October 22, 2013
Tuberculosis bacteria – Mycobacterium tuberculosis (BK, Koch’s bacillus) is the bacteria responsible for tuberculosis .
Belonging to the genre of mycobacteria ( Mycobacterium spp.) as well as the bacillus of leprosy ( Mycobacterium leprae or Hansen’s bacillus ) or the so-called atypical mycobacteria, it was discovered by the German Robert Koch in 1882 and its genome is sequenced in 1998 .
M. tuberculosis bacillus is aerobic strict motionless, straight or slightly curved, 2 to 5 microns of 0.3 to 0.5 microns .
Like other mycobacteria, M. tuberculosis takes quite ordinary dyes and Gram : it is generally colored with Ziehl Neelsen . For ease of reading, and to save time in large series, it is possible to use the auramine reading with a fluorescence microscope, allowing the use of a dry objective and therefore the exploration of a larger field. The coloring is often fragmented or granular bacilli appear pink on blue background Ziehl-Neelsen. In pathological products, they take the form of small clusters in fences or irregular arrangement. A positive direct examination does not necessarily mean the presence of Mr. tuberculosis in a pathological product, indeed all mycobacteria are colorable Ziehl-Neelsen. Review the report therefore mentions the presence of “acid-fast bacilli-resistant” or AFB. In young cultures of virulent strains, M. tuberculosis adopt a provision in braids or strings (“Agreement”).
Its wall is formed from the inside to the outside of a lipid bilayer (plasma membrane), a layer formed of polymers and mycolic acids and a second lipid bilayer 1 .
Mycobacterium tuberculosis bacillus is a very slow growing (2-6 weeks) and requiring special media. The solid medium most used is that of Lowenstein-Jensen or one of its many variants (Coletsos. ..). These are solid media containing eggs, added in varying proportions of asparagine , of glycerine , of malachite green … Culture is also possible in a liquid medium (Middlebrook, Mycobacteria Growth Indicator Tube (MGIT) for example) and automated system.
Colonies appear after 2-4 weeks and are ivory-white, rough and adherent in the middle. They grow slowly, reaching 3-4 mm after 2-3 months. They then have a cauliflower-like appearance.