## Overall procedure
Use a centrifuge tube with a built-in membrane filter (like the tubes pictured above). The overall steps are like this:
1. Load your sample into the concentrator (4 in the figure below)
2. Centrifuge the tube
3. The bacteria now sits on the membrane (5 in the figure below)
4. Resuspend the bacteria in [diluent](/encyclopedia/item-register/vials-flasks-and-liquids/dilution-liquid-5-l.md)
After step 4, you have a concentrated sample vial.
The bacteria stay on the retentate side (5 in the figure below) because the membrane pores are too small to let them through. Liquid and salts pushes through the membrane to the filtrate container (6 in the figure below).
See also [Filter-centrifuge your sample with Vivaspin® 20](/protocols/prepare-your-sample/hit-the-right-concentration/concentrate-your-sample/filter-centrifuge-your-sample/filter-centrifuge-your-sample-with-vivaspin-r-20.md).
## What kind of filter membranes?
We have found that 1 million MWCO membranes work best in terms of performance and recovery of intact bacteria. Several sizing options exist and we list some below.
### Vivaspin® 20
For standard benchtop centrifuges, [Vivaspin® 20](https://shop.sartorius.com/dk/p/vivaspin-20-centrifugal-concentrator-polyethersulfone-12-pc/VS2061) is easy to use. It fits in standard 50 mL centrifuge tube rotors.
You can even combine pressurization (see 2 in the figure above) and centrifugation.
{% content-ref url="/pages/3HZ7N10QDpXjgnlpaLwh" %}
[Filter-centrifuge your sample with Vivaspin® 20](/protocols/prepare-your-sample/hit-the-right-concentration/concentrate-your-sample/filter-centrifuge-your-sample/filter-centrifuge-your-sample-with-vivaspin-r-20.md)
{% endcontent-ref %}
### Vivaspin® 100
[Vivaspin® 100](https://shop.sartorius.com/dk/p/vivaspin-100-ultrafiltration-units/M_Vivaspin_100) has larger sample volume (90 mL). This allows you to concentrate more sample at a time. Like Vivaspin® 20, you can also fit the tube with a pressure cap.
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