Mycobacterium tuberculosis (Tuberculosis)

It’s estimated that about two billion people worldwide are infected with mycobacterium tuberculosis, often just shortened to tuberculosis or simply ‘TB’. Two billion is a ton of people, but even though they’re infected, that doesn’t mean that all those people have symptoms, the vast majority, about 90-95%,  aren’t even aware that they’re infected. And this is because usually the immune system can contain it such that it isn’t able to multiply, and often remains latent, or dormant, as opposed to active, which usually causes symptoms and can be spread to others. If the host’s immune system becomes debilitated at some point down the road, like with AIDS or some other illness, or as a person grows older, it can be allowed to reactivate, or basically wake up and become very serious, especially if it spreads throughout the body.

Mycobacteria are an interesting bunch, they’re slender, rod-shaped, and need oxygen to survive, in other words, they’re “strict aerobes”. They’ve got an unusually waxy cell wall, which is mainly a result of the production of mycolic acid. Because of this waxy cell wall, they’re “acid-fast”, meaning that they can hold on to a dye in spite of being exposed to alcohol, leaving it bright red colored when a Ziehl–Neelsen stain is used.  The wall also makes them incredibly hardy, and allows them to resist weak disinfectants and survive on dry surfaces for months at a time.

Now Mycobacterium tuberculosis is usually transmitted via inhalation, which is how they gain entry into the lungs. Now, we breathe in all sorts of virus and bacteria all the time, but we’ve got defenses that take care of most of them. For one, air that we breathe in is turbulent in the upper airways, and drives most bacteria against mucus which is then cleared pretty quickly. Ultimately, though, TB can avoid the mucus traps and make its way to the deep airways and alveoli where we have macrophages that eat up foreign cells, digest, and destroy them. With TB, they recognize foreign proteins on their cell surface, and phagocytize them, or essentially package them into a space called a phagosome. With most cases, the macrophage then fuses the phagosome with a lysosome, which has hydrolytic enzymes that can pretty much break down any biochemical molecule. TB’s tricky, though, and once inside the macrophage, they produce a protein that inhibits this fusion, which allows the mycobacterium to survive. It doesn’t just survive, though, it proliferates, and creates a localized infection.