THE MYCOBACTERIA
Mycobacteria are aerobic, acid-fast bacilli. They are neither Gram-positive nor Gram-negative because they are stained poorly by dyes used in Gram stain. They are virtually the only bacteria that are acid-fast. One exception is Nocardia asteroids. (Levinson & Jawetz, 2000)
The term “acid-fast” refers to an organism’s ability to retain the carbol fuchsin stain despite subsequent treatment with an ethanol-hydrochloric acid mixture. The mycobacteria are distinguished by their acid-fast staining property, which results in part from the high content of lipids in their cell walls. The high lipid content which is approximately 60% of their cell wall makes Mycobacteria acid-fast. (Levinson & Jawetz, 2000)
The major pathogens are Mycobacterium tuberculosis, the causative agent of tuberculosis and Mycobacterium leprae that cause leprosy. It has been estimated that tuberculosis was the single greatest cause of human disease and death during the past 200 years. Atypical mycobacteria such as Mycobacterium avium-intracellulare complex and Mycobacterium kansaii can cause tuberculosis-like disease but less frequent pathogens. Rapid growing mycobacteria, such as Mycobacterium chelonei, are saprophytes that occasionally cause human disease in immunocompromised hosts. All of these bacteria are from genus Mycobacterium. (Jensen et al, 1995)
WHAT IS TUBERCULOSIS?
Tuberculosis, also known as TB, is a chronic infection with specific type of bacterium which is Mycobacterium tuberculosis that usually affects lungs. (Maureen O’ Sullivan, 2003)
Tuberculosis is the major disease of humans. The organism’s impact is difficult to determine, but historically it would certainly rank among the most devasting of all human diseases. It has been estimated that tuberculosis was the single greatest cause of human disease and death during the past 200 years. (Jensen et al, 1995)
The species Mycobacterium tuberculosis, commonly referred to as the tubercle bacilli, is the major cause of human tuberculosis. This bacterium is a non-spore-forming bacillus measuring about 0.5 x 3 μm. Its appearance is slender rod, and it tends to grow in clumps.
It can be grown on simple culture media in the routine laboratory isolation procedure. However, best results are obtained with a medium containing egg yolk and starch such as Lowenstein-Jensen medium. On the surface of liquid media, their growth appears moldlike, which suggested the genus name Mycobacterium, from the Greek ‘mykes’, meaning fungus. (Jensen et al, 1995)
Mycobacterium tuberculosis is an obligate aerobe. This explains its predilection for causing disease in highly oxygenated tissue such as upper lobe of the lung. It grows much more slowly than most other bacterial pathogens. The generation time of Mycobacterium tuberculosis is 15 to 20 hours compared with less than 1 hour for most bacterial pathogens. Because of the long generation time, visible growth on solid media may take up to 6 weeks. Slow growth may delay diagnosis by culture methods alone. Even using specialized liquid broth media, detection can take 2 weeks or longer. (Engleberg et al, 2007)
These bacteria are relatively resistant to normal staining procedure. When stained by the Ziehl-Neelsen or Kinyoun technique that stains the cell with carbol fuchsin dye, they cannot be decolourized with a mixture of acid and alcohol. Therefore, it is classified as acid-fast bacilli (AFB)..