Review Question - QID 106717

This patient has pulmonary tuberculosis, which can be diagnosed with acid-fast staining to help detect mycobacteria. Acid-fast staining works by applying an aniline dye then decolorizing with acid alcohol.

Mycobacterium tuberculosis (TB) infection is extremely common worldwide. Severe cases present with chronic cough, often productive of bloody sputum, and weight-loss. The mycobacterium cell wall contains high concentrations of mycolic acids (long branched-chain saturated fatty acids), and, as a result, mycobacterium do not appear well on Gram stain. Acid-fast staining techniques (such as the Ziehl-Neelsen stain) have been developed to aid identification of the organism.

Potter et al. discuss the diagnosis of TB. In suspected cases, a sample of fluid should be obtained for acid-fast staining and culture. In suspected pulmonary TB, three induced sputum samples should be obtained on three separate occasions. The acid-fast test requires a relatively high concentration of organisms for positive detection. Culture and subsequent acid-fast staining increases the sensitivity of diagnosis, but this comes at the expense of a longer time for diagnosis.

Ellis et al. describe a method for acid-fast staining. Mycolic acid (in the mycobacterial cell wall) is a complex lipid with long carbon chain lengths. The acid-fast stain relies on the presence of these long carbon chains. The staining process uses a lipophilic agent to allow passage of the aniline dye past the mycolic acid barrier. Following removal of the lipophilic agent, the mycolic acid barrier reconstitutes and acts to prevent subsequent decolorization with acid alcohol. As a result, the aniline dye is retained beneath the mycolic acid layer, allowing microscopic identification.

Figure A is a chest radiograph showing severe bilateral pulmonary TB. Illustration A depicts a microscopic image of an acid-fast stain of TB.

Incorrect Answers:
Answer 1: Crystal-violet is used in Gram staining and helps to identify Gram positive organisms. Gram-positive bacteria have a thick peptidoglycan layer in their cell walls, which holds the crystal-violet stain.
Answer 2: Safranin is also used in Gram staining and helps to identify Gram-negative organisms.
Answer 3: India ink stains the polysaccharide capsule of some fungi.
Answer 5: Periodic acid-Schiff stains glycogen and mucopolysaccharides, used to diagnose Whipple's disease.