The Ziehl-Nielsen acid-fast stain, developed by Franz Ziehl and Friedrick
Nielsen, is a red dye that attaches to the waxy material in the cell walls of bac-
teria such as Mycobacterium tuberculosis, which is the bacterium that causes
tuberculosis, and Mycobacterium leprae, which is the bacterium that causes
leprosy. Microorganisms that retain the Ziehl-Nielsen acid-fast stain are called
acid-fast. Those that do not retain it turn blue because the microorganism
doesn’t absorb the Ziehl-Nielsen acid-fast stain.
Here’s how to Gram-stain a specimen (Fig. 3-5).(^62) CHAPTER 3 Observing Microorganisms
Apply Crystal Violet Stain Apply Iodine Alcohol wash Apply Safranin
Orange
Fig. 3-5. How to Gram-stain a specimen.
Type Number of Dyes Used Observations Examples
Simple stains Uses a single dye Size, shape, and Methylene blue
arrangement of cells Safranin
Crystal violet
Differential stains Uses two or more dyes Distinguish gram-positive Gram stain
to distinguish different or gram-negative Ziehl-Nielsen
types or different Distinguishes the members acid-fast stain
structures of bacteria of mycobacteria and
nocardia from other
bacteria
Special stains These stains identify Exhibit the presence
specialized structures of flagellae Shaeffer-Fulton
Exhibits endospores spore staining
Table 3-4. Quick Guide for Staining Techniques