Table 2. Spontaneous reversion of SJ and Janus cassettes in two backgrounds.

Strain	Growth (CFU/ml) with selection on				Revertant frequency
	Km	Sm	Su	Sm+Su	Smr	Sur	(Sm+Su)r
TIGR4J	1.8×109	3.6×103	1.6×109	5.6×103	2.0×10−6	NA	3.1×10−6
SpnYL001	6.3×108	6.3×103	9.9×103	1#	1.0×10−5	1.6×10−5	1.6×10−9
SpnYL003	1.76×108	6.36×103	1.30×108	5.76×103	3.61×10−5	NA	3.27×10−5
SpnYL004	3.67×108	7.88×103	9.09×103	1#	2.14×10−5	2.48×10−5	2.72×10−9

^#Titer below the limit of detection was denoted as one-half the detection limit

NA: Revertant frequency not calculated because strains showed no Su sensitivity.