Mycobacterium Tuberculosis: The Deadly Killer Worldwide


By Barry C. Fox, M.D.University of Wisconsin

Worldwide, one person is infected with the deadly killer Mycobacterium tuberculosis— or TB—every second. TB has been found in mummies in Egypt, India, and China from thousands of years ago. Scientists think it originated in the Cradle of Africa and migrated, just like people did, out of Africa over time. But another more recent study has suggested it was carried to the New World by an animal—seals.

3D illustration of Mycobacterium tuberculosis.
One person is infected with Mycobacterium tuberculosis every second. (Image: Kateryna Kon/Shutterstock)

The Reason for TB’s High Incidence of Infection

The main reason for TB’s high incidence of infection relates to the highly contagious potential of the bacteria. The size of the TB germ is about three microns in size. That’s exactly the same size as the dust particles in the air. So when someone with active TB coughs, they generate an aerosol of germs that does not sink to the ground in the six- to eight-foot radius, like normal, larger germs. And the TB germs actually ride the air currents and can be spread over long distances.

Now when individuals are exposed to air currents with TB, there’s a high probability that TB will enter the lungs and settle in the furthermost passages of the airways. This exposure begins a complex immune response by several different types of cells, to attempt to contain TB in the lung.

The germ is called Mycobacterium, which differentiates itself from normal bacteria with a special outer membrane made of mycolic acid. This makes it more difficult for the immune system to kill the germ, so TB can continue to live inside of cells.

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Latent Mycobacterium Tuberculosis 

Active primary TB infection from germ inhalation is prone to occur if there are large numbers of TB germs inhaled, if someone’s immune system is compromised, or they have the HIV virus. Under most circumstances, however, active TB does not immediately occur. But the best that the immune system can provide the body with is to wall off TB in a highly organized layer of cells known as a granuloma. This usually leads to a noncontagious disease state known as latent TB.

More than 90 percent of people who inhale TB will carry dormant TB bacteria their entire lives. Their bodies will show an immune response with antibodies and reactive lymphocytes, resulting in a positive tuberculosis skin test. Up to one third of the world’s population carries TB in a latent form.

However, this is not necessarily the end of the story for latent TB cases, especially in the first three years after inhalation; there is still a risk of developing active TB until the immune system gains further control of the original infection.

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When Inactive Mycobacterium Tuberculosis is Re-activated

As the immune system ages, a small percentage of those with latent TB will actually lose containment of the TB that has been inactive for decades. This leads to a condition known as re-activation TB. To reduce this risk with aging, patients with latent TB are usually offered a chance to take TB medications, usually called isoniazid, to reduce the risk of re-activation TB.

At this stage, the bacteria actually leave the lungs, enter the bloodstream, and can settle anywhere in the body as disseminated or military TB. Most patient germs will also return to the lung combining with the site of the original infection, leading to active lung tuberculosis.

To make matters worse, with a large burden of TB in the lung, the lung architecture can be destroyed, leading to cavitary TB, an actual hole in the lung. With a hole in the lung filled with TB germs, medications have difficulty getting to the site of infection. It also makes this individual highly contagious to others, as every time they cough, germs leave the lungs.

Drug Treatment of Mycobacterium Tuberculosis

Image of TB treatment aids such as capsules, pills, and syringes.
Ethambutol, Pyrazinamide, and Rifampin are examples of drug treatment for TB. (Image: ESB Professional/Shutterstock)

Drug treatment of TB is complicated for several reasons. First, Mycobacteria do not rapidly divide like normal bacteria. As a general principle, germs are only vulnerable to antibiotics while they are in the growth stage. Therefore, treatment usually needs to be extended to a minimum of six months but can be as long as two years.

TB usually requires treatment with four medications to start off with, as each medication has a specific target in the cell layers within the granulomas surrounding the TB germs. The drugs are usually Isoniazid, Rifampin, Pyrazinamide, and Ethambutol. 

People who do not follow through with taking TB medications completely usually don’t get well, and they increase the chances of developing drug-resistant TB. Or by taking medications in a haphazard fashion, low concentrations of the drugs will give multiplying germs a chance at genetic mutations that will favor the emergence of resistant strains.

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Vaccination and Treatment of Mycobacterium Tuberculosis

An image of a TB vaccine bottle and a syringe.
BCG TB vaccine is used for small children. (Image: joel bubble ben/Shutterstock)

Rather than containing and treating TB, prevention efforts with vaccination should be a priority. There is a vaccine called Bacillus Calmette–Guérin, or BCG, which is used in many countries for vaccinating small children. This vaccine is a weakened form of a cow strain of TB. Its protective effect may reduce the risk of developing active TB by about 50 percent.

Before the discovery of streptomycin, the first TB antibiotic, a novel idea of treatment for TB emerged in 1864—the ideas of sanatoriums. The most famous U.S. sanatorium was built in the Adirondack Mountains by Dr. Edward Trudeau, who also had TB. 

He built what were called cure cottages to accommodate other TB patients. He put them on a strict regime of three months of bed rest, a strict diet, and exposure to fresh air. While actually not a cure for the disease, it served to isolate the sick from the well. It also ensured that people got rest and nutrition, which enhanced their strength to fight TB.

Common Questions about Mycobacterium Tuberculosis

Q: Why is TB’s incidence of infection so high?

The size of the Mycobacterium tuberculosis germ is very small, about three microns, and it can travel for long distances in the direction of airflow. There’s a high probability that TB will enter the lungs of exposed individuals and settle in the furthermost passages of the airways.

Q: What is latent TB?

Under most circumstances, active TB does not immediately occur as the immune system surrounds the Mycobacterium tuberculosis bacteria with an organized layer of cells called granuloma. This causes a noncontagious disease state called latent tuberculosis.

Q: What was TB treatment before the discovery of streptomycin, the first TB antibiotic?

A novel idea for the treatment of TB that emerged in 1864 was the idea of sanatoriums. The sanatorium was not actually a cure for the disease, and its main focus was on providing patients with adequate rest and nutrition.

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