The genus Mycobacterium contains many members, few of which are pathogens. But when pathogenic, chronic diseases such as tuberculosis and Hansen’s Disease may result.
All members of the genus Mycobacterium have special features that make these species resistant to control. Mycobacteria have a unique bacterial cell wall that contains large quantities of a lipid called mycolic acid. This lipid results in a waxy bacterial cell wall that is directly responsible for the control-resistant features of pathogens in this genus.
The presence of mycolic acid also makes it difficult to reliably stain Mycobacteria with water-based stains, such as the Gram stain, used to help identify groups of bacteria. A special stain, called the acid-fast stain, must be used to identify Mycobacteria. This staining protocol employs heat to permanently drive a pink dye into the waxy cell wall.
The Chronic Nature of Mycobacterial Diseases
Mycobacterial diseases are chronic, developing slowly due to relatively long generation time of these bacteria. Generation time is how long it takes for a bacterial population to double, as individual bacteria divide by binary fission. Whereas it only takes Escherichia coli, a nonacid-fast bacterium, a matter of minutes to double its population, Mycobacterial generation time varies from hours to days.
Pathogenicity of Mycobacteria
The waxy cell wall also protects this type of bacteria from osmotic effects and many antibiotics. Mycobacteria are one of the few bacteria types that are capable of intracellular growth (multiplying within an animal cell); and when engulfed by phagocytic cells of the immune system, Mycobacteria are resistant to degradation by digestive enzymes of these phagocytes.
Tuberculosis Caused by Mycobacteria
Caused mainly by the bacterium Mycobacterium tuberculosis, TB (also known as Tubercle bacillus) is an infectious disease of the respiratory system that can ultimately disseminate from the lungs and result in a body-wide, systemic infection. The waxy cell wall of M. tuberculosis enables the bacteria to remain viable, in dried aerosol droplets, for up to 8 months. Only a small percentage of people infected with this bacterium will develop the disease, but it only takes one bacterium to infect, and left untreated, active TB results in a mortality rate of approximately 50%.
Those in the medical field, and others with a high risk of contracting TB, are routinely tested with the Mantoux test or the tuberculin skin test (TST). The TST involves injection of a very small amount of a purified protein derivative or tuberculin into the inner part of the lower arm. Testing positive does not, however, indicate a definite infection with TB. False positive TB test results are common and require follow up diagnostics.
Leprosy Caused by Mycobacteria
Also known as Hansen’s disease, this dreaded infection is caused by M. leprae. This unusual bacterium grows best in a climate lower than body temperature, a preference evidenced by the peripheral locations on the human body (fingers, toes, lips, earlobes) where the bacteria is most prone to thrive.
The disease manifests in one two forms. Those who only develop the nonprogessive form, called tuberculoid leprosy, have a strong immune response to the bacteria that is able to kill the body cells infected with M. leprae. Individuals with a weak cell-mediated immune response develop the more well-known form of the disease, lepromatous leprosy, which disfigures by slowly destroying infected tissues.
Those infected, who have access to medical care, typically aren’t diagnosed until they display desensitized lesions or disfigurement. At that point, a skin test, similar to that for tuberculosis, is done.
Treatment of Tuberculosis and Leprosy
Pathogenic Mycobacteria are inherently resistant to eradication, due to the protection derived from the mycolic acid. This genera also is quick to develop resistance to antimicrobial medication. Multi-drug therapy is required to eliminate, or sometimes only control, infection.