Membrane filtration for microbiological testing, step by step

The core idea: pass a measured sample volume through a membrane fine enough to retain the organisms of interest, then transfer that membrane onto a nutrient source and incubate. What grows (or doesn't) on the membrane surface is what gets counted or identified.

1. Choose the membrane

Pore size is the first decision—0.45 µm and 0.22 µm are common choices depending on the organism and method being followed. Gridded membranes make colony counting easier under magnification; non-gridded membranes are common where the grid isn't needed or could interfere with a downstream step. Material matters too: mixed cellulose ester (MCE) membranes are widely used for general microbiological recovery, while PCTE's flat, low-background surface is common where a clean field for microscopy or particulate work matters.

2. Filter the sample

Vacuum filtration through a funnel-and-membrane setup is the typical format for water and liquid samples. Funnel and membrane diameter should match, and the filtration hardware needs to be sterile (or sterilized between samples) to avoid introducing contamination that isn't actually in the sample.

3. Transfer to media

The membrane is placed directly onto (or into) a nutrient medium appropriate to the target organism—selective media for a specific organism group, or general media for a broader recovery count. This is where the method being followed (a validated protocol, a pharmacopeial method, an internal SOP) drives the exact medium and incubation conditions, not a generic default.

4. Incubate and read

Incubation time and temperature are method-specific and organism-specific—this is not a step to standardize generically across different testing programs. Reading typically means counting colonies on the membrane surface, sometimes with the help of a grid pattern printed on the membrane itself.

5. Documentation and traceability

For regulated or audited workflows, keep the filter lot, media lot, and any relevant certificates tied to the run. If a specific product's documentation (CoA, sterility certificate) is required for your quality system, request it at the time of quote—Cyverna does not assume every workflow needs the same paperwork.

A note on scope

This is a general orientation to the technique, not a validated protocol. Organism-specific method requirements (compendial methods, regulatory methods, internal SOPs) should be the actual source of truth for pore size, media, and incubation parameters in your workflow.