Hit the right concentration

Samples with high concentration are usually turbid.

Samples with low concentration are usually transparent.

If the concentration in your sample is too high, we dilute it. If the concentration is too low, we concentrate your sample. In either case, we try to hit the target of 0.5–4 million cells/mL. See the table below for our recommendations:

Concentration in your sample	Dilution	How-to guide(s)
5 · 1011 cells/mL	10-6	Three Dilute 1:100 in sequence.
5 · 1010 cells/mL	10-5	Dilute 1:100 000.
5 · 109 cells/mL	10-4	Dilute 1:10 000
5 · 108 cells/mL	1 0-3	Dilute 1:1 000
5 · 107 cells/mL	10-2	Preferred Dilute 1:100, alternatively Dilute 1:1 000 if measurement gives a conductivity error.
5 · 106 cells/mL	1:10	Dilute 1:100. Contact SBT for guidance or review Hit the right conductivity if you receive conductivity errors.
5 · 105 cells/mL	N/A	Dilute 1:10⚠️ You must use custom diluent.
≤5 · 104 cells/mL	N/A	Generally it is not recommended to use BactoBox for very dilute samples. For workarounds, see Concentrate your sample.

You can do combinations to get even high dilution factors. Just like how we do three Dilute 1:100 steps in a row to get a 1:1 000 000 dilution overall.

Quick 1-step dilutions

The above dilution principles rely on integers divisible by 10. This straightforward method typically works well for samples with high bacterial concentrations. However, there are situations where these dilutions may not be optimal.

• For highly conductive broths, some cultivation media may require a dilution greater than 1:100 to achieve acceptable conductivity levels. In such cases, performing a 1:201 dilution by adding 50 µL of the sample to 10 mL of diluent might be beneficial.

• For rapid and rough qualitative assessments, a 1-step dilution is efficient and conserves consumables. For example, you can mix 10 µL of your sample with 10 mL of diluent to achieve a 1:1 001 dilution.

Ensure precise pipetting to maintain accuracy, especially when handling small volumes.

Sample volume	Diluent volume	Dilution
80 µL	10 mL	126
50 µL	10 mL	201
40 µL	10 mL	251
25 µL	10 mL	501
10 µL	10 mL	1 001
8 µL	10 mL	1 251

Target 0.5–4 million cells/mL for fast results

The more things to count, the faster we are certain about the concentration. Give BactoBox® something to work with and it triggers the Fast results feature ⏩.

Make a dilution series and measure the most diluted sample first. Aim for 0.5–4 million cells/mL.

Very low bacterial concentration?

BactoBox® is generally not suitable for very low bacterial concentrations.

Motivation

BactoBox® counts individual particles. If there are too few particles, the concentration estimate is too uncertain. If there are too many particles, we can't tell them apart. Therefore, we have limit of detection (LoD). BactoBox® outputs concentrations in these ranges:

• 30 000–5 000 000 cells/mL (3·10⁴ to 5·10⁵ cells/mL)

• 30 000–5 000 000 total/mL (3·10⁴ to 5·10⁵ total/mL)

See limits of detection for more details.

BactoBox® uses an electric field to measure individual cells. Below the LoD (top), next-to-nothing enters the electric field and the cells/mL concentration is uncertain. Within the LoD (middle), individual cells enter the electric field so we can tell them apart and count them. Above the LoD (bottom), multiple cells enter the electric field at the same time and we can't tell them apart.