Mycobacteria

These bacteria are rods that have high lipid content in their cell walls, which makes them acid-fast on staining. (Red stain is applied and heated to penetrate bacteria, acid alcohol is poured over the smear, then a counter stain methylene blue is applied). Mycobacteria are stained red as their cell walls do not wash off with acid alcohol, thereby retaining the first stain. Mycobacteria and Nocardia are two major acid-fast organisms.

Mycobacterium tuberculosis 
Commonly infects the lungs, causing the chronic disease tuberculosis. These bacteria possess mycosides, lipids that are only present in acid-fast organisms, which are involved in virulence. The formula of a mycoside is a mycolic acid bound to a carbohydrate.

• Mycosides: 
• Cord factor - formed by two mycolic acids and a disaccharide. 
• Sulfatide - like cord factors, but sulfates are attached to the disaccharide. 
• Wax D - complex mycoside that activates the immune system. 
• Stages of tuberculosis: 
• In primary tuberculosis, inhaled bacteria infect the lung, but is usually asymptomatic. Bacteria enter macrophages and then lymphatics and other areas of the body. Cell-mediated immunity will suppress the bacteria, but they can cause secondary tuberculosis.  
• Secondary or reactivation tuberculosis occurs after the bacteria lie dormant. Infection can occur in any organ. 

Mycobacterium leprae 

An acid-fast, rod bacterium. Causes leprosy as it grows on the skin, preferring cooler body temperatures. This is why warm areas such as the armpit and groin are spared. Disease severity depends on one's immune system. 

• Diseases:
• Lepromatous leprosy (LL), where the lack of cell-mediated immune response in patients causes leprosy in its the severest form. The reason may be due to defective T-suppressor cells that block Th cell response to the bacteria. 
• Tuberculoid leprosy (TL), a milder form of leprosy as there is cell-mediated immune response against the bacteria.