MRSA and Multi-Drug Resistant Pathogens

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  • 0:03 Antibiotics and…
  • 2:07 MRSA
  • 3:40 XDR Tuberculosis
  • 6:22 Lesson Summary
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Lesson Transcript
Instructor: Katy Metzler

Katy teaches biology at the college level and did her Ph.D. work on infectious diseases and immunology.

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. 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.

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