Mycobacterium

Mycobacterium is a diverse genus of Gram-positive bacteria within the phylum Actinomycetota. This group contains over 190 different species, many of which belong to the family Mycobacteriaceae. While most species are harmless, some are famous pathogens that cause serious diseases in mammals. For example, Mycobacterium tuberculosis causes tuberculosis, and Mycobacterium leprae causes leprosy. The name itself comes from the Greek prefix "myco-", which means fungus. This name was chosen because the colonies of these bacteria often look like mold on a surface.

The most defining feature of these bacteria is their complex cell envelope. This structure consists of a thick, hydrophobic cell wall made of peptidoglycan and arabinogalactan. Crucially, this wall contains high concentrations of mycolic acid, which is a waxy lipid. This waxy layer acts as a powerful shield for the cell. It helps the bacteria survive environmental stressors like chlorine and allows them to evade a host's immune defenses. Because of this unique chemistry, these bacteria are described as acid-fast. This means they resist being washed away by acid during laboratory staining processes.

Mycobacteria are generally aerobic organisms, meaning they require oxygen to grow. They are typically shaped like small rods, measuring 0.2 to 0.6 micrometers wide and 1.0 to 10 micrometers long. Most species are non-motile, meaning they cannot move on their own. However, one exception is Mycobacterium marinum, which has been shown to move within macrophages. These bacteria are incredibly hardy and can grow with very minimal nutrition. They often use ammonia or amino acids as nitrogen sources and glycerol as a carbon source. Their optimal growth temperatures vary by species, typically ranging from 25 to 45 degrees Celsius.

Scientists have historically used different methods to classify these species. The Runyon classification system was based on growth rates and pigment production. This system divided them into groups like photochromogens, which produce pigment when exposed to light. Other groups, called scotochromogens, produce pigment regardless of light. Some species are "rapidly-growing," meaning they form visible colonies in less than seven days. Others are "slowly-growing" and may take much longer. For instance, Mycobacterium leprae has an extremely long reproductive cycle, requiring 12 days for a single division. In contrast, some E. coli strains can divide in only 20 minutes. Today, researchers rely more on DNA sequencing and computational phylogenetics for precise identification.

The impact of these bacteria on human health is significant. It is estimated that one-third of the global population carries latent tuberculosis. This means the Mycobacterium tuberculosis bacteria stay hidden in the host for decades without causing symptoms. They can reactivate if the host's immune system becomes suppressed. Another serious concern is leprosy, caused by Mycobacterium leprae or Mycobacterium lepromatosis. About 200,000 new cases of leprosy are reported annually, with 80% of cases occurring in Brazil, India, and Indonesia. Additionally, nontuberculosis mycobacteria (NTM) can cause various infections. About 80% to 90% of recorded NTM infections manifest as pulmonary diseases.

Because their cell walls are so unique, standard medicines like penicillin are ineffective. Instead, doctors must use specific antibiotic combinations to target different parts of the cell wall. For example, isoniazid is used to inhibit mycolic acid synthesis. Rifampicin is used to interfere with transcription, while ethambutol hinders arabinogalactan synthesis. Researchers are also studying mycobacteriophages to fight these bacteria. Mycobacteriophages are viruses that specifically target mycobacteria. By hijacking the bacteria's own machinery, these viruses can be used in phage therapy to treat drug-resistant infections.

Genomic studies have provided deep insights into the complexity of this genus. Hundreds of Mycobacterium genomes have been completely sequenced. The sizes of these genomes vary significantly between species. For example, Mycobacterium leprae has a relatively small genome. However, Mycobacterium vulneris has a much larger genome, encoding 6,653 proteins. This is even larger than the roughly 6,000 proteins found in eukaryotic yeast. Understanding these genetic blueprints helps scientists identify virulence factors. These factors include various proteins and lipids that allow the bacteria to survive and cause disease in their hosts.