Microorganisms in the natural environment are rare in the form of single scattered cells, i.e. the so-called plankton, or rather show a tendency for adsorption at the solid-liquid, liquid-gas interface or liquid-liquid. Most often they form clusters called biofilm (or biological membrane) adhering to the surface solids or cell surfaces of other organisms. Biofilm is a multicellular formation composed of microorganisms of one or more species or genera. Both autotrophic and heterotrophic microorganisms have the ability to create biofilm, among which there are saprophytes, as well as pathogenic microorganisms. Among them are ‘next door bacteria’ but also fungi, algae and protozoa. Microorganisms, possibly thanks to their adhesive properties and structure, colonize different surfaces and due to that the resulting biofilm is stabilized by polymeric substances secreted extracellularly, the so-called EPS (extracellular polymeric substances). Biofilm is formed by complex, multicellular structures in which numerous microbial cells are surrounded by a layer of mucus. Cells of microorganisms included in the biofilm are characterized by specialization to perform different functions and exhibit different characteristics than free living cells. Construction of these clusters protects the microorganisms from the unfavorable influence by external factors and makes nutrients easier available. Due to that biofilm can function under the conditions in which single cell survival would be difficult, and in many cases even impossible.