What is Antimicrobial Resistance? – Definition, Testing & Statistics

This lesson is going to define something known as antimicrobial resistance. You’ll learn some of the ways by which it can be detected and tested as well as some incredible statistics regarding it!

Antimicrobial Resistance

Throughout the world, militaries have been having an arms race for millennia. Shields against swords. Walls against cannons. Missile defense systems against ballistic defense systems. And so too, an arms race is occurring on a microscopic scale. The race is between us and our drugs vs. microbes, like bacteria and viruses. These microbes develop defenses to our weapons which help them resist and survive the effects of those drugs. This microscopic arms race is called antimicrobial resistance, and it mainly applies to the way bacteria have become resistant to drugs called antibiotics.

Let’s find out how antibiotic resistance is tested for and some important statistics regarding this problem.

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Broth Dilution

We can test how sensitive and, by extension resistant, bacteria are to antibiotics via numerous ways. One of them is known as the Broth dilution method. Here’s how it works. Imagine there are several tubs of acid. Some tubs of acid have almost no acid in them at all; they are mainly water. That is to say, they are very dilute. Other tubs of acid are almost 100% acid, and so are highly concentrated. If you were to be placed into the dilute tubs of acid, you’d probably survive. If you were to be placed into the highly concentrated tubs of acid, you’d die. The minimum concentration of acid that would kill you is known as the minimum inhibitory concentration.

Broth dilution. Cloudy areas show microbial growth.
Broth dilution. Cloudy areas show microbial growth.

The same kind of logic is used with not you, but bacteria, and not tubs of acid, but tubes of antibacterial agents in order to test the relative sensitivity and resistance of a bacterium to an antibacterial agent.

Disk Diffusion

Here’s another way by which we can test a microbe’s sensitivity or resistance to a particular antibacterial. It’s called the disk diffusion method. In this case, a growth medium (agar) is seeded with bacteria on an agar plate. Onto this plate, are placed antibiotic infused disks. The antibiotics in these disks begin to diffuse into the surrounding growth medium. High concentrations of the antibiotic are found closest to the disks and lower concentrations further from the disk. Bacteria may or may not grow around these disks (and at various distances from each disk), depending on how sensitive or resistant they are to the antibiotics and their concentrations.

Disk Diffusion. Clear areas around a disk show a lack of bacterial growth.
Disk Diffusion. Clear areas around a disk show a lack of bacterial growth.

Other Tests

There are other ways by which we can test for antimicrobial resistance, including via the use of:

  • An ETEST. An ETEST involves the use of a plastic strip that has a decreasing concentration (gradient) of an antibiotic.
  • Automated commercial systems which help identify and test the sensitivity and resistance of bacteria to various antibiotics. These are literally machines that do the work for you!
  • PCR, or polymerase chain reaction, in order to detect the presence of genes that encode for resistance.


Some important and incredible statistics regarding antimicrobial resistance include the following:

  • In the U.S. alone, at least 2 million people a year become infected with bacteria resistant to antibiotics.
  • 23,000 people die every year as a direct result of such infections!
  • Antibiotic resistant infections cause people (in sum) to stay a total of 8,000,000 days a year longer in a hospital setting than they otherwise would have.
  • It costs between $18,000-$30,000 to treat just one person with an antibiotic-resistant infection.
  • Antibiotic resistance costs the entire United States around $55 billion a year in terms of health care costs and lost productivity.

Lesson Summary

Antimicrobial resistance refers to the ability and problem of microbes, such as bacteria, to develop resistance to the therapeutic effects of agents, such as antibiotics, designed to kill them or suppress their growth. While this term can apply to bacteria, viruses, fungi, and parasites, its biggest cause of concern currently lies with bacteria and their resistance to antibiotics.

The sensitivity, or resistance, of bacteria to antibiotics can be tested via numerous ways, including:

  • Broth dilution, where test tubes of various concentrations of an antibiotic are used to determine how sensitive bacteria are to the antibiotic.
  • Disk diffusion, where disks of antibiotics are placed onto an agar plate to see if any bacteria grow around the disks and how far/close to the disks they grow.
  • The use of automated commercial systems.
  • Checking for resistance-conferring genes.

Today, over 2,000,000 people in the U.S. alone are affected with antibiotic resistant infections, and 23,000 of them die directly as a result of it. It’s a very serious issue!


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