The antibiotics penicillin and streptomycin are used in cell culture to prevent bacterial contamination due to their effective combined action against gram-positive and gram-negative bacteria. Penicillin acts by interfering with the turnover of the bacterial cell wall. Streptomycin acts by binding to the 30S subunit of the bacterial ribosome, leading to inhibition of protein synthesis and death in susceptible bacteria. Penicillin/Streptomycin is provided as a ready to use 100x concentrate.