Bacteria isolated from the lung tissue of a 32-year-old Caucasian male fail to decolorize with hydrochloric acid and alcohol after staining carbolfuchsin. Which of the following cell wall components is most likely responsible for this staining phenomenon?

Question

Bacteria isolated from the lung tissue of a 32-year-old Caucasian male fail to decolorize with hydrochloric acid and alcohol after staining carbolfuchsin.  Which of the following cell wall components is most likely responsible for this staining phenomenon?

Quizplus · Accepted Answer

The procedure described above is the acid-fast stain, which is used in the detection of a select few pathogenic organisms (eg, Mycobacterium and some Nocardia species). Although it is less sensitive than culture, the acid-fast stained smear allows for immediate microscopic evaluation. In the acid-fast stain for mycobacteria, the smear is first treated with an aniline dye (eg, carbolfuchsin). The dye (red color) penetrates the bacterial cell wall, where it binds with mycolic acids. The slide is then treated with hydrochloric acid and alcohol. This acid alcohol dissolves the outer cell membranes of nontuberculous bacteria, but the presence of mycolic acids prevents decolorization of mycobacteria. A counterstain (eg, methylene blue) is then applied and taken up by decolorized bacteria. As a result, the carbolfuchsin acid-fast stain produces red mycobacteria (initial stain) and blue non-acid fast bacteria. The cell membrane and cell wall of mycobacteria are most similar to those in Gram-positive organisms, causing mycobacteria to appear weakly positive on Gram stain. However, the mycobacterial cell wall differs from that of the typical Gram-positive organism in that they are encapsulated with mycolic acid, a waxy, long-chain fatty acid that is covalently bound to the sugars within the cell wall. Another organism that will also stain positive with the acid-fast technique is Nocardia. Nocardia is a Gram-positive rod that contains mycolic acid in its cell wall. Because Nocardia possesses less mycolic acid than do mycobacteria, Nocardia is more weakly acid fast. (Choice A) N-acetylmuramic acid and N-acetylglucosamine are the saccharides that combine with an amino acid chain to form the peptidoglycan layer in both Gram-positive and Gram-negative cell walls. (Choice B) Teichoic acid is a molecule linked to the peptidoglycan cell wall of Gram-positive bacteria (but not Gram-negative bacteria). Teichoic acid serves as an antigenic determinant for organism identification in the laboratory and an antigenic target for the human immune system. (Choice C) Lipopolysaccharide (LPS) is a component of the outer cell envelope of Gram-negative bacteria. (Choice E) Unique to fungi, ergosterol is the sterol component of fungal cell membranes. This molecule is not found in human cell membranes, as human cells have cholesterol in their cell membranes instead. Educational Objective: The acid-fast stain identifies organisms that have mycolic acid present in their cell walls, including Mycobacterium and some Nocardia species. Acid-fast staining is carried out by applying an aniline dye (eg, carbolfuchsin) to a smear and then decolorizing with acid alcohol to reveal whether the organisms present are "acid fast."

Bacteria Isolated from the Lung Tissue of a 32-Year-Old Caucasian