Mycobacteria are aerobic, acid-fast bacilli (rods) (Figure 21–1). They are neither gram-positive nor gram-negative (i.e., they are stained poorly by the dyes used in Gram stain). They are virtually the only bacteria that are acid-fast. (One exception is Nocardia asteroides, the major cause of nocardiosis, which is also acid-fast.) The term acid-fast refers to an organism’s ability to retain the carbolfuchsin stain despite subsequent treatment with an ethanol–hydrochloric acid mixture. The high lipid content (approximately 60%) of their cell wall makes mycobacteria acid-fast.
Mycobacterium tuberculosis—acid-fast stain. Long red rods of M. tuberculosis are seen on a blue background. (Source: Dr. George Kubica, Public Health Image Library, Centers for Disease Control and Prevention.)
The major pathogens are Mycobacterium tuberculosis, the cause of tuberculosis, and Mycobacterium leprae, the cause of leprosy. Atypical mycobacteria, such as Mycobacterium avium-intracellulare complex and Mycobacterium kansasii, can cause tuberculosis-like disease but are less frequent pathogens. Rapidly growing mycobacteria, such as Mycobacterium chelonae, occasionally cause human disease in immunocompromised patients or those in whom prosthetic devices have been implanted (Table 21–1). The clinical features of three important mycobacteria are described in Table 21–2.
Additional information regarding the clinical aspects of infections caused by the organisms in this chapter is provided in Part IX entitled Infectious Diseases beginning on page 593.
TABLE 21–1Medically Important Mycobacteria |Favorite Table|Download (.pdf) TABLE 21–1 Medically Important Mycobacteria
|Species ||Growth on Bacteriologic Media ||Preferred Temperature In Vivo (°C) ||Source or Mode of Transmission |
|M. tuberculosis ||Slow (weeks) ||37 ||Respiratory droplets |
|M. bovis ||Slow (weeks) ||37 ||Milk from infected animals |
|M. leprae ||None ||32 ||Prolonged close contact |
|Atypical mycobacteria1 ||Slow (weeks) ||37 ||Soil and water |
|M. kansasii || || || |
|M. marinum ||Slow (weeks) ||32 ||Water |
|M. avium–intracellulare complex ||Slow (weeks) ||37 ||Soil and water |
|M. fortuitum–chelonae complex ||Rapid (days) ||37 ||Soil and water |
TABLE 21–2Clinical Features of Important Mycobacteria |Favorite Table|Download (.pdf) TABLE 21–2 Clinical Features of Important Mycobacteria
|Organism ||Main Site of Infection ||Skin Test in Common Use ||Multiple-Drug Therapy Used ||Vaccine Available |
|M. tuberculosis ||Lungs ||Yes ||Yes ||Yes |
|M. avium–intracellulare ||Lungs ||No ||Yes ||No |
|M. leprae ||Skin, nerves ||No ||Yes ||No |
This organism causes tuberculosis. Worldwide, M. tuberculosis causes more deaths than any other single microbial agent. Approximately one-third of the world’s population is infected with this organism. Each year, it is estimated that 1.7 million people die of tuberculosis and that 9 million new cases occur. An estimated 500,000 people are infected with a multidrug-resistant strain of M. tuberculosis.