What is a biofilm ? In microbiology, knowledge has traditionally been gained from studies of suspensions of cells grown from a single cell in laboratory culture plates. These free-floating or “planctonic” cells have for instance been used in studies of how well antibiotics can kill bacteria. However, microbes can also aggregate as biofilms – i.e. organized layers of cells attached to a surface. In nature, probably 99% or more of all bacteria exist in biofilms. For instance, in an alpine stream there is typically only 10 bacteria per ml., whereas bacteria living in slimy biofilms on nearby rocks can occur in numbers like 5 x 10 per square centimetre. Biofilms, which the bacteria form naturally, provide a way of surviving e.g. UV and drying, as the slimy mass they are living within acts as a protecting layer. When a bacterium attaches to a hard surface in a moist environment gene expression is adapted to the new environment. Some genes are up-regulated whereas others are depressed or turned off.