MicroScan® colistin well can be used as a reliable surrogate for detecting colistin resistance because in this study there was a low rate of categorical agreement between the MicroScan® colistin well and ETEST®. A future study is planned comparing these two testing methodologies with reference broth microdilution.Disclosures All authors: No reported disclosures.