What Is Pen-Strep? Penicillin-Streptomycin in Cell Culture

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.

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.