TABLE 2.

Frequency of Rif^r mutants in E. coli tester strains

Mtz concn (μg/ml)	Viabilitya	Mutation frequency (no. of Rifr colonies/108 CFU)b					Mean ± SD	Fold increase
		CC101	CC103	CC104	CC105	CC106
0	1	0.47	0.23	0.75	0.94	0.64	0.61 ± 0.27
10	10−2	83	88	12	96	52	66 ± 34	∼100
15	10−4	178	383	118	238	137	210 ± 106	∼344

^aDetermined by EOP as described in the text. 

^bMean of three determinations. All strains contained pGS950 (rdxA⁺), and the data were normalized against data from strains containing pBSK and treated in parallel with Mtz. The bacteria were challenged in LB broth for 12 h in the presence of Mtz as indicated.