Minimum Inhibitory Concentration: Units & Test

Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

Doctors sometimes rely on specialized tests to figure out which antibiotic(s) will be most effective at targeting bacteria that have infected a person. This lesson will explain how this is so as you learn about the minimum inhibitory concentration.

Stubborn Infections

When a patient is infected with stubborn bacteria for which run-of-the-mill antibiotics aren't doing the trick, a doctor will need advice from a lab as to which antibiotic(s) may be effective for their patient. How does the lab determine such a thing? The answer depends on testing for something called the minimum inhibitory concentration.

What Is The Minimum Inhibitory Concentration?

Pretend for a minute that you are trying to kill a bunch of weeds on your lawn. What product do you use? How much of that product do you use? You could go to the local home improvement store, get a random weed killer and try randomly spraying it on the weeds. That might work if you're lucky, but if you use the wrong weed killer, it may not work against the specific weeds growing on your lawn. If you use too little of even the right weed killer, it may not do anything either. If you use too much of any weed killer, you might end up killing the grass too.

Lucky for you, companies have done a lot of testing to figure out which herbicide to use for which weeds and how much, just enough of it, to do the job right without killing the grass.

A similar process is used to figure out which drugs may be used, at the recommended and thus relatively safe dosage, to inhibit the growth of a microorganism. It all has to do with the minimum inhibitory concentration.

The minimum (or minimal) inhibitory concentration, MIC, refers to the lowest concentration of a chemical that inhibits the growth of a microorganism. Typically, the chemical is an antibiotic, a drug that kills bacteria. Thus, the microorganism is a specific kind of bacteria.

Test & Units

When trying to figure out the MIC, a specific kind of bacteria is tested for in a lab. The strain of bacteria is then exposed to various antibiotics, much like a specific weed is subjected to various weed killers to figure out which weed killer(s) even work in the first place.

But that's not enough. Not only do we want to determine which antibiotics will work against this strain of bacteria, the tests that determine MIC must also figure out what the minimum inhibitory concentration is for each antibiotic. This concentration is denoted with units of micrograms/milliliter. Again, this is like our weed example. We not only want to figure out which kind of herbicide will work in the first place, we also need to figure out what minimum concentration (strength) of the herbicide we need to use in order to get the job done.

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