Plate harvest

Measure on bacteria from plate harvest

When you prepare a test suspension for e.g., disinfection studies, one of the crucial steps is to know the concentration of viable cells in the test suspension. For most bacteria there is a very good correlation between the cells/mL concentration and colony-forming units (CFUs). Therefore, BactoBox® measurements offer an estimate for the CFU’s when you harvest bacteria directly from an agar plate.

Plate counts on bacterial clumps and chains

Bacterial aggregates are generally a pain to measure. If your bacteria form clumps or chains, neither BactoBox® nor plate counts give meaningful results.

We measure plate counts in colony-forming units (CFUs). However, a colony does not distinguish between single cells and aggregates. What does your CFU number mean then? Answer: "It depends". Not happy with that answer? Neither are we.

Always break up clumps and chains whether you measure with BactoBox® or perform plate counts.

Dilution

A filled 1 μL inoculation loop typically holds roughly 5 · 10⁸ cells/mL. This is beyond the limit of detection for BactoBox®.

First, disperse the bacteria in 9–10 mL peptone water, and you get around 5 · 10⁷ cells/mL.

Second, dilute the sample further by a factor of 1:100 (see Dilute 1:100), and you get approximately 500 000 cells/mL. This is well within the limit of detection.

Typically, 1:100 dilution in SBT-provided diluent also means that your sample has the right conductivity right away. Peptone water is good for cell preservation but has too high conductivity for BactoBox® measurement. On the other hand, SBT's diluent is good for measurements but not suited for long-term preservation. We recommend that you do the analysis within 30 minutes after you dilute in SBT-provided diluent.

Don't know the concentration of your sample?

Make a dilution series and measure the most diluted sample first. This avoids clogging your BactoBox® setup. It also avoids carry-over from your previous sample vial.

Once you are above the lower limit of detection (30 000 cells/mL), target a concentration of 0.5–4 million cells/mL.

Examples

Here is a specific example protocol for plate harvest of E. coli:

Plate harvest with E. coli