Explanation:

Let us take the example of TB.
Almost all of the antibiotics that can be used to treat TB work when the bacteria are actively dividing. TB can be treated effectively by using first line drugs (FLD) isoniazid (INH), rifampin (RIF), pyrazinamide (PZA), ethambutol (EMB) and streptomycin (SM).
However, this first line therapy often fails to cure TB for several reasons.
Relapse and the spread of the disease contribute to the emergence of drug resistant bacteria.
The emergence of multidrug resistant TB (MDR-TB), i.e. which is resistant to at least isoniazid (INH) and rifampicin (RIF), is of great concern, because it requires the use of second-line drugs that are difficult to procure and are much more toxic and expensive than FLDs.
Therefore, the detection and treatment of drug susceptible or single drug resistant TB is an important strategy for preventing the emergence of MDR-TB.