MTB stands for Mycobacterium tuberculosis, the bacterium that causes tuberculosis (TB). It’s one of the deadliest infectious organisms on the planet, responsible for an estimated 10.8 million new infections and 1.25 million deaths worldwide in 2023 alone. Despite being a preventable and treatable disease, TB remains a leading cause of death from a single infectious agent, surpassing even HIV in many years.
How MTB Differs From Other Bacteria
MTB is unusual among bacteria in several ways. Its cell wall is packed with large, waxy fatty acids called mycolic acids that form a thick, lipid-rich barrier around the organism. This fatty armor makes MTB extremely tough. It resists drying out, survives many common disinfectants, and shrugs off the immune system’s first-line chemical attacks. The waxy coating also makes MTB notoriously slow-growing. While common bacteria like E. coli can divide every 20 minutes, MTB takes roughly 15 to 20 hours to reproduce. That slow growth is part of why TB treatment takes months rather than days.
The mycolic acid coat also gives MTB a distinctive laboratory signature. When stained with certain dyes, the waxy layer holds the color even after being washed with acid and alcohol. This “acid-fast” property is one of the oldest and still most widely used ways to identify TB under a microscope.
How MTB Spreads
TB spreads exclusively through the air. When someone with active lung or throat TB coughs, sneezes, shouts, or sings, they release tiny particles called droplet nuclei that carry live bacteria. These particles are incredibly small, roughly 1 to 5 micrometers, which means normal air currents can keep them floating for hours and carry them throughout a room or building. You cannot catch TB from touching surfaces, shaking hands, or sharing food.
The exact number of bacteria needed to start an infection has never been pinned down precisely. What is clear is that prolonged, close contact in poorly ventilated spaces dramatically raises the risk. Brief encounters with an infected person in open air carry very low risk by comparison.
What Happens Inside the Body
Once inhaled, MTB lands deep in the lungs and is quickly swallowed by immune cells called macrophages. In most bacterial infections, macrophages destroy invaders by sealing them inside a compartment and flooding it with digestive enzymes. MTB has evolved a remarkable trick to avoid this fate: it blocks the compartment from maturing into its fully destructive form. Essentially, the bacterium gets swallowed but prevents the macrophage from “turning on” its killing machinery.
Recent research has shown that MTB manipulates the macrophage’s own signaling molecules to accomplish this. The bacterium triggers infected macrophages to release a protein that, in a feedback loop, prevents the digestive compartment from acidifying and activating its enzymes. In laboratory experiments, immune cells lacking this protein destroyed roughly 80% of ingested bacteria, compared to only about 50% in normal cells. This ability to survive and even multiply inside the very cells meant to kill it is central to how MTB establishes infection.
Latent Infection vs. Active Disease
One of MTB’s defining features is that most people who breathe it in never get sick. About 90% of infected individuals develop what’s called latent TB infection: the bacteria are present in the body, held in check by the immune system, but cause no symptoms and cannot spread to others. Only about 10% of people with latent infection ever progress to active, symptomatic TB.
Of those who do develop active disease, roughly half get sick within the first one to two years after exposure, while the other half may not become ill until years or even decades later. The risk of progressing from latent to active TB rises sharply when the immune system weakens, whether from HIV, diabetes, malnutrition, aging, or medications that suppress immune function. Reactivated TB typically appears as cavities in the upper portions of the lungs.
Symptoms of Active TB
Active pulmonary TB, the most common form, causes a persistent cough lasting three weeks or longer, often producing blood-tinged mucus. Other hallmark symptoms include unexplained weight loss, night sweats, fever, fatigue, and chest pain. Because these symptoms develop gradually, many people delay seeking care, which gives the bacteria more time to spread to others.
MTB can also infect parts of the body beyond the lungs, including the kidneys, spine, and brain. These forms of “extrapulmonary” TB are less common and generally not contagious, but they can be serious and harder to diagnose because their symptoms mimic other conditions.
How TB Is Treated
Standard TB treatment uses a combination of four antibiotics taken over six to nine months. The first two months are called the intensive phase, during which all four drugs are taken daily to rapidly kill the bulk of the bacteria. After that, treatment continues with fewer drugs for an additional four to seven months to eliminate the slow-growing bacteria that survive the initial assault.
The long treatment duration is one of TB care’s biggest challenges. Patients often start feeling better within weeks and are tempted to stop taking their medications. Stopping early is dangerous because it allows surviving bacteria, often the hardest-to-kill strains, to multiply and potentially develop drug resistance.
Drug-Resistant TB
When MTB develops resistance to the two most powerful first-line antibiotics, the resulting infection is classified as multidrug-resistant TB (MDR-TB). If it gains additional resistance to key second-line drugs, it becomes extensively drug-resistant TB (XDR-TB). Both forms are far harder and more expensive to treat, often requiring two years or more of therapy with drugs that have more side effects.
Drug-resistant TB is a growing global crisis. In 2023, approximately 400,000 new cases of drug-resistant TB were reported, contributing to an estimated 150,000 deaths. Resistance develops most often when treatment is incomplete, inconsistent, or uses the wrong drug combinations.
The BCG Vaccine
The only existing TB vaccine, known as BCG, has been in use since the 1920s and is given to infants in most countries where TB is common. It is effective at preventing severe forms of TB in children, particularly TB meningitis and disseminated disease. However, its effectiveness against pulmonary TB in adults is highly variable, which is one reason it is not routinely used in the United States. Protection from the childhood dose also wanes over time, leaving adults in high-burden countries vulnerable despite having been vaccinated.
Global Impact Today
TB is often thought of as a disease of the past, but the numbers tell a different story. With 10.8 million new cases annually, it remains one of the world’s most widespread infectious diseases. The burden falls disproportionately on low- and middle-income countries, particularly in Southeast Asia and sub-Saharan Africa, where crowded living conditions, limited healthcare access, and high rates of HIV create ideal conditions for transmission. Even in wealthier nations, TB persists in immigrant communities, homeless populations, and people with compromised immune systems.