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We’ve learned a lot about different kinds of antibiotics and how they work. But there are some pathogens that virtually no antibiotics work against. In this lesson, we learn about two of these pathogens, and how they threaten to take us back to a pre-antibiotic era.

Antibiotics and Microbe Evolution

In the 1830s, Charles Darwin took an amazing 5-year voyage to various remote parts of the world, where he encountered all kinds of plants and animals that he’d never seen before.

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During this journey, he developed his theory of evolution by natural selection. You may have heard of this before; survival of the fittest, right? Whichever individuals have traits that help them survive and reproduce in their environment will pass on their genes to their offspring, shaping the next generation.Natural selection is why the most common color of peppered moths shifted from white to black during the Industrial Revolution. The black ones were better camouflaged on the dark, sooty bark of trees, so they could avoid being eaten by birds. The white ones used to be well-camouflaged before all that soot got there, but now they stuck out like a sore thumb and got gobbled up.

It turns out that not all selection happens naturally. For example, dogs have been selectively bred for hundreds of years to produce the characteristics that distinguish the various breeds. Pretty cool, right? Well, yeah, but did you know that we humans are also responsible for causing a much more sinister kind of evolution? I’m talking about the evolution of antibiotic resistance in bacteria.When people have bacterial infections, they get antibiotics so that they can get better. But at the same time, antibiotics put bacteria under very strong selection. The bacteria causing the infection want to survive and pass on their genes just like the rest of us, and the only ones that can do that are the ones that happen to be antibiotic-resistant.

Unfortunately, even though they work hard to maintain sterile conditions, hospitals are a major source of multi-drug-resistant bacteria today. This is because there are so many sick people in hospitals who are treated with many different antibiotics. The antibiotic-resistant bacteria in and on their bodies can colonize surfaces in hospitals.In this lesson, we’ll talk about two of the most frightening antibiotic-resistant pathogens that are around today: methicillin-resistant Staphylococcus aureus, or MRSA, and extensively drug-resistant tuberculosis.


Staphylococcus aureus are common and usually harmless bacteria that often live on the skin and in the noses of perfectly healthy people.

They usually only cause problems when they get into cuts or wounds; then they can cause pimples, boils and other skin infections, as well as more severe infections in the lungs, heart and blood.Patients with staph infections are given antibiotics. And this used to work just fine, until recently, when Staph aureus became resistant to all of the most effective antibiotics used to treat it. Methicillin-resistant Staphylococcus aureus, MRSA, is the name given to any strain of Staph aureus that is resistant to various beta-lactam antibiotics, such as penicillins, cephalosporins and methicillin. You can learn about these and other types of antibiotics in other lessons.

What do you do when a bug becomes resistant to all of the antibiotics that are usually used to treat it? Try another antibiotic. Doctors were relieved at first when they found that the antibiotic vancomycin was able to treat MRSA infections. Even its name sounds hopeful, like it could vanquish all of those severe infections.But perhaps unsurprisingly, MRSA are already developing resistance to vancomycin. There have already been a handful of VRSA, or vancomycin-resistant Staph aureus, cases in the United States. So we try another antibiotic, right? The major problem is that bacterial antibiotic resistance is developing faster than we can invent new antibiotics, so Staph infections may go back to being untreatable, just as they were in the pre-antibiotic era.

XDR Tuberculosis

Another global antibiotic resistance problem is drug-resistant tuberculosis.

You may have heard that tuberculosis, or TB, is a lung disease caused by Mycobacterium tuberculosis. M. tuberculosis hides inside of macrophages, which are a kind of white blood cell in our immune system. They can remain dormant, or latent, for long periods of time and then suddenly begin to cause active infection.

Symptoms of active TB include coughing up bloody sputum, weight loss, weakness and fatigue.You may think of tuberculosis as an old-fashioned disease from a long time ago. It’s true that it’s not very common in the United States nowadays, but TB is actually still a global pandemic. It’s estimated that one third of the world’s population is infected with this pathogen. And 30-90% of TB patients also have HIV, the human immunodeficiency virus that causes AIDS.So how do we treat TB? There are four so-called first-line drugs that are used to treat TB infections.

They are the best TB drugs that are available today: isoniazid, ethambutol, pyrazinamide and rifampin, and you can learn how they work in other lessons. Many antibiotics only work against actively growing cells, and since M. tuberculosis grows very slowly, it is hard for antibiotics to work against it. This means that antibiotic treatment for TB has to go on for a minimum of 6-9 months.Many patients fail to precisely follow this long and complicated drug regimen, and in many areas of the world, the first-line TB drugs are not reliably available.

If antibiotics are not taken consistently and for a long enough period of time, the patient becomes a breeding ground for antibiotic-resistant bacteria.So what antibiotic-resistant TB strains are out there already? The first is called multi-drug-resistant tuberculosis, and this class of bacteria is defined by being resistant to isoniazid and rifampin, which are the two most effective first-line TB drugs. The second class is even worse. They are called extensively drug-resistant tuberculosis, or XDR TB, and on top of being resistant to isoniazid and rifampin, they’re also resistant to at least one of the three available second-line drugs.

XDR TB is basically untreatable and is emerging globally. It is so much of a problem that in some cases, patients with XDR TB can be forced into quarantine, in effect locking them up to keep them from spreading the disease to others. What’s more, as we heard before, many TB-infected patients also have HIV. If an HIV-infected person is also diagnosed with XDR tuberculosis, their life expectancy is only three months.

Lesson Summary

Today, we’ve learned that antibiotic treatment, especially when it’s not carried out consistently or for a long enough period of time, puts bacteria through selection that is strong enough for them to evolve antibiotic resistance. It’s the same survival of the fittest that Darwin figured out in the 1800s.

We learned about MRSA, or methicillin-resistant Staphylococcus aureus, and VRSA, or vancomycin-resistant Staphylococcus aureus. These two bacteria are resistant to nearly all available antibiotics, and it’s inevitable that they will keep heading towards resistance to the remaining drugs. Staph infections used to be relatively harmless because they were easily treatable. MRSA and VRSA threaten to take us back to the pre-antibiotic era, when nothing could be done for bacterial infections.Finally, we learned about the global pandemic of tuberculosis. Because of TB’s slow growth, complicated antibiotic regimens have to be followed for at least 6-9 months to cure this disease.

Often, patients fail to comply with these drug regimens, which has led to the evolution of multi-drug-resistant tuberculosis and extensively drug-resistant tuberculosis, or XDR TB, strains. These two strains are defined by which antibiotics they are resistant to. XDR TB is such a big problem that people that have it can even be forced into quarantine to prevent them from spreading it to others.

Learning Outcomes

After you have finished this lesson, you should be able to:

  • Recognize the misuse of antibiotics as a cause for resistant MRSA
  • Describe how Staphylococcus aureus has become drug resistant
  • List the four first-line drugs used to treat tuberculosis
  • Explain how drug-resistant tuberculosis developed

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