Science stuff ya probably need to know - Serial Dilution

So, when working in science, you may often get yourself landed with a sample of a test liquid in order for you to estimate many of its properties. For example, it may be a water sewage sample from a water treatment plant, from which, you might want to know how many E. coli bacteria are present in it.
One method to do this is to plonk a known volume (usually 1ml) on a petri plate that’s been covered with a growth media that the E. coli are well known to thrive upon. (The idea is that each separate E. coli (or other bacteria of interest) forms on a growth media to form a separate observable colony what can be counted later). They are then left in an incubator (a posh oven) at 37oC (their favourite growth temperature) for them to do their thing. Which is great, cos after a day you should be able to see the growth of the bacteria on the surface of the media, which is likely to be a complete layer if the number of E. coli in the test sample was really high (10 x 10(9) colonies per ml) which is usually the case with these things, and thus, does not enable us to be able to count the number of E. coli colonies hanging out in the original sample. To be able to do this, we must dilute the starting sample maybe 5 or 6 orders of magnitude, which is done by serial dilution.
OK, so what you have to do here is, you simply take 1ml of your initial sample and add it to 9ml of distilled water – mix thoroughly… then bingo - you have a 1:10 dilution.
To go further, you then take 1ml of this 1:10 solution and add it to 9ml of distilled water, mix - and this’ll give you a 1:100 dilution. And there you have it - just continue like this taking 1ml of the preceding dilution, adding it to 9ml of distilled water for 1:1000, 1:10000… it could just go on forever, but you can stop when you reach your desired level of dilution!
Then, simply pop 1ml of your chosen dilution on a growth media, incubate, and count the separate colonies – usually, for best results, we aim for a coverage of between 30 and 300 colonies per plate.

© Willow Science 2021