Mycobacterium tuberculosis (MBT)

The bacterium Mycobacterium tuberculosis (MBT) is the causative agent for tuberculosis.18 The lungs are the most common site of infection. MBT is transmitted via inhalation of droplets and droplet nuclei generated when infected persons cough, sneeze, shout, sing, or talk; and less frequently by contact with freshly contaminated articles and environmental surfaces.

Clinical Features

Two MBT-related conditions exist: latent TB infection and TB disease. People with latent TB infection have a positive tuberculin skin test reaction or a positive blood test, but they have no symptoms, no radiographic abnormalities compatible with tuberculosis, all bacteriologic studies are negative, and they are not contagious. In some people, MBT may remain inactive for a lifetime; in others, however, the bacteria become active, multiply, and cause TB disease.

TB disease in the lungs may cause symptoms such as a bad cough that lasts 3 weeks or longer, pain in the chest, and coughing up blood or sputum. Other symptoms of TB disease are weakness or fatigue, weight loss, no appetite, chills, fever, and night-sweats. Infected sputum may cause tuberculous tracheitis; laryngitis (hoarseness, coughing, and pain); ulcers on the tonsils (dysphagia) and nasal cavity (obstruction, perforation, nasal discharge); and oral lesions.


Immunization with Bacille Calmette-Guérin (BCG) is not widely used in the United States, but it is a common preventive measure to control tuberculosis worldwide. Administered to newborns in a single dose, it prevents severe disease and reduces mortality among children from miliary (multi-organ) and meningeal disease. However, BCG does not protect against pulmonary tuberculosis in either children or adults.

The risk for progression from latent TB infection to TB disease is highest during the first two years after infection and is often predicated on concomitant medical conditions that alter the ability of the immune system to maintain the isolation of MBT. Isoniazid, given for nine months in a single daily dose, is the drug of choice for the treatment of latent TB infection. Persons exposed to isoniazid resistant MBT may be treated with rifampin for four months.

Medical Management

Until susceptibility results are available, the empirical phase of treatment consists of isoniazid, rifampin, pyrazinamide, and ethambutol. When the infection proves to be caused by fully susceptible strains of MBT, the targeted phase of treatment continues for two months with isoniazid, rifampin, and pyrazinamide. Predicated on the results of sputum cultures at two months, the continuation phase of treatment may last for an additional four to seven months.