TABLE 1.

Colistin broth-disk elution with and without EDTA result summary for 85 Enterobacteriales

Isolate type	No. of isolates	CBDE colistin MIC (μg/ml) without EDTA (no.)	CBDE with 1 mM EDTA
			No. (%) of isolates with a reduction of colistin MIC	MIC (μg/ml) doubling dilution reduction (no.)
mcr-producing isolatesc	12	2 (2)	2 (100)	≥1
		4 (7)	7 (100)	≥2
		>4 (3)	3 (100)	≥2 (2)
				≥3 (1)
CREd	73	≤1 (55)	NA	NAe
		2 (2)	3 (16.7)	≥1 (1)a
		4 (1)		≥2 (1)b
		>4 (15)		≥3 (1)b

^aOne E. cloacae isolate tested as 2 μg/ml by the CBDE method and as ≤1 μg/ml by the EDTA-CBDE method, resulting in a ≥1 doubling dilution difference.

^bTwo K. pneumoniae isolates tested as >4 μg/ml and 4 μg/ml by the CBDE method and as ≤1 μg/ml by the EDTA-CBDE method, resulting in ≥2 and ≥3 doubling dilution differences, respectively.

^cThe 12 MCR-producing isolates included 7 Escherichia coli isolates (6 mcr-1 and 1 mcr-2), 2 Salmonella enterica serovar Typhimurium isolates (mcr-3 and mcr-4), 1 Salmonella enterica serovar Enteritidis isolate (mcr-1), and 1 Salmonella enterica serovar Oslo isolate (mcr-3) (see Table S1 in the supplemental material).

^dThe clinical carbapenem-resistant Enterobacteriales (CRE) included 39 Klebsiella pneumoniae isolates, 16 Enterobacter cloacae complex isolates, 8 E. coli isolates, 4 Citrobacter freundii isolates, 3 Serratia marcescens isolates, 1 Klebsiella (formerly Enterobacter) aerogenes isolate, 1 Klebsiella oxytoca isolate, and 1 Proteus mirabilis isolate. The CRE were previously characterized for carbapenemase production and included 35 carbapenemase-producing CRE (29 KPC, 3 NDM, 2 KPC and NDM, and 1 NDM and OXA-48) (see Table S1).

^eNA, not applicable.