Others have very complex food requirements. At the other extreme are the disease bacteria, which are parasitic. They rely on their host to supply their food in a predigested form. The saprophytes live on decaying remains of plants or animals. The anaerobes can grow without air because they obtain their oxygen from their food.
Many kinds can grow either with or without air.
Many bacteria are able to move through liquids, some very rapidly, by means of hairlike appendages called flagella. Others are carried about on animals or insects or through the air on dust. Like other forms of life, bacteria are attacked by smaller forms. A group of small viruses cause disease in bacteria and may kill them. Bacterial viruses are called bacteriophages.