TABLE 2.

Reversion frequency of S. pneumoniae mutants containing duplications in the clpX region of the chromosome

Strain	Selection during growtha	CFU/mlb	No. of colonies screened	No. (%) of Ermr and Erms isolates following spontaneous resolution of duplications
				Ermr	Erms
EL873	None	4.7 × 106	31	4 (13)	27 (87)
	ERY	4.9 × 105	31	31 (100)	0 (0)
EL1392	None	2.0 × 107	130	129 (99.2)	1 (0.8)
	ERY	7.9 × 106	61	61 (100)	0 (0)

^aBacteria were grown statically in BHI containing or lacking 0.3 μg of erythromycin (ERY) per ml for ≈7 h at 37°C in 5% CO₂, serially diluted in 0.9% (wt/vol) NaCl, and spread onto TSA-BA plates containing or lacking 0.3 μg of erythromycin per ml, respectively. Random colonies were screened for erythromycin resistance (Erm^r) or sensitivity (Erm^s) by being patched onto TSA-BA plates containing or lacking 0.3 μg of erythromycin per ml.

^bNumber of CFU per milliliter of culture from which cells were recovered at the time of plating.