Explain Counter Stain - Ziehl-Neelsen Method?

Methylene blue is used to stain previously decolourized cells. All non acid-fast bacteria de-stained by acid alcohol now absorbs counter stain and takes on its blue colour. All acid-fast bacteria remained red in colour due to primary stain.

Genus Mycobacterium shows acid fastness. These are aerobic, slightly curved or straight rods that sometimes branch or form filaments. These filaments readily break down into rods and coccoid bodies, when disturbed. Some grow very slowly. Some are free-living saprophytes and some are pathogens, e.g. M. bovis could cause tuberculosis in cattles, M. tuberculosis causes tuberculosis in humans, M leprae causes leprosy in humans. Acid-fast stain has a diagnostic value in identifying these pathogenic mycobacteria.