TABLE 2.

MICs and levels of MarA-mediated MDR in the wild-type and parental strains

Strain	Description or genotype	MIC (μg/ml)a,b
		FOX	NOR	CHL	MIN
BW25113	Wild type	4.0	0.09	10.7	3.3
CR2000	BW25113 ΔmarRA (MarA-free parental strain)	3.2	0.07	8.1	2.4
CR1000	BW25113 ΔmarR (MarA-overproducing parental strain)	10.6	0.21	23.9	4.9

^aThe results are presented as the average MICs (the standard errors of the means were 0 to 15% [n = 3 to 9]) for cefoxitin (FOX), norfloxacin (NOR), chloramphenicol (CHL), and minocycline (MIN). MarA mediated low-level resistance to all four antibiotics, as shown by the significant increase in MICs in the ΔmarR parental strain compared to the levels for the ΔmarRA parental and wild-type strains. MarA also mediated resistance to other antibiotics tested (ampicillin, cephalothin, nalidixic acid, rifampin, erythromycin, tetracycline, and doxycycline) but had no effect on susceptibility to aminoglycosides, trimethoprim-sulfamethoxazole, imipenem, or fosfomycin (data not shown).

^bThe CR1000 MIC/CR2000 MIC ratios were as follows: for FOX, 3.3; for NOR, 3.1; for CHL, 3.0; and for MIN, 2.0. These ratios represent 100% MarA-mediated resistance to each antibiotic (see Table 3), that is, the increase in the MIC resulting from physiological overexpression of marA in the ΔmarR parental strain (CR1000) compared to the MIC for the ΔmarRA parental strain (CR2000).