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Antimicrobial Peptides: Definition and Use Against Microbes

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  • 0:05 Plasma Membranes
  • 0:41 General Mechanism of Action
  • 2:05 Peptide Antibiotics
  • 2:59 Antimicrobial Peptides…
  • 4:15 Selective Toxicity
  • 6:50 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.

If you could poke holes in a bacterium's membrane, would it be like a water balloon, leaking all over the place until it died? In this lesson, learn how short proteins called peptides can specifically damage microbial plasma membranes.

Plasma Membranes

Microbes - they are tiny, simple creatures that don't even have brains, but they can still be formidable enemies when they infect us! These little boogers have ingenious ways to take control of our bodies and use them as breeding grounds. When an infection happens, we have to weaken them in any way possible!

What better way to weaken a cell than to destroy its membrane? A membrane is what separates a cell from the rest of the outside world and keeps all of a microorganism's goodies inside! Today, we'll talk about how antibiotics and even our own natural proteins can kill microbes by damaging their plasma membranes.

General Mechanism of Action

The molecules we are going to talk about in this lesson are peptides, meaning they are short proteins that are approximately 12-50 amino acids long. There is still a lot of research being done on these peptides to find out exactly how they work, but for now, we know that their general mechanism is to interact with membranes and disrupt them. Some of the peptides may form pores or holes in the membrane, others may change the membrane structure by poking into it in many places, and there may even be other mechanisms that scientists haven't even imagined yet. But, the bottom line is these peptides damage membranes and make them more permeable.

When a microbe's membrane is too permeable, it will die. One reason is that it can lose important metabolites and ions, and molecules from the environment that the microbe would rather keep out can come right in. This would be kind of like if all the doors and windows of your house got broken or ripped out. A lot of your possessions might suspiciously go missing, and people, animals, and weather conditions would have free access into your home.

In cells, other crucial everyday processes can stop because of membrane damage. One of these processes is cellular respiration, which is how the cell makes its main energy source, ATP. Without ATP, a microbe won't be able to get anything done at all. This would be like adding insult to injury: on top of the problems you're dealing with in your windowless and doorless house, the electricity stops working. Not fun.

Peptide Antibiotics

Let's look at a few examples of peptide antibiotics, which are peptide molecules that are used as antibiotics. One important example is Polymyxin B, which damages the plasma membranes of Gram-negative bacteria. Polymyxin B used to be an important antibiotic for treating pseudomonas infections but is now mainly found in antibiotic creams you might use at home for minor wounds.

Daptomycin is a newer peptide antibiotic. Instead of just being a peptide antibiotic, it is a lipopeptide antibiotic, meaning that it is a peptide with a lipid chain attached to it. This allows it to very effectively attack the Gram-positive bacterial membrane. Like other peptide antibiotics, daptomycin changes the plasma membrane structure, forming holes that make the membrane more permeable. It is used to treat infections with multidrug-resistant pathogens like methicillin-resistant Staphylococcus aureus, MRSA for short.

Antimicrobial Peptides in Immunity

In addition to the peptides that are used as antibiotics, many organisms naturally make and use antimicrobial peptides (AMPs) as part of their normal immune systems. Antimicrobial peptides have been found in various different organisms, such as amphibians, birds, insects, mammals, and even plants. They may be a part of an evolutionarily ancient system for immune defense.

Let's look at a few examples. The magainins are antimicrobial peptides that are found in the skin glands of amphibians, such as frogs. Our own cells have antimicrobial peptides, too. Neutrophils, an important type of white blood cell in our innate immune system, produce alpha-defensins, and another antimicrobial peptide called dermcidin is present in our sweat. All of these antimicrobial peptides can disrupt the membranes of bacteria and sometimes even fungi and other eukaryotic pathogens.

A really cool fact about antimicrobial peptides is that, even though they've been around for a very long time through evolution, very little resistance has developed. Perhaps it is too difficult for microbes to evolve new types of membranes that would not be affected by these peptides. Since antibiotic resistance is a major problem in medicine today, scientists are continuing to research antimicrobial peptides in the hopes of creating new antibiotics with similar strategies.

Selective Toxicity

But hang on a minute; we've been missing something really important here. All cells have plasma membranes, which are all composed in pretty much the same way. Remember that plasma membranes are lipid bilayers formed of many phospholipid molecules arranged so that their hydrophilic, water-loving head groups are on the outsides and the hydrophobic, water-fearing tails are all hanging out together in the middle of the membrane.

So, how on Earth could these membrane-disrupting peptides be selectively toxic if our cell membranes are so similar to microbial cell membranes? Again, scientists don't know for sure how this works, but one important idea is that the surface charge of microbes could be a major factor. Our cell membranes tend to have a neutral charge on the outside, but bacterial membranes are more likely to have negatively charged phospholipids on the outside. In addition, bacteria have other negatively charged molecules on their surfaces as well, like lipopolysaccharide (LPS) in Gram-negative bacteria and teichoic acid in Gram-positive bacteria.

The bottom line is bacterial surfaces tend to be negatively charged. How convenient, then, that antimicrobial peptides tend to have a positive charge. The electrostatic attraction brings the positively charged peptide to the negatively charged bacterial surface, where it can poke into the bacterial membrane or otherwise damage it. If antimicrobial peptides were made of Velcro and bacterial surfaces were coated in the other type of Velcro, they could stick together. Our cells, on the other hand, don't have any Velcro on them at all, so the antimicrobial peptides won't attach to them very well.

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