A biofilm is a spatial structure in which clusters of microorganisms sticking to each other form a colony surrounded by a specific matrix. This 'structure' separates the whole colony from other microorganisms, changes its phenotype and increases the resistance of the whole structure. This is of great importance both for the survival of the microorganism and for the patient with a biofilm-based infection. The immunity of the bacteria thus raised makes them resistant to treatment and the host's defence mechanisms.