Microbiological Characteristics

Obligate Aerobic

• Why the lung is preferentially affected.
• Why the secondary infections start at the apex of the lung.

Acid-Fastness (For Mycobacteria, Gram Positiveness is Only Academic)

• Acid fast bacteria have the ability to retain dyes (fuchsin) when treated with acid solution (Ziehl-Neelsen stain).
• Useful for the detection of M. tuberculosis in clinical specimens (sputa, pus or tissues) by microscopy.

Special Nutritional Requirement (Lowenstein-Jensen Medium)

• TB cultures are not routinely done! Need special culture media.

Slow Grower

• Requires several weeks to form visible colonies! (DON'T CALL CLINICAL LABS FOR THE RESULT OF CULTURE NEXT DAY!)

Highly Resistent to Desiccation

• This is why most infections (TB) are initially caused by inhalation of contaminated dusts!

Facultative Intracellular Pathogen

• Neutrophils or resident macrophages can not kill this organism. M. tuberculosis prevents the acidification of phagolysomes, thus resists the intracellular microbicidal mechanisms of phagocytes.

Unique Cellular Constituents (Lipids)

• Stimulate cell-mediated immune response (delayed type hypersensitivity) of the host (see the pathogenic mechanisms and tuberculin test below).
• þ The constituents listed below are found largely in cell walls. Mycobacterial cell walls can induce delayed hypersensitivity, and replace whole mycobacterial cells in Freund's adjuvant.