Antimicrobial Peptides

Antimicrobial peptides are a type of molecule found throughout the body in the innate immune system. They are one of the first lines of the body’s defense, killing microbes similarly to the complement system. How they do this is by binding to the membranes of microbes and bacteria that invade the body and rip the membrane apart, killing the cell. 

The structure of antimicrobial peptides and how they kill microbes is very different to other methods of defense as they are not alive. These molecules float around in the body waiting to be triggered by invading cells. They then are attracted to the cells due to the general nature of bacteria and their anionic or hydrophobic membranes. They use one of three methods to break through the membrane of the cell and kill it. The first method is the barrel-stave method. This method has the peptides make a hole in the cell membrane. As the amount of holes increases and the insides of the invading cell empty out, it dies. The toroidal pore model is similar, both methods involving making a hole in the cell. However, in this case, the membrane curls into the hole instead of generally being unaffected by it. Finally is the carpet model, whereby the antimicrobial peptides cover the surface of the cell, and then disintegrate the membrane all at once. 

Antimicrobial peptides don’t just interfere with and destroy the cellular membrane, they can also disrupt DNA, protein synthesis, protein folding, and cell wall synthesis by invading bacteria. Once a hole has been made into bacteria, the antimicrobial peptides that surround the bacteria infiltrate the cell and disrupt processes such as the ones listed above.

However, we still get sick as the system isn't invincible. For one, though some peptides act against viruses, most act against bacteria and fungi. No defense system is perfect, and this is especially true against microbes. They have very quick reproduction rates, and as a result, mutation rates. Some microbes have developed tactics against this system, like enzymes that disrupt the initial binding process or protective layers. There’s also some problems in the system itself. Antimicrobial peptides are very short-lived and often dissolve before doing a lot of damage to the inside of the bacteria. They are also usually concentrated in certain parts of the body, leaving other areas free for bacteria to take over. Like most immune responses, when the system starts its assault, it’s very taxing on the body. This is because human cells also get targeted by the system as they have anionic membranes similar to that of bacteria.  Additional factors of the host such as age, genetic variation, and chronic illness all play a factor into how effective antimicrobial peptides are. So, it’s a good thing they’re only a small part of a much larger immune system.

References:

TG;, A. J.-P. A.-M. P. (2017). Antimicrobial peptides (amps): Ancient compounds that represent novel weapons in the fight against bacteria. Retrieved from https://pubmed.ncbi.nlm.nih.gov/27663838/ 

Author links open overlay panelJ.M. Ageitos a, a, b, c, AbstractAntimicrobial peptides (AMPs) are short peptidic molecules produced by most living creatures. They help unicellular organisms to successfully compete for nutrients with other organisms sharing their biological niche, Epand, R. M., … Shiba, T. (2016). Antimicrobial peptides (amps): Ancient compounds that represent novel weapons in the fight against bacteria. Retrieved from https://www.sciencedirect.com/science/article/abs/pii/S000629521630301X?via%3Dihub 

HJ;, N. L. E. (2011). The expanding scope of antimicrobial peptide structures and their modes of action. Retrieved from https://pubmed.ncbi.nlm.nih.gov/21680034/