Interventions for Tuberculosis Control and Elimination
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Monotherapy with either streptomycin or para-amino salicylic acid led to rapid emergence of resistance in a large fraction of patients. Combining the two drugs, overcame this problem.
If mutations conferring resistance to one drug occur at a frequency of 1 in 1 million then the joint probability of a mutant simultaneously resistant to both drugs is 1 in 10^12. As the number of bacilli in a cavitary lesion may be 10^8 to 10^9, it is an unlikely event that such a mutant is occurring.
