How Does It Work?: Penicillin

In the grand tradition of great sites like HowStuffWorks.com, I decided to start a regular feature with the terribly derivative title How Does It Work?  Why?  Well, because microbiology isn't confined to textbooks or research laboratories.  We interact with microbes every day.  We are carrying them in our guts, on our skin, inside our nose (yes, gross but true).  We eat and drink things that microbes help produce (bread and beer, for example).  Sometimes they help us and sometimes they hurt us.  So, it makes sense for us to know a little more about the different ways we study them and how we try to control them for our own purposes.

For our very first How Does It Work? today, I'll talk about the antibiotic penicillin, which was one of the first antibiotics discovered and then mass produced for treatment of bacterial infections.  Penicillin is credited with dramatically reducing the number of military deaths due to infection during World War II.  Although penicillin itself is not often prescribed these days, there are a number of antibiotics derived from penicillin that are still used in clinical therapy today, and these antibiotics work in the same general way as penicillin.

So, what is penicillin?  It is a chemical compound that is naturally produced by a fungus called Penicillium.  Below is a great picture of what Penicillium looks like under a microscope; the long filaments are called hyphae and the broom-like structures sprouting off from the hyphae are made up of columns of round spores.

Many of our antibiotics are derived from compounds produced by fungi or bacteria.  These compounds are the result of an evolutionary arms race between the different microbes that occupy the same environments.  In the case of Penicillium, this fungus lives in the soil, and it evolved to produce penicillin as a way of competing against other soil-dwelling microbes for resources.  Scientists discovered that what works for Penicillium can also work for us to combat bacterial infections.


How does penicillin work?  This may not be something you think about when you take an antibiotic.  Usually, we're just happy to take a pill and feel better.  But when you take your first course of an antibiotic, an incredible, microscopic drama starts to unfold within you.  Here's how it goes...

 Let's say you come down with pneumonia caused by a bacterium called Streptococcus pneumoniae.  This bacterium is growing inside of your lungs.  About every 30 minutes, each individual Streptococcus cell divides into two cells...you can see that this is not a great situation for your lungs and your overall health.  Penicillin to the rescue!  The trick of penicillin is that it prevents the bacteria from building cell walls.  Hm?  Cell walls?  What's that, you say?  Well, for almost all species of bacteria, their cells are surrounded by a layer of something called peptidoglycan, which is a combination of proteins and sugars.  

This cell wall is a very important part of the bacterial cell.  It helps the cell maintain its shape, and it also helps prevent the cell from bursting due to the influx of water by osmosis.  What's osmosis?  When I was in college, a lot of people used to joke that if you slept with your head on a textbook, you might manage to learn by osmosis.  The idea here is that all the knowledge moves from a place of high concentration (the textbook) to a place of low concentration (my brain...on many days).  Inside of a bacterial cell, there are an incredible number of proteins and other compounds (otherwise known as solutes) and a low concentration of water compared to the outside of the cell.  To keep up with our analogy, the water is like the knowledge in the textbook example I just mentioned.  The natural state of things is to have an equilibrium, so water will always tend to move into the cell (from high to low concentration) to try to reach this equilibrium.  Without a rigid barrier like the cell wall, the cell would burst due to the influx of water.  Below, there's a neat video demonstrating osmosis by putting a wilted piece of lettuce into a bowl of water.  You can easily see what happens as the water moves from high concentration (the bowl) to low concentration (inside the lettuce).  Much like bacterial cells, lettuce cells have cell walls also, which prevents the lettuce from exploding into little lettuce bits as a result of too much water rushing into the cells.

Bacterial cells are normally spared an explosive fate by their rigid cell walls.  But, when you take your penicillin pill, the clock starts ticking for those bacterial cells!  Penicillin interferes with cross-linking between the proteins of the bacterial cell wall.  These cross-links reinforce the bacterial cell wall, and when penicillin disrupts them, it weakens the cell wall.  Eventually, the cell wall is so weakened that water moving into the cell causes it to swell, burst and die.  And this is how penicillin works!  So, when you take a course of penicillin (or a penicillin-related antibiotic such as amoxicillin), it basically bursts all the invading bacterial cells, such as the Streptococcus pneumoniae that was causing your pneumonia in our example here.  Because our cells don't have cell walls, penicillin doesn't cause our cells to burst.


Effective antibiotic development is all about finding targets that are specific to pathogenic bacteria or fungi.  That is why it is important for us to understand the differences between our cells and bacterial or fungal cells.  We may be able to use that knowledge to develop new antibiotics!

More licensing stuff:
The picture of Penicillium is from WikiMedia Commons and is used in compliance with the Creative CommonsAttribution-Share Alike 3.0 Unported license. 
Thanks to khymos for the YouTube video!