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Name: Tracy L.
Status: Student
Age: 16
Location: N/A
Country: N/A
Date: N/A 

I have done a chemistry experiment related to pH levels (using NaOH and H2SO4)on bacteria in soil. After incubating agar plates for two days, the percentage cover of bacteria grown is estimated for each plate.

I have repeated the experiment about four times and results show no correlation. Are bacteria affected by pH levels at all?

Yes they are, some bacteria can grow at high pH only, some at low pH, some have a broad pH range and others a narrow range. I guess the reason your repeated experiments do not show similar results is that your inoculum was not standardized. In other words, if you used (a standard amount of) soil from different batches you would get different results every time you did the experiment. Try it this way: take one sample of soil and do the experiment in 4-fold all at once, make sure that every plate gets exactly the same amout of soil-derived bacteria all from the same batch. Then you should see an effect of pH.

By the way, did you pretreat the soil with low and high pH first, and then plate out on identical plates? or did you try to change the pH of the plates with NaOH and H2SO4? You should be aware that most bacterial growth media are strongly buffered. Have you checked that the high and low pH you wanted to induce were indeed achieved?

Good luck with the experiments.
Trudy Wassenaar

It is difficult to answer your question not knowing what the bacteria species are -- no doubt some are more sensitive the pH than others -- and what the range of pH is. If the pH is high enough, or low enough -- whatever that might mean for your particular case -- certainly the growth of the bacteria would be affected. Try increasing/decreasing the pH in increments of one unit and see what happens. In addition, the soil may act as a buffer that will require you to make a more robust change in the pH.

Vince Calder

Yes, bacteria ARE affected by pH levels. You are assuming that they are all affected the same way however. If you are looking only at total growth, you may see no difference because when one type of bacteria are killed, it leaves more room for other bacteria to grow. You would have to isolate the different kinds of bacteria and test them separately.


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