Country: United States
Date: January 2006
As to an answer about the E coli in the NEWTON archives stating
that after incubating for 24 hours, the culture should be turbid and
contain 10^8 and 10^9 cells/ml. My question is how can we determine
You have to obtain a Petroff-Hauser counting chamber. This is a glass slide
with very fine etchings and a coverslip. Whan viewed under a microscope (40X
objective), each "square" represents 0.00000005 mL. After placing a drop of
the bacterial culture on the slide, you count the number of bacteria in
several squares (at least 100 cells) and then calculate the average number
of cells per square and then multiply the average by 20,000,000 to find the
number of cells/mL.
A countin Petroff-Hauser counting chamber can be purchased from VWR
Ron Baker, Ph.D.
One could do a serial dilution. Since we don't know exactly how many
organisms are there, we have to make many dilutions of the broth until we
reach a dilution that can be counted. We dilute the broth 1/10, 1/100,
1/1000 etc. usually 6-7 times. We plate 1 ml of each dilution and
incubate. We count a plate that has between 30-300 organisms. Then we
multiply back times the dilution factor. So for instance, if you count 125
organisms on the 1/10,000 plate, you multiply 125 x 10,000. That is how
many organisms were in the original broth.
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Update: June 2012