Friday, March 25, 2011

Daily Silver: Here I am writing about peptidoglycan for my roommate.

So, every time I ask my roommates what I should write a blog post on, my one roommate L says "peptidoglycan."  Since it was her birthday this week, I'm going to write about peptidoglycan (not too much I promise, as she is the only person I've ever met that thinks it's interesting).

I know there are some of you who are staring at the computer screen right now, eyes slowly glazing over, saying to yourself "what the heck is peptidoglycan?"  Peptidoglycan is a carbohydrate-protein scaffolding (called a polymer) that is a major component of some bacterial cell wallsAll living things are made up of cells, and animal cells are different from plant cells are different from bacterial cells, and so on.  One [of the many] major differences between animal cells and bacterial cells is that bacteria have what is called a cell wall, which is pretty much exactly what it sounds like: it's a stiff boundary that surrounds the cell and helps it keep its shape.  That's why we animals are so squishy, our cells don't have walls, they only have membranes which are fluid (bacteria also have cell membranes, in addition to the wall).

Peptidoglycan isn't very exciting on the whole, except for one thing: penicillin.  You may know parts of this already, but a quick refresher: penicillin is an antibiotic, meaning it kills bacteria.  Actually, it's kind of THE antibiotic, because it was the first substance used therapeutically (medically, to treat disease) as an antibiotic, and the story is a study in serendipity.  In 1928, Scottish scientist Alexander Fleming was culturing some bacteria.  When you culture bacteria, you coat the bottom of Petri dish with a medium like agar (something with sugars and other nutrients in it that growing bacteria need) and then you spread bacteria from somewhere--another culture, some lake water, your mouth--over the medium with a Qtip or a plastic rod or something.  Then you stick it in an incubator (many bacteria require temperatures similar to body temperature to grow) and wait a while.  But when Fleming came back and took his plates out of the incubator, he noticed far less bacteria growing than he'd expected.  He also noticed that there were spots of fungus growing on the plates, and there was a circle around the fungus where no bacteria were growing.  Voila; penicillin!  The fungus was Penicillium notatum, and he found that when it was grown under the right conditions it secreted a substance that acts as an antibiotic; he called it penicillin.

Nowadays, the term is used to describe a group of naturally derived antibiotics, so when the doctor asks if you're allergic to penicillin she's actually asking if you are allergic to the fungus P. notatum and/or substances derived from it.  What does this have to do with peptidoglycan? you ask (and if you weren't asking it just pretend you were).  Glad you asked.

Penicillin is antibiotic because it inhibits the formation of the peptidoglycan wall.  It stops links from forming between individual peptidoglycan subunits and results in an overall weakening of the wall and even holes in places.  Because bacteria are constantly remodeling and repairing their cell walls, penicillin acts pretty quickly, stopping the individual parts from linking up.

Remember how I said that the wall helps the cell retain its shape?  Cells are basically bags of fluid and protein and sugars, and they're surrounded by more fluid and protein and sugar.  At all times, the weights of the water inside and outside the cell are fighting.  Think of when you swim all the way to the bottom of the pool and you can feel the water squeezing you harder the deeper you go.  It's a complicated and dynamic process, keeping enough water pressure inside the cell to keep it from collapsing, and at the same time having little enough pressure that the cell doesn't burst, and the cell wall makes it a little bit easier, itself pushing both in and out.  But when the wall is weak, the cell is not equipped to handle all those conflicting pressures by itself, so it bursts.  Peptidoglycan has, however indirectly, killed the cell.

So that's the main reason for focusing on peptidoglycan.  We're always looking for faster, better, easier ways to rid people of disease.  Not all bacteria contain peptidoglycan cell walls, but knowing about the penicillin mechanism offers new ways to go about looking at other potential systems for killing bacteria in our water, our waste, and in our bodies.

There's peptidoglycan in a nutshell.  Happy birthday, L!

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