This is where dilution saves the day. Not just dilution, but serial dilution… meaning dilution over and over again. Why do we dilute? To have fewer colonies to count and to obtain a more accurate count.
Why do we do it repeatedly? So we might do 5 dilutions and grow up 5 plates. Sound wasteful? We use serial dilutions to create decreasing concentrations of the original sample that are then plated so that a plate will be created with a low enough number of bacteria that we can count individual colonies.
From that number, we can calculate the original cell density in the broth. A turbid broth tube contains millions of bacteria. If we transferred directly from that tube, we would get a plate that had a lawn of bacteria. This plate would be uncountable and we could not use it to estimate bacterial cell numbers.
Therefore, we need to dilute our original sample before plating. Therefore, we do a dilution series to create a number of plates, with a range of dilutions, in the hope of producing a countable plate.
We then divide this by how much we diluted the original sample to get the estimated concentration of bacteria in the original tube:. Fill in the chart below before proceeding you do not need to turn this in. Using the magic of fire to bend a glass Pasteur pipette into a spreader. Place it in your beaker of alcohol. Flame your glass spreader, wait for ALL the alcohol to evaporate and repeat this 3 times.
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