Acid-fast positive and acid-fast negative Mycobacterium tuberculosis: The Koch paradox

Mycobacterium tuberculosis possesses a unique cell wall architecture that is distinct from both Gram-negative and Gram-positive bacteria. The cell wall consists of a thick, lipid-rich outer layer composed primarily of mycolic acids (1) (Fig. 1). This lipid layer lies on top of a layer of peptidoglycan and the polysaccharide arabinogalactan, which, in turn, are anchored to the inner lipid membrane common to all bacteria (2 - 4). The overall thick waxy coat renders acid-fast (AF) mycobacteria resistant to Gram staining. When stained with alternative dyes, the cell wall is resistant to decolorization with acid alcohol, thus giving these bacteria their sobriquet "acid-fast." This unique AF property has been the basis for the continuous development of staining procedures over the past century and remains the cornerstone for the diagnosis of tuberculosis (TB), especially in low-income and middle-income countries where more than 90% of TB cases occur (5). The Ziehl-Neelsen (ZN) stain, also known as the AF stain, which is used in microscopic detection of M. tuberculosis, was originally developed independently by Ziehl and Neelsen, who improved on the early work of Koch, Rindfleisch, and Ehrlich (see below).