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The results of Ziehl–Neelsen staining is variable because many fungal cell walls are not acid fast. [11] An example of a common type of acid-fast fungus that is usually stained with Ziehl–Neelsen staining is called Histoplasma (HP). [12] Histoplasma is found in soil and the feces of birds and bats. [13]
[1] [2] Once stained as part of a sample, these organisms can resist the acid and/or ethanol-based decolorization procedures common in many staining protocols, hence the name acid-fast. [ 2 ] The mechanisms of acid-fastness vary by species although the most well-known example is in the genus Mycobacterium , which includes the species ...
This coating makes the cells impervious to Gram staining, and as a result, M. tuberculosis can appear weakly Gram-positive. [3] Acid-fast stains such as Ziehl–Neelsen, or fluorescent stains such as auramine are used instead to identify M. tuberculosis with a microscope.
Other mycobacteria are also acid-fast. If the smear is positive, PCR or gene probe tests can distinguish M. tuberculosis from other mycobacteria. Even if sputum smear is negative, tuberculosis must be considered and is only excluded after negative cultures. [citation needed]
Mycobacterium tuberculosis group bacteria are 1.0-4.0 μm long by 0.2-0.3 μm wide in tissues. In culture, they may appear as cocci, or as bacilli up to 6-8 μm long. [citation needed] The bacteria stain Gram-positive, acid-fast. The cell wall contains as high as 60% lipid, giving the mycobacteria their hydrophobic characteristics, slow growth ...
Since MTB retains certain stains even after being treated with acidic solution, it is classified as an acid-fast bacillus. [14] [58] The most common acid-fast staining techniques are the Ziehl–Neelsen stain [60] and the Kinyoun stain, which dye acid-fast bacilli a bright red that stands out against a blue background. [61]
Some bacteria have cell walls which are particularly adept at retaining stains. These will appear positive by Gram stain even though they are not closely related to other gram-positive bacteria. These are called acid-fast bacteria, and can only be differentiated from other gram-positive bacteria by special staining procedures. [25]
Acid-fast organisms display a reddish-yellow fluorescence. [2] Although the auramine–rhodamine stain is not as specific for acid-fast organisms (e.g. Mycobacterium tuberculosis or Nocardia ) as the Ziehl–Neelsen stain , it is more affordable and more sensitive, therefore it is often utilized as a screening tool.