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Differentiation of E. Coli and Enterobacter Aerogenes

These two bacterial species are identical in many superficial ways. They both produce white colonies at 37 degrees Celsius on nutrient agar. They both ferment lactose producing acid and gas. They are both Gram negative and virtually identical microscopically.

To distinguish them we must resort to biochemical tests. They metabolize sugars and other organic compounds, like Citrate, differently. Below are some of the tests we do to distinguish E. coli and E. aerogenes. Note how clearly they differ when we use biochemistry to tell them apart!

Below on the left E. coli and Enterobacter aerogenes are growing on the surface of Eosin-Methylene Blue Agar. There is glucose in this medium which both bacteria metabolize. But they both produce different byproducts.

E. coli produces a byproduct of glucose metabolism which reacts with the basic dyes in the medium producing a "green metallic sheen" in reflected light. Enterobacter produces normal colonies. The EMB test differentiates between E. coli and E. aerogenes sharply.


E. coli is seen on the left and E. aerogenes on the right

The Indole Test tests for the presence of a double carbon ringed molecule called indol, which is a product of the enzymatic splitting of the amino acid Tryptophan. If indol is produced from tryptophan Kovac's reagent reacts with it producing a deep red color. Below are Echerischia coli and Proteus vulgaris in Sulfide Indole motility medium tested with Kovacs. Both are positive for Indole.

Figures 1 and 2. On the left see the thin red layer at top of each tube. Both organisms are positive for Indol. Above right is the MR test. In a glucose broth, specially buffered, E. coli drops the pH to around 5. Methyl Red indicator turns red at this pH and the test for coli is MR+. E. aerogenes is the yellow tube with a pH above 5.5.

Below is a further simple test we can do to differentiate E. coli and E. aerogenes. One of the two (aerogenes) can use citrate, (a four carbon molecule), as an energy source while the other cannot. If the bacteria can use Citrate the blue indicator (bromo Thymol Blue) turns from green to blue as the pH changes. Also, we see growth on the surface of the slant.

Note both Below

E. Bollenbach NCCC Fall 2001