Edible Acids and Bacteria Growth Science Fair Projects

Methods/Materials

Five 50 ml samples of LB broth were buffered with acetic acid at pH 4 and 5, with MES at pH 6 and with HEPES at pH 7 and 8. Each sample of medium was transferred to a 250 ml flask and 0.5 ml of a saturated overnight ulture of E. coli was added to each flask. A sample was taken from each flask at times 0, 0.5, 1, 2, 3, and 4 hours. The amount of bacteria in each sample was measured using a spectrophotometer at 595 nm. The OD(595) and time data were plotted to show growth curves at each pH tested.

Results

My first experiment showed that the bacteria grew at identical rates at pH 6, 7 and 8 and did not grow at pH 4 and 5. Since I used a different buffer at pH 4 and 5 I was concerned that the failure to grow at these pH levels was due to a contaminant in the buffer. I repeated the experiment with a differnet lot of acetic acid to test this idea. My second experioment gave the saame result as the first: no growth at pH 4 and 5 and equal growth at pH 6, 7 and 8.

Conclusions/Discussion

I conclude that E. coli bacteria grow best at a near neutral pH of 6-8. They do not grow at a more acidic pH of 4 or 5. The advice to add acids to food appears to be a valid way to prevent the growth of E. coli. My experiments used a single non-pathogenic strain of E. coli and should be repeated with pathogenic strains of E. coli and other bacteia species to reach a more general conclusion on the value of low pH for preventing food contamination. It is possible that acetic acid is toxic to E. coli and therefore my experiments should be repeated using a different buffer at pH 4 and 5. 3rd party contributor