Physical characteristics

Mycobacteria are most usually found as Bacilli, or rod-shaped bacteria. Their cell walls are surrounded by thick waxy coat, which protects them in harsh environments. Sometimes, they shed their cell wall. In this case they are known as the Spheroplast form of the bacteria.

Mycobacteria are extremely hardy. They are highly resistant to germicides, because of their thick protective cell wall. They are highly resistant to drying, and can exist outside the body without moisture for long periods.

Testing for mycobacteria

The standard stain test for bacteria, the Gram Stain, does not work for mycobacteria, because their waxy coat will not take the dyes involved in the stain. A different staining method must be used, namely the Ziehl-Neelsen Stain, a method which removes the waxy coat with detergents before staining them. This staining method works for the bacillus form of the bacteria, but does not work the spheroplast form. There is no known stain test for the spheroplast form of mycobacteria.

To test for spheroplast form of these bacteria requires genetic testing, using the Polymerase Chain Reaction. Please refer to the page "How are bacteria identified?" for information on genetic testing of bacteria.

Culturing Mycobacteria

The optimum temperature at which to grow most mycobacteria is 37 C, the temperature of the human body. They have complex nutritional requirements, and researchers have for years sought the optimal nutrient mix in which to grow these bacteria. The mix used usually contains mycobactin. Mycobacteria grow extremely slowly, taking up to 24 hours for each generation. This means that to culture, in a dish, a sample large enough to be detected takes an amount of time in the order of months, i.e. up to two months for the tuberculosis bacterium. It is not always possible to culture mycobacteria in the laboratory. Mycobacterium leprae, an obligate pathogen which causes leprosy, has never been cultured in the laboratory. The only place where it is known to grow outside of the human body is in the footpads of Armadillos, a South American quadruped. The inability to culture Mycobacterium leprae in the lab has greatly hindered research into disease caused by this bacterium.

Pathogenesis of Mycobacteria

The number of mycobacteria required to establish an infection is extremely low. Mycobacterium tuberculosis, an obligate pathogen, can establish infection with as few as one to ten bacteria. Compare this to the one hundred thousand bacteria required to establish a Salmonella infection. This indicates that the immune system can be very unsuccessful at combatting these organisms.

When pathogenic mycobacteria enter the body, they are very successful at evading the bodies defenses. Their waxy coat helps to protect them against the high acidity of the stomach. The surfactant (soap-like) qualities of bile salts are not powerful enough to wash away their waxy coat. Also, they are not susceptible to the anti-biotic chemicals produced by the normal flora.

Once at their preferred site of infection, pathogenic mycobacteria tend to allow themselves to be phagocytosed(swallowed) by Macrophages(white blood cells), within which they are able to resist destruction, and are able to multiply. The host immune reaction to the organism varies widely between different individuals. Some individuals are able to control the infection, whereas others go on to develop a chronic infection. This latter group further breaks down into two groups, depending on the type of immune response they mount. For more information on the different types of immune reaction, see the page "Human immune reactions to mycobacteria"