FIG 1.

Colistin broth disk elution method. CBDE is performed with four 10-ml cation-adjusted Mueller-Hinton broth tubes per isolate, to which 0, 1, 2, and 4 colistin disks (10 µg) are added, generating final concentrations of 0 (growth control [GC]), 1, 2, and 4 µg/ml, respectively. (A) Tubes for a non-carbapenemase-producing carbapenem-resistant Klebsiella pneumoniae isolate with a colistin MIC of ≤1 µg/ml. (B) Tubes for an mcr-1-producing Escherichia coli isolate (CDC AR Bank accession number 493) with a colistin MIC of 4 µg/ml.