The two dilutions I commonly make are 1 in 100 (1 part of sample into 99 parts diluent) and 1 in 10. When I plate the sample I plate 0.1 ml so that adds another 1 in 10 dilution in the end. I multiply the number of colonies on the resultant plates by the number of times I diluted and it gives me the number of cells per ml in the original sample. Not diluted enough the bacteria form a smear across the plate, if too diluted then only a few colonies if any are present, I want approximately 20-250 colonies to have what is considered statistically significant results. And at risk of losing all of my beloved readers and having Baby Sibling make fun of how incredibly dorky I am I have made a video of me doing a dilution set and spread plating it:
You are still here??? Great! Now that you have gotten to laugh at me how about giving me a laugh or two (with you not at you), it is time for the next Guess That Gizmo!
Sorry about the not so sticky biohazard sticker, they just don't make 'em like they used to I guess:
And remember, this is a semester long contest so new readers can participate too, here are the rest of the Fall 2008 Semester GTGs: