Antibiotic Effectiveness: MICs, Time- and Concentration-Dependent Antibiotics

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  • 0:07 Drug Susceptibility
  • 0:35 Diffusion Tests
  • 3:30 Broth Dilution Tests
  • 4:59 Time- Vs.…
  • 6:30 Drug Synergy and Antagonism
  • 7:47 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.

With so many antibiotics available, not to mention so much antibiotic resistance in bacteria, how is a doctor supposed to choose which drug to prescribe? In this lesson, learn about tests to determine which antibiotics will be most effective against a microbe.

Drug Susceptibility

With all you've learned about bacteria and other microbes, it might seem almost like they're supernatural beings or something. They have virulence factors that help them set up infections better, they have ways to hide from our immune system, and they even have tons of ways to resist the antibiotics and other drugs that we throw at them. This lesson is about finding microbes' weak spots: finding out which antibiotics a particular microbe is especially susceptible to and exactly how much of a drug is needed so that we can destroy it and all its little buddies as soon as possible.

Diffusion Tests

As we've learned in other lessons in this chapter, different kinds of drugs work better for different kinds of organisms. If the organism causing an infection is known, doctors can choose an appropriate drug pretty easily. But often, the organism is unknown. Or it could be a type of bacteria that is often antibiotic-resistant. In these cases, it's a good idea to test which antibiotics it is susceptible to.

The first way we'll talk about is called the disk diffusion test / Kirby-Bauer test. First, you take your microbe of choice - for example, one that you've cultured out of a patient's infected lung. You spread a large amount of this microbe all over an agar plate in a Petri dish. There should be enough bacteria there that they'd cover the entire plate after you let them incubate at body temperature for a while.

But before you incubate them, you need to place a few paper disks that contain known amounts of various antibiotics on the plate. The drugs are going to diffuse gradually out of the disks and into the agar, hence the name 'diffusion test.' The further away from a disk you get, the lower the concentration of antibiotic will be there.

So let's fast forward until after we've incubated the bacteria on the plate with the antibiotic disks. What we'll see if an antibiotic worked against our organism is a so-called zone of inhibition. This is an empty area on the plate surrounding an antibiotic disk. It's empty because all of the bacteria that were there were killed or were unable to grow because of the action of the antibiotic. In general, the bigger the zone of inhibition, the more effective that drug was because it could work at the lower concentrations found further away from the disk. If there is no zone of inhibition, the organism was not susceptible to that antibiotic.

One caveat to this method is that some drugs aren't very soluble, meaning they don't dissolve well in the agar plate. That means the antibiotics won't be able to diffuse very far away from the disks. So the zone of inhibition might look misleadingly small, even if the drug was effective.

There's also a more advanced version of a diffusion test that provides more detailed information to the microbiologist. It's called the E test. The E stands for 'epsilometer.' In this test, you put plastic strips onto the plate of bacteria instead of the disks.

The key thing that makes the E test better than the Kirby-Bauer test is that the plastic strips have a precise, known gradient of antibiotic concentrations on them. So when you see the zones of inhibition, you will also know the minimal inhibitory concentration (MIC) of the antibiotic that works on the microbe in question. That is, you'll know the lowest amount of antibiotic that can be used to inhibit your bug.

Knowing the MIC allows physicians to avoid promoting antibiotic resistance by not using a high enough dose to control the bacteria. Also, they can minimize the toxic side effects that their patients might get from taking higher doses than necessary.

Broth Dilution Tests

The diffusion methods we've learned about so far can only tell us whether a drug inhibits a microbe, not whether it's bactericidal or bacteriostatic. Remember that bactericidal antibiotics kill bacteria directly, while bacteriostatic antibiotics just stop bacteria from growing.

The broth dilution test overcomes this problem. Several different dilutions of a drug are made into liquid media in a special plate that has many sample wells. Then the same amount of bacteria is added to each well. Then we have to incubate again to let the bacteria grow and multiply.

After incubation, the wells with no bacterial growth had antibiotic concentrations that were above the MIC of the drug. Now comes the best part: to check if the drug was bacteriostatic or bactericidal, you can culture a sample from those wells in media without the drug.

If there's growth in this fresh, antibiotic-free media, you know that the drug was only bacteriostatic since it did not kill the bacteria in the well - they were still alive and able to replicate once they were away from that pesky antibiotic. However, if nothing grows in the media without antibiotics, you know those little bugs were already completely dead because the antibiotic was bactericidal.

From these results, a microbiologist can determine the minimal bactericidal concentration (MBC) - that is, the lowest concentration of the drug that can be used to actually kill the bacteria instead of just stop their growth.

Time- vs. Concentration-Dependent Drugs

Once you've figured out which drug your patient's bacteria are most susceptible to, as well as the MIC and maybe the MBC, you're good to go, right? Actually, there are a couple other considerations to think about when it comes to antibiotic effectiveness. For example, some antibiotics are time-dependent and some are concentration-dependent.

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