Pen-Strep is a combination of two antibiotics: penicillin and streptomycin, commonly used in cell culture to prevent bacterial contamination. Penicillin inhibits the synthesis of bacterial cell walls, while streptomycin interferes with bacterial protein synthesis. This combination helps maintain the sterility of cultures by preventing the growth of a wide range of Gram-positive and Gram-negative bacteria. Pen-Strep is often added to growth media for routine cell culture to protect cells from bacterial infections.