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