TABLE 4.

Minimum time required for development of INH resistance for a subculture (H37Rv) exposed to INH in 12B medium^a

Exposure time (day)	0.1 μg/ml INH			0.4 μg/ml INH
	CFU/ml in 12B medium	AST susceptibility result after subculture in INH at:		CFU/ml in 12B medium	AST susceptibility result of subculture in INH at:
		0.1 μg/ml	0.4 μg/ml		0.1 μg/ml	0.4 μg/ml
0	30,000	S	S	30,000	S	S
1	6,275	S	S	2,050	S	S
2	1,130	S	S	520	S	S
3	125	S	S	45	S	S
4	55	S	S	20	S	S
5	55	R	R	10	S	S
6	30	R	R	10	R	R
7	475	R	R	445	R	R

^aTo determine the minimum time required for emergence of INH resistance, daily subcultures of H37Rv from INH-exposed cultures at 0.1 and 0.4 μg/ml were tested for susceptibility to INH at 0.1 and 0.4 μg/ml and CFU.