Question

You have a tube with 5ml of infected blood that has a concentration of bacteria at 10(8) cells per ml. You wish to make a serial dilution of the blood so when you take 100 ul of the last dilution and spread it on a Petri dish you will have between 30-300 colonies growing on the plate. You have plenty of empty 1.5 ml microfuge tubes, a bottle of sterile water, a p1000 pipettor (100-1000ul) and tips, for the dilutions, BUT you can only have ONE petri dish. Clearly diagram how you would make these dilitions, add them to the plate, and prepare for incubation. Also, show exactly how many colonies you would expect to have on the plate when using your procedure. Don't forget to clearly label the amounts measured and transferred. One of your goals is to make these dilutions with the fewest possible numbers of tubes and materials.

Answer 1

Initial concentration of bacteria in infected blood is 10⁸ bacteria/ml. As we have only p1000 pipettor we can make highest 10 fold serial dilution. Therefore, the serial dilution will be as follows-

I would expect 100 colonies on the plate.