Materials and surfaces can give off substances which serve as a food source for microorganisms such as bacteria, algae or fungi. Microorganisms could accumulate and multiply. However, plastics could not only represent a food source to microorganisms, materials and surfaces may also be altered by the products of metabolism generated by the microorganisms (e.g. discoloration). This may affect the service life of materials and enable microorganisms to gain uncontrolled access to critical areas.