I am trying to test the pH drop by oral microbes in the medium. Most biological buffers are designed for slightly different working pH ranges.
Any suggestions how does Citric Acid – Na2HPO4 buffer or sodium citrate/citric acid, effect the oral microbial growth or change any metabolic properties?.
I plan to try a glycine-HCl system, but I'm not too confident about it, the HCl doesn't sound cell-culture friendly
Thanks in advance
For assessing the buffering capacity you just have you check out the pKa values of your intended buffer compounds. Citric acid has 3 pKa values at 3.13, 4.76 and 6.40 and phosphoric acid has also three at 2.15, 7.2, and 11.9, while glycine has only two at 2.35 and 9.7. The pKa of HCl is irrelevant because it is a strong acid and will always fully dissociate in water. Every pKa value provides a buffering plateau in the respective mixture, therefore a citrate/H2PO4 system has a very good buffering capacity from pH 2 to about pH 8. If you use the Gly/HCl system, you have only buffering around pH 2.35, but an unbuffered system at all other pH values.
As for the potantial toxic effects of the buffer compounds, that depends always on the bacteria you work with. However, most oral bacteria should be "used" to see citric acid, phosphoric acid, glycine and HCl all the time. They all are common nutrients and occur in food sources, sometimes in high concentrations (e.g. phosphoric acid in Coca Cola). It may still be a matter of concentration to become inhibiting, therefore I would recommend to keep the concentrations as low as possible from your experimental setup. E. coli does not mind phosphate buffer concentrations (arond pH 7) of 100 mM or more, but some other environmental bacteria are known to stop growing at phosphate concentrations of 30-40 mM. For organic compounds like citrate, uptake and metabolism may create additional problems, but this is just a matter of trial and error. As for HCl, you don't have to worry about tox effects because it will equilibrate into H+ and Cl- in solution and accumulate only at very acidic conditions which will be unsuiotable for growth anyway. However, as a strong acid it will not contribute to buffering the pH itself. If you want a wide area of pH buffered, it is therefore better to use a weak acid and a weak base with overlapping pKa values. If you don't like the citrate/phosphate system, get a pKa table, select another system along the lines outlined above, and try whether your intended experiment works with it.
Dear Johann Heider, thank you very much for the answer. I am studying the electrochemistry of some oral microbes so i need a good buffer range (2 to 7), to observe the pH drop. As you said i was worried too about the effect of HCL/citric acid on oral microbes metabolism although it depends on the concentration levels.
Any way i will just try and see the microbe activity and compare with my previous results.
Thanks alot again
Best regards
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