
Hello! I am teaching 6th11th grade sciences. I
have a B.S. in science, but only took a year or so
of microbiology in college. Every year we have
students who want to do microbe projects for
science fair. We have good success growing
microbes in petri dishes. As yet, my best method
for measuring microbe growth is to give students a
5mm x 5mm grid which they lay on top of the petri
dish and have them estimate how much of these
boxes are filled with the microbe that has grown.
Then they add up the portions of the boxes or
whole boxes filled by the microbes and arrive at
an mm squared value. One science fair judge asked
about the height of microbe growth. I guess
students could arrive at an estimate of height by
the same method. I have done online searches
which have only yielded dilution methods. We do
not have any specialized equipment for observing
microbes in dilution, nor do I have stains for the
various microbes to show up on a microscope slide.
Any websites
or suggestions you can give a generalist 6th12th
grade science teacher would be very much
appreciated. Thank you!   Answer 1:
The most accurate way to quantify microbes
on a petri dish is to dilute them to the point
that you expect about less than 100 microbes.
Then you let them grow and count them taking into
account your dilution. Each little dot you see
will have about 100 million to a billion bacteria
(assuming you are plating bacteria).
Unfortunately, if you don’t dilute the microbes
somehow, you’ll get “lawns” which are smears of
cells. There is really no good way to count the
number of cells in a lawn. Using the area covered
by the cells is not that accurate because cells
will grow until they get to close to other cells
which gives you a very inhomogeneous result of
bacterial growth. Also, it’s possible that some
parts of the dish may be more suitable for growth
than other parts which would give you clusters of
cells in certain parts. The height of the microbes
is irrelevant because they will generally only
grow on the top of the dish in a very thin section
that you would have trouble measuring.
If you are starting with a dish with microbes,
then you could scrape off the top and put them
into a liquid that won’t kill the microbe, but it
won’t grow too much in (phosphate buffered saline
is usually best). Then make sure the solution is
well mixed. You could then use a pipet to transfer
a small amount of the liquid to a large amount of
liquid to dilute it, and plate the result. Let the
plate incubate overnight (best is around 100
Fahrenheit). Keep diluting until you get a dish
with circular, well separated bacterial colonies
that you can count. Take into account the dilution
used to get those well separates bacteria and then
you can have an idea of the total amount of
bacteria on the original plate. This would be a
lot more work intensive, but also a lot more
accurate than using area of microbes.
  Answer 2:
While you could use a ruler, I doubt you would get
any precise measurements, so I'm not sure how you
could measure the volume of the bacterial growth.
Depending on the bacteria, you may be able to
detect metabolism by the dissolution of the agar,
if you have a very fine scale to weigh them on
(you would need that, though).
  Answer 3:
In our lab, we measure microbial growth
using a spectrophotometer to estimate the density
of cells in solution. The more turbid the
solution gets, the more microbial growth we have.
I understand that you probably don’t have this
instrument in your classroom, and I imagine it’s
probably pretty difficult to measure how many
cells are on a petri dish. Can you see
individual colonies, or do the bacterial form into
big globs? If you can’t see individual
colonies, it’s hard to arrive at a quantitative
result of bacterial growth. But, you can still use
a dilution method and petri dishes to arrive at a
useful estimation! You can do this with a colony
forming unit assay, which is used to estimate the
number of bacteria cells in a sample.
For this assay, a solution of bacteria at an
unknown concentration is serially diluted in order
to obtain a plate with a countable number of
bacteria (spots of bacteria colonies). The
standard formula to obtain a colony count is CFU =
total dilution used to make the plate x the volume
of cell solution added to the plate. When
I’ve done this assay, I’ve often had to dilute it
by 1,000,000,000 just to get a countable number!
A useful website explaining this protocol can
be found:
here
I hope this helps!
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