Bacteria are single-celled living organisms. They are considered to be among the earliest forms of life and have been on Earth for about 4 billion years. The word "bacterium" is derived from the Greek wording meaning "cane" because the first to be discovered were shaped like rods.
Each bacterium cell is composed of a cell wall, cell membrane, cytoplasm and nucleus. Bacteria can be classified according to how the cells join to form clusters. Diplococci are two spherical bacterial cells paired together. When more cells are clustered they are called staphylococci and when they form a chain they are known as streptococci. Some bacteria have hairs called flagella, which allow them to move. Rod-shaped bacteria can contain spores.