Mycobacterium tuberculosis

Mycobacterium tuberculosis is a species of pathogenic bacteria. It is the specific agent that causes tuberculosis.

This bacterium belongs to the family Mycobacteriaceae. It is a small, rod-shaped bacillus. While many bacteria live in various environments, humans are the only known reservoirs for this specific species. It primarily targets the mammalian respiratory system. Once it enters the lungs, it can cause significant damage to tissue.

The bacterium is famous for its unusual cell surface. It possesses a thick, waxy coating made of lipids. This coating includes mycolic acid and a substance called cord factor glycolipid. This waxy layer acts as a shield. It helps the bacteria resist desiccation, which means drying out. This allows the cells to survive in a dry state for several weeks. The coating also makes the bacteria resistant to weak disinfectants. Because the wax repels most dyes, scientists cannot use a standard Gram stain. Instead, they must use acid-fast stains like Ziehl–Neelsen or fluorescent stains like auramine.

Inside a host, the bacterium uses clever mechanisms to survive. When inhaled, it is swallowed by immune cells called alveolar macrophages. Normally, these cells would digest the germ. However, M. tuberculosis prevents this process. The cord factor glycolipids in the cell wall inhibit the fusion of the phagosome with the lysosome. The lysosome is the part of the cell that contains antibacterial factors. The bacterium also produces a substance called diterpene isotuberculosinol. This prevents the phagosome from maturing. It even secretes a nucleoside called 1-tuberculosinyladenosine to act as an antacid. This helps it neutralize pH levels and induce swelling in lysosomes.

This survival strategy allows the bacteria to stay in a latent state. They find a safe place to replicate inside the host. The body often responds by creating granulomas. These are organized aggregates of immune cells. Granulomas serve a dual purpose. They help regulate the immune response and minimize tissue damage. However, they can also help the infection expand. During an infection, the body may increase levels of a protein called PPM1A. This increase can inhibit apoptotic pathways. Apoptosis is the process where a cell undergoes programmed death to clear a pathogen. By suppressing this, the bacteria maintain a protected niche.

Growing M. tuberculosis in a laboratory requires great patience. Most common bacteria, like Escherichia coli, can divide every 20 minutes. M. tuberculosis is much slower. It divides only once every 18 to 24 hours. Because of this slow rate, visible colonies on agar plates can take several weeks to appear. Scientists use various media to grow them. These include liquid Middlebrook 7H9 or 7H12. They also use egg-based solid media called Löwenstein-Jensen.

Some labs use Mycobacteria Growth Indicator Tubes. These contain a gel that emits fluorescent light if the bacteria are growing.

History has played a major role in our understanding of this pathogen. Robert Koch first discovered the bacterium in 1882. This was a massive breakthrough in microbiology. Later, in 1998, the genome of the H37Rv strain was sequenced. The genome is about 4 million base pairs long. It contains 3,959 genes. Scientists found that 250 of these genes are involved in fatty acid metabolism. This explains how the bacteria use host-derived lipids, like cholesterol, as a food source.

Understanding these genes helps researchers study how the bacteria survive during the chronic phase of infection.

Managing the disease is difficult because of antibiotic resistance. Some strains are considered multidrug-resistant, or MDR TB. This means they are resistant to both rifampicin and isoniazid. The most severe type is extensively drug-resistant, or XDR TB. These strains resist isoniazid, rifampin, a fluoroquinolone, and at least one injectable second-line drug. Resistance happens when mutations accumulate in the bacterial genes. Researchers are looking at ways to target the PPM1A-JNK signaling axis. If they can restore the macrophage's ability to undergo apoptosis, they might improve how chemotherapy works.