Bacterial transformation and counting colonies for grade school students

Our experimental setup: Microbe plushies, Lego DNA, plates, iPhones and makeshift stand, ready to go.
Our experimental setup: Microbe plushies, Lego DNA, plates, iPhones and makeshift stand, ready to go.
A few weeks ago, our elementary school held its annual science fair. Owing to the greater-than-usual number of scientists among the parents, the halls of this event were lined with tables staffed by said parents, showing off the wonders of science, tech, and especially biotech. There were at least three stations devoted to various aspects of stem cell research, and the table next to us had kids run simple nucleic acid extractions from wheat germ using detergent and alcohol – my son loved that one, as he pulled out the stringy goop with a q-tip at the end of the process.

My wife and I contributed to the festivities by putting together a presentation on bacterial transformation. I was just about finished working on a colony counter iPhone® app for Promega, so I figured why not try it out in the field: Print out some colorful ersatz bacterial plates, have the kids count the colonies using the app (yay, touch screens!) and maybe teach them something about genetic engineering along the way.

Our setup turned out to be a lot of fun to run, and quite popular to boot. It went roughly like this:

Step 1: Plushies and legos. Way back when my wife was finishing her bacteriology degree, I bought her a set of incredibly cute microbe stuffed toys (a.k.a. plushies). So we retrieved them out of our kids’ bedrooms and set them up on the table: A yeast, a Staphylococcus, an E. coli, and a rhinovirus (I couldn’t find the Epstein-Barr virus in time for the festival). The kids passing by took instant notice, and never failed to ask what those were. This would be my cue – I would first ask the kids what they thought the plushies were, and what they thought that DNA was, and would latch onto any part of their explanation that made any sense whatsoever (“You’re totally right! DNA tells you what hair color you have!”) I’d then follow-up warning kids to wash their hands after they petted the cold virus.

Next came the transformation bit. Keeping to the spirit of microbes as plushies, we co-opted legos to represent DNA: A double-layer of yellow legos for (very, very schematically) bacterial plasmid DNA, a small gray set of blocks as the insert – our gene of interest. Finally, a red pair of blocks stood in for the antibiotic-resistant co-insert.

Step 2: Explaining the problem of bacterial transformation. I’d then ask the kids: does putting these lego bricks into the plushies look easy? (Here, I tried stuffing the lego bricks in, a-la the Monty Python gumby sketch) No! The kids would laugh at this. Well, I continued, it turns out that putting DNA inside bacteria is almost as challenging. But we have tricks to do it: I explained electroporation, in which a bacterial cell is subjected to a mild electric shock to allow its membrane to become somewhat permeable. But even then, how do we know we got the DNA in?

Step 3: The antibiotic resistant gene trick! Most of molecular biology is a collection of truly fiendish tricks (although I’m no molecular biologist, I love reading up about the field for this precise reason). Before we try to insert our gene of interest into the bacteria, we first add to it ANOTHER gene, this one providing resistance to a particular antibiotic. Then, we transform the bacteria with plasmids that contain both these genes together. So, either the bacteria have NEITHER gene, or BOTH genes together.

Depending on the age of the kids I say “antibiotic” or simply “poison”. There were some 4th graders who understand what antibiotics were (I was sure to ask them first to explain it to me – nothing like letting a 4th grader show off their knowledge), and there were some kindergardeners who… really didn’t. They still enjoyed watching me horse around with plushies and legos.

Step 4: Plate our transformed bacteria onto petri dishes. Here, I would swish the fuzzy hairs of my E. coli plushie around on a plate, making kids laugh again. (I love this job!) And finally, mimic pouring in the antibiotic/poison. Bacteria that have neither gene in them, I’d explain, “don’t do so well” (sad E. coli plushie). While those that did get the DNA inserted do fine and make more copies of themselves. These form colonies that grow, and grow, and grow, so much so that after a bit, you can see them as little spots on the plates.

Then I switch up and show the kids our fake bacterial plates (while assuring the parents that these are just paper printouts glued to the bottom of the plates, not real bacteria – although goodness knows there’s plenty enough of those on all the grade schoolers’ hands). The idea is, the spots come from single bacteria that flourished in spite of the poison (antibiotic, for grades 3 and up) that we put on the plates. So they must have the antibiotic resistant gene, and therefore our gene of interest inside there too (I know I said it before, but repetition is good in this business). Now all we have to do is just count how many of these spots there are, and we can figure out how successful we were at the transformation.

Step 5: The counting. As I mentioned previously, I had just been wrapping up a simple colony counter for the iPhone as a work project (it’s now for sale here on the app store) so I loaded up a couple of instances of that for a grade-school beta test, to see what the students thought of it. To make everything a bit simpler, I had built a simple stand out of poster board and paper that shielded the plate being counted from direct light (and thus glare) at the same time ensuring that the iPhone was situated at the right height from the plate. The kids took images of the plates with the iPhone perched on top of its stand, let the automatic count do its first approximation (not bad, but it would miss a few colonies here and there), and then have fun adding in the colonies missed by the auto count by touching on them.

Over the course of the morning, we had about 25 kids come by our table and try counting colonies on our fake plates. Other than a couple of notable outliers, the final counts were within 7% of the true value. The kids seemed to have fun doing the manual adjustments, and as an added bonus, I spotted a display bug in the app that we had a chance to fix before its release to the general public.

The real benefit, though, was that the kids seemed genuinely interested in the concepts. I can only hope that they’ll retain some vague memory of plush bacteria, lego DNA, electric shocks and iPhones when they learn principles of genetic engineering for real, and perhaps it’ll help convince them that it’s really not all that magical, and can be understood with a modicum of effort.

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Maciek Smuga-Otto


  1. Maciek, Can’t help but wonder: does a sad bacterial plushy have little pili that stick up in the air as it lays on it’s back?
    Science fair sounded like lots of fun! The blog sure was fun to read. -Kari

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