Treating mycobacteria with antibiotics.

Mycobacteria are susceptible to some antibiotics. For example, Mycobacterium tuberculosis is usually treated with drugs like rifampin, ethambutol and isoniazid. Mycobacterium leprae is treated with dapsone. Newer antibiotics, known as macrolide antibiotics, are more effective against intracellular mycobacteria than standard anti-tuberculosis drugs. One of these macrolide antibiotics, clarithromycin, has been shown to be effective against Mycobacterium avium complex infection (known as MAC) in AIDS patients.

Multi-drug regimes.

The mutation rates of mycobacteria are very high, meaning that they quickly adapt to their changing environment. Genetic mutation can develop as often as once in every million divisions(1,000,000). Each time a genetic mutation appears, there is a small chance that the mutation will result in immunity to antibiotic drugs that might otherwise have killed the mycobacterium. Tubercles (granulomas in the disease tuberculosis) can contain up to one hundred billion mycobacteria (100,000,000,000), so the chances of the mycobacteria developing resistance to any individual drug is very high.

If a patient with a mycobacterial infection is treated with only one drug, the disease may appear to improve temporarily, as most of the bacteria are destroyed. However, the disease will usually return, when the bacteria with resistance to that one drug have had a chance to multiply. Remember that only a very small number of viable mycobacteria are necessary to establish infection. Use of that one drug in that same patient is then useless, since the infecting bacteria are resistant to it.

For this reason, mycobacterial infections are almost always treated with a combination of two or more drugs. The chances of a genetic mutation resulting in resistance to two or more drugs at the same time are extremely small.

Long treatment times.

Since mycobacteria are extremely hardy, drug treatment times are extremely long. Many species of mycobacteria can become "inactive" for long periods of time, i.e. they do not metabolise. Antibiotics can only be effective against a bacterium if that bacterium is metabolising. To completely eradicate a mycobacterial infection with antibiotics requires that the course be long enough to act against bacteria that are "hibernating" as well as active bacteria.

To eradicate a pulmonary(lung) tuberculosis infection takes between six and nine months of treatment. Tuberculosis outside the lung may take even longer to treat. If the antibiotic drugs are not taken until the infection is eradicated (e.g. the patient stops taking the drugs after two months, because s/he is feeling better), this gives the bacteria a chance to develop resistance to the drugs and increases the probability that the patient will relapse with the disease. In this case, the drugs used can not be used again, since the infecting bacteria will be resistant to them.