Bacteria feed on materials as varied as soybean oil, sugar, starch, sulfur, amino acids, iron, milk, meat and even wood compounds. Some types of bacteria are photosynthetic, making their own food from sunlight. Others absorb nutrition from the surface where they live.
Bacteria exist as a single cell and there are thousands of species. The diet of bacteria is generally determined by their metabolic category. The categories are broad but loosely fall into one of three groups: lithotrophs, organotrophs or phototrophs. Lithotrophic bacteria consume inorganic material, while organotrophic bacteria get their energy from organic compounds. The process of bacteria breaking down food to get energy is called respiration. Phototrophic bacteria get their energy directly from the sun. Some bacteria feed on decaying matter and help break down environmental waste. Others get their food by breaking down chemicals in their surrounding environments. Some even consume harmful products such as oil, arsenic and nuclear waste. Bacteria do not consume food through chewing or swallowing in a mouth the way that other living organisms consume food. Instead, they absorb nutrients through channels in the walls of membranes and cells. Bacteria can feed alone or in groups that cluster together, forming chains, squares or various pairs.