Bacterial ecology is concerned with the interactions between bacteria and their biological and nonbiological environments and with the role of bacteria in biogeochemical element cycling. Many fundamental properties of bacteria are consequences of their small size. Thus, they can efficiently exploit very dilute solutions of organic matter and their potential growth rates are very high. Bacteria do not have a cytoskeleton and they are covered by a rigid cells wall. Therefore they can only take up dissolved low-molecular-weight compounds from their surroundings; when bacteria exploit polymeric compounds these must first be undergo extracellular hydrolysis. Bacteria have a great diversity with respect to types of metabolism that far exceeds the metabolic repertoire of eukaryotic organisms. Bacteria play a fundamental role in the biosphere and certain key processes such as, for example, the production and oxidation of methane, nitrate reduction and fixation of atmospheric nitrogen are exclusively carried out by different groups of bacteria. Some bacterial species – ‘extremophiles’ – thrive in extreme environments in which no eukaryotic organisms can survive with respect to temperature, salinity or pH. Key Concepts:Key Concepts: * Fundamental properties of bacteria are related to their small size and lack of cytoskeleton. * Bacteria display a great diversity in types of metabolism. * Bacteria play a key role in the biosphere in terms of transfer of matter and energy. * A number of fundamental biogeochemical processes are carried exclusively by bacteria. * Bacteria play an important role in all types of habitats including some that cannot support eukaryotic life.