he antibiotics penicillin and streptomycin are used to prevent bacterial contamination of cell cultures due to their effective combined action against gram-positive and gram-negative bacteria. Penicillin was originally purified from the fungi Penicillium and acts by interfering directly with the turnover of the bacterial cell wall and indirectly by triggering the release of enzymes that further alter the cell wall. Streptomycin was originally purified from Streptomyces griseus. It acts by binding to the 30S subunit of the bacterial ribosome, leading to inhibition of protein synthesis and death in susceptible bacteria. This solution contains 10,000 units/mL of penicillin and 10,000 µg/mL of streptomycin in a 10 mM citrate buffer (for pH stability).