Question

1.Describe the structure and appearance of an acid-fast cell wall

2..Which of the following is the defining characteristic of acid fast bacteria?

• A thin cell membrane made of phospholipids
• A thin layer of peptidoglycan
• A thick layer of mycolic acids
• A polypeptide layer surrounding the cell membrane

Answer 1

1. The different layers in an acid fact cell wall are cytoplasmic membrane followed by a peptidoglycan layer, arabinogalactan (AG) polysaccharide, outer membrane containing mycolic acid, glycolipids, waxes, sulfolipids and glycerophopspholipids. The AG polysaccharide acts as the link between the inner peptidoglycan layer and the outer membrane. The acid-fast staining results from the presence of mycolic acids in the outer membrane of the cell wall.

Acid fast staining is a technique used in identification of microorganisms. Different types of stains can be used in this method. The most common stain used is Ziehl–Neelsen stain. Depending on the outcome of this staining method, microorganisms can be classified as acid fast group and non-acid fast group.

Acid fast group of microorganisms have lipoid capsule cell wall which stains with the carbol-fuchsin dye. However, this stain is not removed when washed with diluted acid as the lipoid capsule does not allow water-based components to enter the cell wall. Thus, the acid-fast organisms retain a red colour from the carbol-fuchsin dye.

Non-acid fast bacteria do not have the lipoid capsule and hence becomes detained when washed with mild acids. When a counterstain like methylene blue or malachite green is applied, these bacteria appear blue or green respectively.

2. the defining characteristic of acid-fast bacteria is the presence of a thick layer of mycolic acid which are very long chain 2-alkyl-3-hydroxy fatty acids. This layer of mycolic acid along with the glycolipids are responsible for the acid-fast property of these organisms.