Pen-Strep is a combination of two antibiotics—penicillin and streptomycin—commonly used in cell culture to prevent bacterial contamination. Penicillin inhibits bacterial cell wall synthesis, while streptomycin interferes with bacterial protein synthesis. Together, they help prevent the growth of a wide range of Gram-positive and Gram-negative bacteria. Pen-Strep is routinely added to culture media to protect cells from bacterial infection and maintain sterile conditions.