The slow, waxy microbe behind tuberculosis hides inside the cells sent to eat it
Most bacteria split every few minutes; E. coli can manage roughly every 20. The germ that causes tuberculosis takes 18 to 24 hours. What it lacks in speed it makes up for in armour: a waxy, fat-rich coat that shrugs off ordinary stains, resists drying out and helps it survive inside immune cells.
Robert Koch identified the bacterium in 1882, and it is still sometimes called Koch's bacillus. It is a tiny curved rod, about 2.71 micrometres long, that needs plenty of oxygen, which is one reason it favours the lungs. Its outer layer is rich in mycolic acid, so the standard Gram stain barely takes, and laboratories use special acid-fast or fluorescent dyes instead. Under the microscope the cells often cling together in strands, a look known as cording, like the fibres of a rope. Grown in a lab dish, it needs several weeks to form visible colonies.
Humans are its only known reservoir, and it spreads mainly through airborne droplets when an infected person coughs, sneezes, speaks or sings. Sharing toothbrushes, cups or toilet seats is a common misconception about how it passes on. Once in the lungs, it is swallowed by immune cells called macrophages, which normally destroy invaders by fusing them with compartments full of digestive chemicals. The bacterium blocks that fusion, neutralises some of the cell's chemical weapons and even coats itself in a compound that works like an antacid. It also suppresses the cell's self-destruct programme, giving itself a safe place to multiply or lie dormant for a long time.
Its genome, sequenced in 1998, holds about 4 million base pairs and 3,959 genes. Some 250 are devoted to handling fats, 39 of them to building the waxy coat, a sign of how vital that armour is. Inside a host it prefers to feed on fats, and it can even live on cholesterol alone.
Some strains have become resistant to several drugs at once. Genetic fingerprinting of strains, now based on 24 repeated stretches of DNA, lets investigators test whether one person really caught the disease from another.
Source: Mycobacterium tuberculosis
