page_title: Mycobacterium tuberculosis
section_title: Strain variation
hierarchical_section_title: Mycobacterium tuberculosis / Pathophysiology / Strain variation
caption_attribution_description: Русский: "Косы" (проявление корд-фактора) на люминисцентной микроскопии препарата культуры штамма H37Rv Micobacterium tuberculosis, выращенной на жидкой питательной среде Миддлбрука 7Н9 (окр. аур.-родам., ув. 80х) English: Chording mycobacterium tuberculosis (H37Rv strain) culture on the luminescent microscopy (magn.80x, fluorochr. auramine-rodamine). Culture grown on Middlebrook 7H9.
context_page_description: Mycobacterium tuberculosis is a species of pathogenic bacteria in the family Mycobacteriaceae and the causative agent of tuberculosis. First discovered in 1882 by Robert Koch, M. tuberculosis has an unusual, waxy coating on its cell surface primarily due to the presence of mycolic acid. This coating makes the cells impervious to Gram staining, and as a result, M. tuberculosis can appear either Gram-negative or Gram-positive. Acid-fast stains such as Ziehl-Neelsen, or fluorescent stains such as auramine are used instead to identify M. tuberculosis with a microscope. The physiology of M. tuberculosis is highly aerobic and requires high levels of oxygen. Primarily a pathogen of the mammalian respiratory system, it infects the lungs. The most frequently used diagnostic methods for tuberculosis are the tuberculin skin test, acid-fast stain, culture, and polymerase chain reaction.
  The M. tuberculosis genome was sequenced in 1998.
context_section_description: Typing of strains is useful in the investigation of tuberculosis outbreaks, because it gives the investigator evidence for or against transmission from person to person. Consider the situation where person A has tuberculosis and believes he acquired it from person B. If the bacteria isolated from each person belong to different types, then transmission from B to A is definitively disproved; however, if the bacteria are the same strain, then this supports (but does not definitively prove) the hypothesis that B infected A.
  Until the early 2000s, M. tuberculosis strains were typed by pulsed field gel electrophoresis (PFGE). This has now been superseded by variable numbers of tandem repeats (VNTR), which is technically easier to perform and allows better discrimination between strains. This method makes use of the presence of repeated DNA sequences within the M. tuberculosis genome.
  Three generations of VNTR typing for M. tuberculosis are noted. The first scheme, called exact tandem repeat, used only five loci, but the resolution afforded by these five loci was not as good as PFGE. The second scheme, called mycobacterial interspersed repetitive unit, had discrimination as good as PFGE. The third generation (mycobacterial interspersed repetitive unit – 2) added a further nine loci to bring the total to 24. This provides a degree of resolution greater than PFGE and is currently the standard for typing M. tuberculosis. However, with regard to archaeological remains, additional evidence may be required because of possible contamination from related soil bacteria.
  Antibiotic resistance in M. tuberculosis typically occurs due to either the accumulation of mutations in the genes targeted by the antibiotic or a change in titration of the drug. M. tuberculosis is considered to be multidrug-resistant (MDR TB) if it has developed drug resistance to both rifampicin and isoniazid, which are the most important antibiotics used in treatment. Additionally, extensively drug-resistant M. tuberculosis (XDR TB) is characterized by resistance to both isoniazid and rifampin, plus any fluoroquinolone and at least one of three injectable second-line drugs (i.e., amikacin, kanamycin, or capreomycin).
caption: Cording M. tuberculosis (H37Rv strain) culture on the luminescent microscopy
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