Table 1. Minimal inhibitory concentration of quinolones.

Strain*	MIC (μg/ml)†
	NAL		LVX			OFX			CIP
E. coli MG1656	3	S	0.023	S		0.006	S		0.004	S
E. coli MG1656/pDIJ09-518a	>256	R	0.19	S		0.25	S		0.094	S
E. coli MG1656 + CIP	2	S	0.023	S		0.006	S		0.004	S
E. coli MG1656/pDIJ09-518a + CIP	>256	R	0.25	S	×1.3	0.38	I	x 1.5	0.19	S	×2
E. coli MG1656 + TOB	2	S	0.032	S		0.006	S		0.006	S
E. coli MG1656/pDIJ09-518a + TOB	>256	R	0.38	S	×2	0.38	I	x 1.5	0.25	S	×2.7

^*+CIP and +TOB stand for strains exposed to sub-MIC of ciprofloxacin and tobramycin, respectively, prior to MIC assessment.

^†Susceptibility testing categories according to EUCAST clinical breakpoints. Nalidixic acid: R > 16 μg/ml. Levofloxacin: S ≤ 0.5 μg/m, R > 1 μg/ml. Ofloxacin: S ≤ 0.25 μg/ml, R > 0.5 μg/ml. Ciprofloxacin: S ≤ 0.25 μg/ml, R > 0.5 μg/ml. Fold-change increases of MIC are shown in comparison to the QnrD-producing WT strain.

Similarly, the MICs increased for the qnrD-carrying E. coli exposed to sub-MIC of tobramycin as compared to growth in antibiotic-free medium (Table 1): 2-, 1.5-, and 2.7-fold higher for levofloxacin, ofloxacin and ciprofloxacin, respectively. These results showed that the aminoglycoside-induced SOS response increased quinolone (nalidixic acid and fluoroquinolones) MIC in line with the increased expression of qnrD in E. coli.