Tuberculosis is caused by Mycobacterium tuberculosis (M.TB.), has a unique cell wall, making it difficult to kill and stain. Three matjor components compose the cell wall: mycolic acids, cord factor, and Wax-D. M.TB. is also hard to culture and can only thrive in the body. It is an obligate aerobe and an acid-fast bacteria. Tuberculosis
can be diagnosed through the Mantoux test and by staining methods such as the Ziel-Neelsen method. Chest x-rays are also used in the diagnosis process to detect any damage done to the lungs.
Tuberculosis is caused by M.TB. This bacterium
is hard to kill because of its cell wall, which is 60% lipid. M.TB. is also difficult to stain and culture (Todar, Kenneth paragraoh 14). Skin tests and microbiologic smears and cultures are the main methods used to diagnose TB, though the full diagnosis consists of many more tests.
M.TB. is a rod-shaped bacterium related to the Actinomycetes. The rods are 2 to 4 um long and 0.2 to 0.5 um wide. This bacterium is an obligate aerobe, an organism that needs oxygen to survive. It is a tough bacterium that can withstand weak disinfectants and can survive in a dry state for weeks. M. TB. is a slow-growing bacterium that divides every 16 to 20 hours, which is extremely slow compared to other bacterium, which have division times measured in minutes (Wikipedia paragraph 1). Although M.TB. contains peptidoglycan in their cell wall, it is neither Gram-positive nor Gram-negative because it lacks the chemical characteristics of either. The Mycobacterium species is classified as acid-fast bacteria because it is virtually impermeable to certain dyes and stains, but once stained
, the dye is permanent. The Ziehl-Neelsen method is one method used to stain Mycobacterium tuberculosis. When this stain is used, the bacterium is stained with carbol-fuchsin and the acid-fast bacilli appears pink, clearly standing out
against a blue background. The carbol-fuchsin can only be removed with acid-alcohol. Acid-fast bacilli can also be seen by using fluorescent microscopy or with auramine-rhosamine stain. M.TB.’s resistance to stains is caused by its cell wall (Wikipedia paragraph 6-7).
The cell wall structure of M.TB. stands out in comparison to other prokaryotes. The cell wall contains peptidoglycan and over 60% of it is lipid. The lipid portion of the cell wall consists of three major components: mycolic acids, cord factor, and Wax-D. “Mycolic acids are alpha-branched lipids found in cell walls of Mycobacterium and Corynebacterium” (Todar, Kenneth paragraph 15). They make up half of the mycobacterial cell envelope. Mycolic acids are hydrophobic molecules that affect the permeability properties of the cell surface, which is why Mycobacterium tuberculosis is difficult to stain. Mycolic acids also prevent attacks by cationic proteins, lysozyme and oxygen radicals in the phagocytic granule. Mycolic acids also protect extracellular mycobacteria from complement deposition in serum. Cord Factor is responsible for serpentine and is abundantly produced in virulent strains of Mycobacterium tuberculosis. It is toxic to mammalian cells and inhibits PMN migration. The high concentration of lipids in the cell wall of this bacterium causes resistance to stains, dyes, many antibiotics, acidic and alkaline compounds, osmotic lysis, through complement deposition, and to lethal oxidations and survival inside macrophages (Todar, Kenneth paragraph 15-18).
The complete diagnosis for tuberculosis consists of the patient’s medical history, a physical examination, a TB skin test, a serological test, a chest x-ray, microbiologic smears and sputum cultures (Wikipedia paragraph 27). However, the skin tests, smears, and cultures are the most common methods. The acid-fast bacilli in sputum must be
detected using the Ziel-Neelsen method. Sputum is coughed up from the lungs and may include mucus and blood (McKesson Corporation paragraph 9). A sample of sputum may be examined through a microscope to see if it contains TB bacteria. The TB-causing organisms must be cultured from sputum. First, the sputum is treated with sodium hydroxide (NaOH) to kill all other bacteria. Culturing Mycobacterium tuberculosis may take 4 to 6 weeks to yield visible colonies. As a result, the BACTEC system is more commonly used, which contains radio-labeled palmitate as the sole carbon source. “As M.TB multiplies, it breaks down the palmitate and liberates radio-labeled CO2” (Todar, Kenneth paragraph 44). The growth of M.TB can be detected in 9-16 days using the BACTEC system ( Todar, Kenneth paragraph 42-44).
The Mantoux test, a skin test, can also be used to diagnose TB. In this method, PPD (purified protein derivative) is used as an antigen. PPD is generated by boiling a culture of M.TB. 5 TU (tuberculin units), which is equal to 0.0001 mg, of PPD in a 0.1 ml volume is injected in the patient’s forearm. The test is read within 48 to72 hours. The test is positive if the diameter of the resulting lesion is greater than or equal to 10 mm and the lesion will be red and show signs of swelling. False positive tests usually cause weaker reactions, which indicate prior exposure or infection with other Mycobacteria or vaccination with BCG. Where the vaccine is not used, lesser reactions are suspicious. False negative tests are rarer than false positves, but are common in AIDS patients because of their impaired CMI response. (Todar, Kenneth paragraph 45-48).
While the skin test and Mantoux test are used to detect TB, the chest x-ray is used to detect any damage that TB may have caused in the lungs. “If you have inhaled TB bacteria but have fought off the infection, your lungs may not be damaged and your chest
x-ray may be normal. However, if you have an active infection and bacteria have attacked tour lungs, the damage will show on a chest x-ray.” (McKessen Corporal paragraph 8)
Mycobacterium tuberculosis is hard to kill and stain because of its cell wall, which contains peptidoglycan and lipids. The lipid layer consists of three parts: mycolic acids, cord factor, and Wax-D. M.TB. is classified as an acid-fast bacilli because of its impermeability to stains. The Mantoux skin test, Ziel-Neelsen method, BACTEC system, and the chest x-rays make up the most common methods used to diagnose tuberculosis. Although M.TB. is difficult study under a microscope, the understanding of the cell wall structure helps scientists to invent treatments for tuberculosis.
“Tuberculosis.” Todar, Kenneth. Todar’s Online Textbook of Bacteriology.
2006 University of Wisconsin-Madison, Department of Bacteriology.
2007 23 July 2006
“Tuberculosis.” Wikipedia. 27 July 2006 Wikipedia Foundation, Inc. 22 July 2006
“Tuberculosis.” University of Michigan Health System. 14 March 2005 University of Michigan Health System. 22 July 2006