Mycobacterium tuberculosis: characteristics and pathophysiology

Mycobacterium tuberculosis is a potentially deadly bacterium that can come into contact with humans and create an unpleasant scenario. Because of the potential danger that this bacterium holds, numerous diagnostic tests exist that will help to accurately identify this particular strand. The first test that is done on an unidentified bacterium is the Gram Stain. This stain allows for the determination of whether the bacterium that is being tested is either gram-positive or gram-negative (Murray and Baron, 544). This is important as almost all of the bacteria in the world fall into these two categories.

The difference between gram-negative and gram-positive bacteria is in the composition of the bacterial wall. Gram-positive bacteria have a thick peptidoglycan layer that is made up of polysaccharides; however, gram-negative bacteria have a thinner peptidoglycan layer, but it has an extra lipid layer surrounding the outside of its cell wall (Brooks et al., 232). This difference is pertinent for deducing the type of bacterium that is being tested. The gram stain also allows for the determination of morphology and size. In the case of M. tuberculosis, it is neither gram-negative nor gram-positive, and are bacilli or rod-shaped (Murray and Baron, 544). Because of an inconclusive gram test, subsequent diagnostic testing is necessary to identify this bacterium.

The acid-fast stain is conducted on bacteria that do not fall into a gram-positive or a gram-negative category. Very few bacteria will actually test positive for the acid-fast test, but those that do are essentially extremely dangerous. When a bacterium does not take up the dyes used in the gram stain, then that is the first indication that further acid-fast testing needs to be done in order to confirm the identity of the particular bacteria (Schaechter et al., 257). An acid-fact test uses stronger acids and detergents to break down the bacterial wall in order to allow for the penetration of the dyes. Mycobacterium tuberculosis tests positive for the acid-fast test. This means that their cell wall is composed of a waxy acid, mostly mycolic acid that prevents that penetration of many substances (Brooks et al., 233). However, the acid stain will be able to successfully stain the bacterium for further identification.

Endospore stains are used to identify bacteria that are able to sustain themselves without nutrients for large amounts of time. These bacteria include lethal microorganisms such as Bacillus anthracis. Endospores are genetically identical bacterial structures that develop asexually when the bacterium is preparing for times of residing in a high stress environment (Murray and Baron, 543). A piece of the bacteria pinches off, allowing for the species to maintain an identical copy in order to prevent the bacterium from being extinguished. However, endospore activity is not found in M. tuberculosis and tests negative for this particular test, leading into further and proper identification (Murray and Baron, 575).

Another diagnostic test that is routinely done in order to attain the possible identification of unknown bacterium is the urease test. Once a Mycobacterium genus has been identified, in order to further categorize it into its appropriate species, a urease test can be conducted. A urease test is used to determine whether a bacterium produces the enzyme to break down urea into carbon dioxide, water, and ammonia (Murray and Baron, 575). This will be visible through a change in pH color once the acidic urea is turned into an alkaline substance with the urease. If there is no growth in the medium, then the test will be negative. In the case of M. tuberculosis, the urease test is negative therefore no production of urease is detected (Murray and Baron, 578)

An oxygen sensitivity diagnostic test can also be performed in order to determine whether Mycobacterium tuberculosis is able to thrive in environments with insufficient or over-sufficient amounts of oxygen. Mycobacterium are obligate aerobes, which means that they require the presence of atmospheric oxygen for successful growth and requires oxygen as the final hydrogen or electron acceptor to finalize the degradation of energy molecules such as glucose in order to complete metabolism (Brooks et al., 232). However, M. tuberculosis can also thrive in limited environments with the presence of carbon dioxide (Schaechter et al., 259). The oxygen sensitivity test can also determine mobility in a species. If the bacterium tested has moved away from the area of inoculation and toward an area where more oxygen is available, then the particular species is able to move, making it mobile. In the case of M. tuberculosis, the bacterium is non-motile (Murray and Baron, 544).

A diagnostic test conducted in order to also determine the specific identity of a particular species of the Mycobacterium genus is the catalase test. This test is used to determine a bacteria's ability to produce catalase, a powerful enzyme that degrades hydrogen peroxide into water and free oxygen (Murray and Baron, 575). This is visible through the massive production of bubbles that result from the release of the free oxygen after degrading the hydrogen peroxide. A positive result for catalase production is the formation of bubbles, while a negative result would have no visible result. Mycobacterium tuberculosis is negative for the catalase test (Murray and Baron, 575).

The nitrate test can also be done in order to further diagnose the particular bacteria. This test is done in order to determine which bacteria are capable of reducing nitrates into nitrites, and at times beyond the nitrite stage (Brooks et al., 132). Aerobic bacteria that are able to reduce nitrates do so because in times when oxygen is not available as the final electron receptor, a microorganism is able to use the reduction of nitrate to nitrites as a replacement for the final oxygen molecule. A partial reduction of nitrate would entail nitrite and water as the final result, while a complete reduction can include the further reduction of nitrite to ammonia or nitrate to molecular nitrogen (Murray and Baron, 577). This would be visible on a diagnostic test by the presence of a dark color or a light color if zinc powder is added. Mycobacterium tuberculosis tests positive for this particular diagnostic test (Murray and Baron, 577).