For my science fair project, I did an experiment on how paracetamol and ibuprofen affected the growth of bacteria. By day 5, however, parts of the paracetamol blackened (image below). I am wondering if E. Coli can metabolize paracetamol and if so, what operon in the K-12 genome is responsible for this.

image of petri dish

EDIT: I realized it may be metabolizing other ingredients in the medicine I used. I am including a list of the ingredients in the medicine I used.

Acetaminophen 160 mg in each 5 mL (duh)

Other ingredients (33.015873 mL):

  • Anhydrous citric acid
  • FD&C red no. 40
  • glycerin
  • high fructose corn syrup
  • microcrystalline cellulose and carboxymethylcellulose sodium
  • purified water
  • sodium benzoate
  • sorbitol solution
  • sucralose
  • xanthan gum

I thought it might be the fructose, but is the byproduct really black? Still confused...

  • $\begingroup$ I thin, you are using blood agar plates, isn't it? In this case, it could also be the heme which can cause the black colour. $\endgroup$
    – SeRe
    Nov 20, 2017 at 10:34
  • $\begingroup$ No, the medicine I used colored the agar red. I used regular agar. $\endgroup$ Nov 20, 2017 at 15:00
  • $\begingroup$ Is it possible the dye changes colour as the pH decreases? $\endgroup$
    – canadianer
    Nov 20, 2017 at 20:02

1 Answer 1


Sorry that my answer is not a real answer, but this would be a good starting point for some more science.

Several of the questions you might ask and could answer (with some more experiments):

  • Does this happen in plates without E.coli?
  • You have a contamination on the right side, perhaps also on the black part. Could this be the culprit? Only half the plate is black, the rest is (still?) red.
  • You used a paracetamol tablet with red dye and a lot of sweeteners, does the same thing happen if you use one without the red dye? A dye would be a likely source, because it already contains an absorbing moiety. It only needs to be slightly modified to absorb different wavelengths.
  • What kind of agar are you using exactly? There are many kinds, often selecting for certain types of micro-organisms, with indicators for pH or other colored markers to detect and identify strains.

For your metabolic question you can take a look at https://ecocyc.org/ where you can find a database with all known metabolic pathways for E.coli K12. Unfortunately I couldn't find anything for paracetamol (or synonyms) but you could try to see if it produces a colored product for any of the additives.


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