Coke vs. Pepsi: The E. coli Test

The following is an article from The Annals of Improbable Research.

(Image credit: Flickr user Jeff M for Short)

by Ella Rotman
Department of Microbiology
University of Illinois, Urbana-Champaign

Here I demonstrate that bacteria can be used as low-cost, “guinea pig” substitutes for human beings for testing new (or old) food products. I tested the ability of an Escherichia coli bacteria strain to grow in a Coca Cola medium. I then determined whether the bacteria preferred Coke or Pepsi.

Getting E. coli to grow on Coke
E. coli are finicky eaters. To grow them under laboratory conditions sometimes involves following elaborate recipes. To test E. coli‘s preference for one versus another cola product, I first had to create a suitable growth medium.  I adapted a well-established formula, similar to that for Gatorade. It provides sugar for energy and various salts and metabolites required for growth. I added a solidifying agent, so the bacteria could be purified to single colonies in a Petri dish. Because the two main ingredients of Coca Cola are water and high-fructose corn syrup, I simply replaced the formula’s prescribed water and sugar components with an equivalent volume of soda.

My “soda medium” had an additional benefit: E. coli cannot grow in pure Coke because of the acidity (pH of ~2-3). In my medium, the salts buffered the solution to the nearly neutral pH of approximately 6.5-7.

Figure 1. Training E. coli to like Coke. PLATE A: E. coli struggling on a caffeine-free Coca Cola plate. PLATE B: E. coli adapted to Coca Cola (streaked on the lower half of the plate).

I still had to coax the E. coli to grow, though. At first the cells grew quite poorly, and I thought it might have been due to caffeine toxicity. However, when I streaked the cells on a similar medium, but prepared with caffeine-free Coke rather than regular Coke, I got the same results (see Figure 1A). Since I was unable to remove any of the other potentially growth-inhibitory ingredients (e.g. caramel color or the mysterious “natural flavors,” which the Coca-Cola Company declines to identify), I hoped the cells could be trained to eat, and perhaps even enjoy, Coke. Happily, when I used cells that had been previously grown on the Coca Cola medium, I was able to obtain discrete colonies (see Figure 1B). This suggests that Coca Cola is an acquired taste.

The Pepsi Challenge
Once I had the bacteria cells growing on soda, I tested to see whether they preferred Coke or Pepsi. I inoculated the E. coli into liquid media made from each beverage, and then tracked their growth over the course of the day. I used a spectrophotometer to measure the density of cells in the tubes over time.

The results of the experiment are shown in Figure 2. Clearly, the bacteria grow faster on Pepsi than on Coke. On solid plates, the E. coli also formed larger colonies on the Pepsi medium than on the Coke medium (data not shown).

Figure 2. Growth of E. coli (strain MG1655) in caffeine-free Coca Cola medium vs. caffeine-free Pepsi Cola medium. The results are an average of three independent trials with error bars representing the standard error of the mean.

I have shown that, when given a choice between Coke and Pepsi, E. coli prefer Pepsi. It is also interesting to note that, although the bacteria were initially trained to like Coke, they still preferred the competition.

I thank the Kuzminov Lab for use of their reagents and equipment, as well as Brian Budke for helping to measure the growing cultures. I also thank Caryn Wadler for her assistance in making the stuffed bug.


The article above is from the January-February 2007 issue of the Annals of Improbable Research. You can download or purchase back issues of the magazine, or subscribe to receive future issues. Or get a subscription for someone as a gift!

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