Lesson Ideas: Genetic Engineering Coupled with an In-Depth Look at a Fascinating Mutualism

Recently, my general Biology students engineered glowing bacteria, photographed by my colleague, Ms. Amy Bonner.  I appreciate her recent post, commenting on the often overlooked connection between art and science.  The glowing phenotype was achieved by transferring genes from a bioluminescent marine bacterium, Vibrio fischeri, into E. coli, which is easy to grow in the lab.  While the experiment is designed to teach students about bacterial transformation, the laboratory also provides a perfect opportunity to take an in-depth look at an awe-inspiring mutualism between Vibrio fischeri and the bobtail squid.  Here, I will detail a few practical aspects regarding the experiment and how I have used this laboratory as a starting point to explore this fascinating symbiosis.

If you haven’t undertaken a molecular biology experiment with your students, don’t be intimidated.  Although the set up is more time intensive than the average high school lab, this kit from Carolina Biological Supply has detailed instructions and provides ready-made LB agar nutrient media to simplify preparations.  I have tried similar kits from Ward’s Natural Science, but I like that Carolina provides enough extra materials to account for occasional student errors.  While transformations can be tricky, I have always gotten good results with this kit.  It is helpful to have a water bath, although I have managed with a good old-fashioned hot plate, beaker, and thermometer.  Notably, Carolina’s student worksheet accompanying this lab is extremely well-written.  It emphasizes important concepts about positive and negative controls and walks students through a calculation of transformation efficiency.

Before beginning the transformation experiment, students watch this clip introducing them to the bobtail squid:

The clip (and the images below) are taken from Dr. Bonnie Bassler’s 2009 HHMI Holiday Lecture, “Shedding Light on an Invisible World.”  After watching the clip, we discuss the fact that, while the bobtail squid hides during the day, it must come out at night to hunt.  It inhabits shallow water.  Thus, moonlight can reveal the outline of the squid to predators lurking below.  A predator’s view would look like this:

As a way of camouflaging itself in its environment, the bobtail squid has evolved a unique symbiosis with Vibrio fischeri.  The squid assumes the expense of feeding the bacteria, housing them in a specialized light organ.

With the bioluminescent bacteria in tow, the outline of the squid from a predator’s perspective is more difficult to make out:

The squid does not have the genes necessary to produce this adaptive glow.  Thus, it provides nutrients and a protected environment for the resident bioluminescent bacterium, whose lux genes generate this glowing phenotype.  These bacterial genes have been engineered into a plasmid, included in the kit from Carolina that I discussed above.  Once my students understand where these genes come from, we move on to replicating the beautiful glow in the lab by moving the plasmid  into easy-to-grow E. coli.

With more advanced classes, I discuss the daily cycle of bacterial growth within the light organ.  As it turns out, the bacterium’s stay within the squid is temporary.  The bacteria are sent packing as day breaks since it is too energetically expensive for the squid to house large amounts of bacteria for a full twenty-four hour period.  As nightfall approaches, bacteria reproduce, increasing in number within the light organ.  What’s more, the bacteria are polite guests and wait to produce the expensive glowing phenotype until it is dark and the squid needs the camouflage.  How do bacteria sense when it is time to produce this glow?

It is all about population density, and bacteria can gage their numbers within their environment.  This is a phenomenon known as quorum sensing, which is a ubiquitous form of communication among bacteria.  The phenomenon can be studied easily in Vibrio fischeri because of the visible phenotype produced when quorum sensing is at work.  I hope to post more about Bonnie Bassler’s fascinating studies on quorum sensing in the future, which have piqued my interest in recent weeks.

The intricate symbiotic relationship between the bobtail squid and Vibrio fischeri is but one example of the remarkable ways in which organisms have evolved to work together.  Do you have a favorite mutualism?


1 Response to “Lesson Ideas: Genetic Engineering Coupled with an In-Depth Look at a Fascinating Mutualism”

  1. 1 artwithmsb January 26, 2012 at 7:07 pm

    Thanks for including me. This was a really fascinating project even though I’m still not completely sure how it works. Here is the link to my post with the photos… http://artwithmsb.wordpress.com/2012/01/25/glowing-bacteria/

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