Why are multiple antibiotics used for the treatment of TB disease?

Why are multiple antibiotics used for the treatment of TB disease?

Compared with a patient with a positive test but no disease, two features of M. tuberculosis make the organism difficult to eradicate after infection has been established. First, mycobacteria replicate slowly and may remain dormant for prolonged periods, but they are susceptible to drugs only during active replication. Second, drug-resistant organisms exist naturally within a large population, even before the initiation of therapy. These features render the organism—when it is present in significant numbers—extremely difficult to eradicate with a single agent.