Tuesday, January 26, 2010

Quorum call, without voices

Is there anybody out there?
When it comes to political intrigue, it’s important to know whether you stand united or alone. Are you surrounded by friends or enemies? Is it time to lead the charge or wait for reinforcements? Tallying how many supporters you have—or how many enemies are waiting in your midst—can be critical to your success.

For bacteria, things aren’t much different. Bacteria survey their surroundings for others of their kind, and others NOT of their kind. They may wait until they have sufficient numbers to launch an attack on your body, or to coordinate effective mining of a nutrient source. But bacteria don’t have eyes, ears, or a mouth; they cannot see, hear, or speak. So how do they tell who is out there?

The peril of the tennis ball
Time to use your imagination, folks. Imagine you are in a windowless room. The door is closed. You are wearing a blindfold and earplugs. You cannot speak. In your hand is a fuzzy tennis ball. When you throw the tennis ball, it bounces off the walls until, eventually, it stops. Think about the chances of the ball ricocheting off a wall and hitting you in the head.

Now remember: you cannot speak, you cannot hear, you cannot see. But, if I put another person in the room with a tennis ball to throw, do you think you’d be able to tell you were not alone in the room? Maybe. How about if I dropped in TEN people, each with one tennis ball? How often do you think you’d be smacked square in head?

It probably seems strange to imagine all this blindfolded tennis ball throwing, and I’m pretty sure it won’t give you a better understanding of human politics. But if you understand how you’d be able to sense the presence of more people in the room based on how often you got a tennis ball concussion, you understand the microbial behavior called quorum sensing.

Bacteria don’t throw tennis balls, they “throw” chemical signals
The tennis ball in our thought-experiment represents a quorum-sensing signal. These are chemical compounds that bacteria can “throw” out into the environment, and “catch” if they hit the cell. In this way, bacteria can sense if they are relatively alone, or if they are surrounded by thousands of others.

If you play around with the tennis ball room scenario, you can start to envision more complex issues with quorum sensing. Can you imagine ways to trick the bacteria into thinking they were not alone? Or trick a crowd of bacteria into thinking they were all alone?


  1. What a great way to explain a complex concept so that anyone can understand. I have never imagined bacteria as people throwing tennis balls before, but it works great for this scenario.

  2. It is a great visual metaphore. Raises hand. Question. You listed several different functions of quorum sensing. Are there different color tennis balls or do the tennis balls change color dependant on what they impact?

  3. @Nick: Great question! I'd say it is more like different color tennis balls (different signal structures) as opposed to one type of tennis ball changing colors as it hits different locations. Each tennis ball has a receptor that senses it. It would be like having one eye that sees red and a separate eye that sees blue. Does that make sense?

  4. dude - i have one eye with a contact and the other eye's contact won't stay in. So I have one eye for seeing and another one for screwing things up.