Mycobacteria: Members of this genus have capacity to retain dye such as carbol fuchsin stain even after decolorization with acid alcohol so the bacteria of this genus are said to be acid fast bacilli. Classification of mycobacteria: can be classified as in the following: I-Mycobacterium tuberculosis complex: 1-Mycobacterium tuberculosis. 2-Mycobacterium bovis. 3-Mycobacterium africanum.
II-Mycobacterium other than tuberculosis:(MOTT) or tuberculoid bacilli (atypical mycobacteria): 1-M.avium-intracellular complex: 2-M.kansasi. 3-M.scrofulaceum. 4- M.marinium, M.ulcerans. III-Rapid grower Mycobacteria: (M.fortuitum-chelonae complex). IV-M.leprae: they are strict pathogens cause leprosy and cannot be cultured in vitro. V-Saprophytic Mycobacterium.
The bacilli appear red in sharp contrast to the blue counter stained background.
if smear is prepared from culture the bacilli appear short, thick and tend to arranged in a sort of bundles (cord formation).Cultivation of Mycobacteria: Mycobacteria need special media for growth & there are three types of culture media for cultivation: 1-Agar base media (oleic acid albumin media) e.g. Dubos medium and Middle brook 7H10, these media contain defined salts, vitamins, co-factor, oleic acid, albumin, glucose, glycerol, these media used for the observation of the colonies morphology ,& used for susceptibility test.
2-Egg-base media: Lowenstein Jensens medium(L-J medium)& Stonebrink s medium, contain defined salt, glycerol & complex organic material in form of fresh egg, egg yolk, potato flour & other ingredient in various combinations.Malachite green is included in the media to inhibit other bacteria .This media used for primary isolation of Mycobacteria because they need small inoculum size. 3-Liquid media:These media enriched with either ascitic fluid or bovine albumin fraction V.e.g. Sula medium.
Growth characteristics:Mycobacteria are obligate aerobes. CO2 increase the growth.Incubation at 37 C for up to 8weeks. According to the growth rate on culture media mycobacteria can be classified into:Slow grower: Need more than 7 days to grow & these slow grower can be differentiated according to the type of the growth on solid media e.g. Egg base media:(L-J media):M.tuberculosis that need 2-3 weeks to grow & growth describe as tough, rough, easily detach from the surface of the culture media creamy colonies, (Eugonic growth) .
M.bovis need 5-6 weeks to grow & the growth describe as dysgonic growth with easily emulsify adhere to the surface & have smooth surface. The other slow grower MOTT can be differentiated according to the pigment production in relation to the light exposure , here we have three groups: Photochromogen: forming yellow pigments after exposure to light, e.g. (M.kansasi) Scotochromogen: a yellow pigment will be formed in the dark, which change to red after exposure to light. e.g.(M.scrofulaceum). Non-chromogen: non pigmented slow grower mycobacteria under all conditions of incubation. e.g. (M.avium-intracellular) complex
Rapid grower: need less than 7 days to grow & the colonies appear with in few days. e.g. M.fortuitum Saprophytic Mycobacterium: these mycobacterium are found free living & not associate with human disease (M.pheli,M.gordonae,& M.smegmatus),first two species present in plants , water, soil, while the third one present in the sebaceous secretion of the human beings.