Cell culture media allow not only cells but also foreign germs, such as bacteria or fungi, to grow ideally. This growth of foreign organisms, which is also referred to as “contamination”, results nearly always in the loss of the respective cell culture. Only the preventive use of antibiotics or sterile working conditions may prevent contaminations. Originally, antibiotics was the term for low-molecular metabolites of microorganisms that act as growth inhibitors against other microorganisms or kill the latter. Today, the term antibiotics comprises all substances (also virostatics and chemotherapeutics) that are effective against any type of microorganism (bacteria, viruses, fungi), no matter if they are of low or high molecular nature or if they were produced synthetically or by the microorganisms themselves.