A biofilm is a community of microorganisms adsorbed to a surface.  Microorganisms in biofilms are enclosed in a polymeric matrix consisting of exopolysaccharides, extracellular DNA and proteins.  Seconds after a surface (usually metal) is placed in a solution, inorganic and organic molecules adsorb onto the surface. These molecules are attracted mainly by Coulombic forces (see above section), and can adhere very strongly to the surface. This first layer is called the conditioning layer, and is necessary for the microorganisms to bind to the surface. These microorganisms then attach reversibly by Van der Waals forces, followed by irreversible adhesion through self-produced attachment structures such as pili or flagella.  Biofilms form on solid substrates such as stainless steel.  A biofilm's enclosing polymeric matrix offers protection to its microbes, increasing their resistance to detergents and cleaning agents.  Biofilms on food processing surfaces can be a biological hazard to food safety.  Increased chemical resistance in biofilms can lead to a persistent contamination condition.