Antibiotics and Antimicrobial Drugs: Selective Toxicity, Classes and Mechanisms

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  • 0:05 Paul Ehrlich and the…
  • 2:14 Selective Toxicity
  • 3:14 General Classes of…
  • 6:27 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.

How can antibiotics kill bacteria but not harm our own cells? In this lesson, learn about the selective toxicity of antibiotics and antimicrobial drugs and some basic mechanisms for their activity.

Paul Ehrlich and the 'Magic Bullet'

In the late 1800s and early 1900s, when Paul Ehrlich was studying microorganisms and how to stain them for microscopy, he imagined the idea of a 'magic bullet' - a chemical that could specifically kill microbes but not harm the host's own cells.

How could such a 'magic bullet' work? Aren't all cells vulnerable to toxic chemicals? What kinds of chemicals could be selectively toxic only for microorganisms like bacteria, fungi and parasites and not damage our own cells?

To find antimicrobial drugs, you'd have to exploit the differences between our cells and microorganisms. It's true that a lot of the most important proteins involved in basic cellular functions have been conserved throughout evolution, which means there are many similarities among all types of cells in the world. However, our cells are still very different from microorganisms in many ways, and these differences provide drug targets that we can make use of.

Antimicrobial drugs are chemicals that are intended to have selective toxicity against microbes, meaning that they kill microbial cells but not the host's cells. Antimicrobial drugs include antibiotics, which were originally defined as substances produced by microorganisms that inhibit other microorganisms. The name 'antibiotics' comes from the word 'antibiosis'. Unlike symbiosis, where two organisms live together in a way that is often mutually beneficial, antibiosis is when one microorganism tries to kill another one.

Nowadays, the definition of antibiotics has changed a bit. When people say 'antibiotics,' they usually mean substances that inhibit bacteria. And instead of always being derived from microorganisms, many antibiotics nowadays are made synthetically or semi-synthetically. This means that all or part of the drug molecule is made by chemists in the lab.

Antimicrobial drugs also include antifungal drugs, antiviral drugs and antiparasitic drugs. These are chemicals that inhibit fungi, inhibit viruses and inhibit parasites.

In this lesson, we will learn about the problem of selective toxicity and some different ways that antimicrobial drugs achieve it.

Selective Toxicity

Imagine that you are a scientist trying to design a 'magic bullet' that will target a microbe but not cause any damage to the host's own cells. In other words, you are trying to design a drug that is selectively toxic. Remember that selective toxicity is the specific inhibition of some types of cells but not others. In the case of antimicrobial drugs, we want the drug to be selectively toxic for whatever microorganism is infecting us and not kill our own cells.

How would you go about designing this drug? First, you would think about the differences between microbes and human cells. As you might imagine, selective toxicity is easier to achieve for bacteria than for eukaryotic pathogens. That's because bacteria, being prokaryotes, are very different from our own cells.

Since eukaryotic pathogens, such as fungi and parasites, are much more similar to animal cells, you would have a harder time designing drugs against these microbes. If you did find some, they would usually cause some degree of toxicity to the patient as well, which means there might be unwanted side effects when using these drugs.

General Classes of Antimicrobial Drugs

Nowadays, we know a lot more about microbes than people did in Paul Ehrlich's day, so I think you're going to have no problem designing a selectively toxic antimicrobial drug.

What's a good target to start with? Hmmm… How about the cell wall? Bacteria need intact cell walls in order to survive, and animal cells don't even have cell walls. Sounds perfect! This strategy really works. Many successful antibiotics that are used today inhibit cell wall synthesis. When bacterial cell walls are weakened, the bacteria lyse, meaning they break open and die.

Okay, what could we try next? What other important cellular properties are different in microbes and in animal cells? How about the plasma membrane? If you damage a cell's plasma membrane, important contents can leak out and the cell can die. But wait a minute… all cells have plasma membranes, right? That's true, but there are some major differences that you could exploit for your 'magic bullet.' For one thing, bacteria tend to have a different surface charge than animal cells, because many bacterial surfaces are covered with negatively charged molecules, like lipopolysaccharide or teichoic acid. For this reason, plasma membrane-disrupting antibiotics that are positively charged bind more easily to bacterial surfaces than to animal cell surfaces. And fungal and animal cell plasma membranes share many similarities because they are both eukaryotes, but they are composed of a few different lipids. Many modern antimicrobial drugs target microbial plasma membranes using these different characteristics.

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