Tuberculosis: microbiology and pathogenesis after exposure

What it is

Tuberculosis describes a broad range of clinical illnesses caused by Mycobacterium tuberculosis. It can affect virtually every organ, most importantly the lungs, and is typically associated with granuloma formation.

The organism is an aerobic, non-spore-forming bacillus. The mycolic acid in its cell envelope confers the ability to resist destaining by acid alcohol after being stained by certain aniline dyes, leading to the term acid-fast bacillus (AFB). The M. tuberculosis complex comprises at least seven species in the genus Mycobacterium, and the species M. tuberculosis causes the vast majority of human tuberculosis worldwide. Humans are the only reservoir for it.

Causes and risk

Almost all infections with M. tuberculosis are due to inhalation of droplet nuclei, 1-5 microns in diameter: infectious particles from a person with pulmonary tuberculosis, aerosolized by coughing, sneezing or talking, which dry while airborne and remain suspended for long periods.

• The air in a room occupied by a person with pulmonary tuberculosis may remain infectious for approximately 30 minutes even after that person's absence.
• Infection does not generally occur out of doors, because M. tuberculosis is killed by ultraviolet light.
• Prolonged exposure and multiple aerosol inocula are usually required to establish the infection.
• The most important determinants of infection of tuberculin-negative persons are closeness of contact and infectiousness of the source.