Tuberculosis (TB) is a major global health concern, affecting millions worldwide. This infectious disease is caused by a bacterium that primarily targets the lungs but can also impact other body parts. Understanding its unique characteristics is key to comprehending how it causes illness and how medical science combats it.
The Tuberculosis Bacterium
The bacterium responsible for tuberculosis is Mycobacterium tuberculosis (M. tuberculosis). This small, rod-shaped bacillus is strictly aerobic, requiring oxygen to survive and multiply. This explains its prevalence in the well-aerated upper lobes of the lungs.
M. tuberculosis has a notably slow growth rate, taking 15-20 hours to double. It is an intracellular pathogen, living and reproducing primarily inside human cells, particularly within macrophages, immune cells that engulf foreign invaders. This characteristic contributes to its resilience and evasion of the host’s immune system.
Unique Cell Wall Structure
The cell wall of Mycobacterium tuberculosis is highly complex and distinct from most other bacteria, contributing to its survival and pathogenicity. A defining feature is mycolic acid, a long-chain, branched fatty acid that gives the cell wall a waxy, hydrophobic nature. This waxy coating is responsible for the bacterium’s “acid-fast” property, allowing it to resist decolorization by acids after staining, a characteristic used in laboratory identification.
The cell wall also contains peptidoglycan, which provides structural rigidity, and arabinogalactan, a highly branched polysaccharide linking the peptidoglycan to the mycolic acid layer. This unique composition creates a low-permeability barrier, protecting the bacterium from immune defenses and making it inherently resistant to many common antibiotics.
How the Bacterium Causes Disease
Infection with Mycobacterium tuberculosis begins with airborne transmission, as infected individuals release tiny droplets containing bacteria into the air. When inhaled, these bacteria settle in the lungs. Once there, M. tuberculosis is taken up by macrophages. However, due to their unique cell wall and other properties, they survive and replicate inside these immune cells, rather than being destroyed.
The body’s immune system attempts to contain the infection by forming granulomas, small, organized clusters of immune cells that wall off the bacteria. This containment can lead to latent TB infection, where bacteria are alive but dormant, causing no symptoms and preventing spread. If the immune system weakens, the bacteria can overcome these defenses, multiply, and cause active TB disease, leading to symptoms and potential transmission. Active TB can affect the lungs, causing symptoms like a persistent cough, or spread to other parts of the body such as the brain, spine, or kidneys.
Identifying and Targeting the Bacterium
Understanding the distinctive features of Mycobacterium tuberculosis is important for diagnosing and treating the disease. Diagnostic methods leverage the bacterium’s unique characteristics. Sputum smear microscopy identifies acid-fast bacilli using specialized stains. While rapid and inexpensive, it may not distinguish M. tuberculosis from other mycobacteria and has lower sensitivity in cases with fewer bacteria.
Culture-based diagnosis, the gold standard, involves growing bacteria from samples, which can take several weeks due to slow growth. Molecular tests, such as nucleic acid amplification tests (NAATs) like GeneXpert, offer faster detection of M. tuberculosis DNA and can identify drug resistance within hours.
Treatment for TB requires multi-drug therapy, typically a combination of antibiotics over a prolonged period, often at least four months. This extended approach is necessary because the bacterium’s complex cell wall impedes drug penetration, and its slow growth rate means drugs must be present for an extended time to eliminate all cells. The long treatment duration also helps reduce the risk of drug resistance development.