Nitrifying bacteria are classified as obligate chemo-lithotrophs. This simply means that they must use inorganic salts as an energy source, and generally cannot utilize organic materials. They must oxidize ammonia and nitrites for their energy, and fix inorganic carbon dioxide (CO2) to fulfill their carbon requirements. They are largely non-motile and must colonize surfaces such as gravel, sand or synthetic bio-media, for optimum growth. They secrete a sticky, slime matrix, which they use to attach themselves.