Table 1.

MICs for the P. aeruginosa isolates used in this study

Isolate	MIC (mg/liter)a		Cephalosporinase and carbapenemase typing	MDRb
	Colistin	Imipenem
ATCC 27853c	1	2	Negative	No
19147 n/m	128	4	IMP and CTX-M positived	Yes
19056 muc	0.5	4	Negative	Yes
20509 n/mc	0.5	1	Negative	No
19271 n/mc	2	32	Negative	Yes
20891 n/mc	1	16	Negative	Yes

^aCLSI breakpoints for colistin were ≤2 mg/liter for susceptibility, 4 mg/liter for intermediacy, and ≥8 mg/liter for resistance. For imipenem, the breakpoints were ≤4 mg/liter for susceptibility, 8 mg/liter for intermediacy, and ≥16 mg/liter for resistance (10).

^bDefined as diminished susceptibility to ≥2 of the following 5 drug classes: antipseudomonal cephalosporins, antipseudomonal carbapenems, β-lactam-β-lactamase inhibitor combinations, antipseudomonal fluoroquinolones, and aminoglycosides (40).

^cColistin heteroresistant. Heteroresistance to colistin was defined as the existence, in an isolate for which the colistin MIC was ≤2 mg/liter, of subpopulations able to grow in the presence of >2 mg/liter colistin (55).

^dContains genes encoding an IMP-type carbapenemase and a CTX-M-type ESBL.