Cell culture
Rinsing cells with calcium- and magnesium-free PBS removes serum and medium before passaging. Serum can inhibit trypsin, and divalent ions keep cells attached. PBS is also used to wash pelleted cells and to resuspend them briefly before counting or plating.
Immunoassays: ELISA, Western blot and immunostaining
PBS is the base of many wash and blocking buffers. Adding a mild detergent such as Tween-20 (often 0.05 to 0.1%) gives PBST, which lowers background binding. Blocking buffers are often PBS with a protein such as BSA or nonfat milk.
Diluting antibodies and proteins
Its neutral pH and physiological ionic strength make PBS a common diluent for antibodies, enzymes and other proteins. Check compatibility first: phosphate can inhibit some enzymes, including alkaline phosphatase, and it precipitates with calcium and some metal ions.
Tissue and sample handling
PBS is used to rinse dissected tissue, to perfuse and wash samples, and as the base for fixatives such as paraformaldehyde in PBS.
Flow cytometry
Staining and wash buffers for flow cytometry are usually PBS with added protein (for example BSA) and sometimes EDTA to prevent clumping.
When not to use PBS
- Workflows sensitive to phosphate, such as some kinase and phosphatase assays
- Solutions that also need calcium or zinc at meaningful concentrations, which can precipitate
- Long incubations of live cells, where a full culture medium is better
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