# Plate harvest

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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.

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## 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.

Plate counts are measured 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.
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## Dilution

A filled 1 μL inoculation loop typically holds roughly 5 · 10<sup>8</sup> cells/mL. This is beyond the [limit of detection](https://help.sbtinstruments.com/encyclopedia/measurements-explained/limits-of-detection) for BactoBox®.

First, disperse the bacteria in 9–10 mL peptone water, and you get around 5 · 10<sup>7</sup> cells/mL.

Second, dilute the sample further by a factor of 1:100 (see [dilute-1-100](https://help.sbtinstruments.com/protocols/prepare-your-sample/hit-the-right-concentration/dilute-1-100 "mention")), and you get approximately 500 000 cells/mL. This is well within the limit of detection.

Typically, 1:100 dilution in [SBT diluent](https://help.sbtinstruments.com/encyclopedia/item-register/vials-flasks-and-liquids/dilution-liquid-5-l) 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 diluent.

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## Don't know the concentration of your sample?

Make a dilution series and [**measure the most diluted sample first**](https://help.sbtinstruments.com/protocols/hit-the-right-concentration/dilute-1-100#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 0.5–4 million cells/mL.
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## Examples

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

{% content-ref url="enumerate-bacteria-from-plate-harvests/protocol-exemplified-with-e.-coli" %}
[protocol-exemplified-with-e.-coli](https://help.sbtinstruments.com/protocols/prepare-your-sample/applications/enumerate-bacteria-from-plate-harvests/protocol-exemplified-with-e.-coli)
{% endcontent-ref %}