Page 72 - MNUmicrobiology practical 2025

General Microbiology & Immunology (PM 401) second level Semester 4 2024/2025

Learning outcomes

• Describe the difference between acid-fast bacteria and non-acid-fast bacteria.

• Explain how the acid-fast stain works comparing acid-fast and non-acid fast

bacteria.

• Identify the genera of bacteria that are acid-fast, and two examples of diseases
caused by these species.
• Differentiate the cell wall structures of acid-fast and non-acid fast bacteria.

Objective

To understand the chemical basis of acid-fast staining and differentiation of bacteria

into acid-fast and non-acid-fast groups.

Principle

• Bacteria of the genera Mycobacterium and Nocardia have unusual cell walls that

are waxy and nearly impermeable due to the presence of mycolic acid, and large

amounts of fatty acids, waxes, and complex lipids. These organisms are highly

resistant to disinfectants, desiccation and are difficult to stain with water-based

stains such as the Gram.

• Because the cell wall is so resistant to most compounds, acid-fast organisms

require a special staining technique involving heat to drive stain into their waxy

cell wall.

• The primary stain used in acid-fast staining, carbol fuchsin, is lipid-soluble and

contains phenol, which helps the stain penetrate the cell wall. This is further

assisted by the addition of heat in the form of heat (steam). Steam helps to loosen

up the waxy layer and promotes entry of the primary stain inside the cell. The

General Microbiology & Immunology (PM 401) Practical note Page 61