TABLE 2.

Frequency of single- and second-step mutations to low-level and high-level rifampin resistance in clinical isolates of S. pneumoniae

Mutation and strainb	MIC (mg/liter)	Mutation frequency (log10) with rifampin at concn (mg/liter) ofa:
		0.5	4	10	50
First-step mutation
104.72-I	0.032	−6.49 ± 0.14	−7.30 ± 0.14	−7.49 ± 0.22	−7.51 ± 0.09
111.46-I	0.016	−7.09 ± 0.20	−7.43 ± 0.52	−7.59 ± 0.42	−7.79 ± 0.37
111.81-I	0.032	−6.52 ± 0.01	−7.30 ± 0.12	−7.65 ± 0.03	−8.12 ± 0.03
204.26-I	0.016	−7.06 ± 0.24	−7.74 ± 0.67	−7.96 ± 0.44	−7.97 ± 0.55
777-I	0.032	−6.33 ± 0.39	−7.43 ± 0.20	−7.24 ± 0.13	−7.58 ± 0.35
Second-step mutation
111.46-II	0.5		−6.15 ± 0.36	−6.50 ± 0.39	−6.63 ± 0.48
204.26-II	0.75		−6.12 ± 0.24	−6.60 ± 0.64	−7.25 ± 0.50

^aThe values represent the means and standard deviations of four independent experiments for each strain except strain 111.81, for which the values represent the means and standard deviations of two experiments. The results of statistical analyses (unpaired Student's t test for all comparisons) were as follows: (i) for first-step mutations, frequencies of mutations for resistance to rifampin at 0.5 mg/liter compared to the frequencies of resistance to higher rifampin concentrations, P < 0.001; (ii) for comparison of mutation frequencies between mutants with first- and second-step mutations, for rifampin at 4 mg/liter, P < 0.001; for rifampin at 10 mg/liter, P < 0.001; for rifampin at 50 mg/liter, P = 0.007.

^bWild-type strains are labeled with the suffix I; mutants with low-level resistance used for second-step experiments are labeled with the suffix II.