For the cross-check, you will also need the plate count results. The cell concentration provided by BactoBox® measurements gives you a good estimate of how much to dilute the culture. We refer to this as the assumed optimal dilution.
We include the vials next to the expected optimal dilution to account for potential differences between BactoBox and plate counts. We refer to this as Bracketing dilutions.
The strategy for hitting the optimal dilutions are detailed below. We assume that 1 mL is plated. More information is available in Plate smarter.
Dilution series for plate counts. The 15 mL centrifuge vial has a concentration of 1×1010 cells/mL. A 1:10 dilution is prepared by transferring 1 mL sample to 9 mL buffered peptone water (or similar). The lowest numbers shows the cells/mL for the individual vials. Eight 1:10 dilutions are needed to dilute to 100 cells/mL. We recommend Bracketing dilutions. Use the cyan bracket when plating 1 mL. Use the magenta bracket when plating 0.1 mL.
Step by step
1
Write concentration with thousand separators
Write cells/mL × dil. with thousand separators.
If the cell concentration was 1×1010 cells/mL, you would write 10 000 000 000 .
2
Determine the assumed optimal dilution
Determine how many zeros to remove to reach 25–250 cells/mL.
In the above example you remove eight zeros from 10 000 000 000 to get 100.
In this example it requires eight 1:10 dilutions to dilute to 25 - 250 cells/mL. 10-8 is therefore the assumed optimal dilution when you plate 1 mL.
3
Bracketing: Include the dilutions next to the optimal dilution
Include the two vials next to the assumed optimal dilution for plating. This "bracketing" approach is a precaution for potential differences between BactoBox® and plate counts.
In the illustrated example we would include the 10-7 and 10-9 vial. In this example we would plate the 10-7, 10-8, and 10-9 vials (cyan bracket).
Use one dilution less if you are only plating 0.1 mL
The above calculations assume you use 1 mL for plating. This corresponds to the cyan bracket in the illustrated example.
Use one dilution less if you are using the spread plate method with only 0.1 mL. This corresponds to the magenta bracket in the above illustration.
Summary
You have now determined which dilutions to use for plating. Next step is Plate dilutions
