Bacteria and pH
Name: Tracy L.
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
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.
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.
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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Update: June 2012