Pen-strep is a combined antibiotic supplied as a 100X stock, giving a working concentration of 100 units per mL penicillin and 100 micrograms per mL streptomycin. It acts on bacteria only and has no effect on yeast, fungi or mycoplasma. Most quality guidance advises against routine use, because it masks low level contamination and lets poor aseptic technique go unnoticed.
Penicillin-streptomycin, usually shortened to Pen-Strep, is the most widely used antibiotic combination in cell culture. Researchers add it to media to guard against bacterial contamination during routine work. This guide explains what Pen-Strep is, how it works, how to use it, and when it is better left out.
Key Facts
- Type: Antibiotic mix (penicillin + streptomycin)
- Covers: Gram-positive and Gram-negative bacteria
- Working conc.: 1X (~100 U/mL pen, 100 ug/mL strep)
- Note: Does not control mycoplasma
What is Pen-Strep?
Pen-Strep is a ready-to-use mix of two antibiotics: penicillin, which targets Gram-positive bacteria, and streptomycin, which targets Gram-negative bacteria. Together they cover a broad range of common bacterial contaminants. It is usually sold as a concentrated stock (commonly 100X) that is diluted into complete media.
How does Pen-Strep work?
Penicillin blocks construction of the bacterial cell wall, causing susceptible bacteria to burst. Streptomycin binds the bacterial ribosome and interferes with protein synthesis. Because they act on structures that mammalian cells do not have, the combination controls bacteria while leaving cultured cells largely unaffected at standard concentrations.
How to use Pen-Strep
Pen-Strep is typically added to complete media at a 1X working concentration, giving common final levels of about 100 units/mL penicillin and 100 micrograms/mL streptomycin. Add it when you prepare complete media, along with serum and glutamine. Aliquot and freeze the concentrated stock to preserve activity, and thaw only what you need.
When to avoid antibiotics
Good aseptic technique, not antibiotics, is the real defense against contamination. Routine antibiotic use can mask low-level contamination and encourage resistant organisms, and some sensitive cells (and some experiments, such as certain transfections) do better without them. Many labs keep antibiotic-free cultures as a check on their sterile technique. Pen-Strep also does not control mycoplasma, which needs dedicated detection and treatment.
Order from PurMa Biologics
PurMa Biologics manufactures Penicillin-Streptomycin and other cell culture antibiotic mixes in the USA, each lot backed by a Certificate of Analysis. See our Penicillin/Streptomycin Antibiotic Mix (100X) or browse the full range of cell culture antibiotics. Questions or need a quote? Contact our team.
