There are five primary shapes of bacteria found in nature: coccus, spirillum, vibria, bacillus and spirachaetes. These shapes can be found in a variety of configurations that help identify the bacterial species.
Bacteria are single cell organisms found all over Earth, and they group together in colonies. It is estimated that the number of bacterial cells inside the human body is 10 times greater than that of human cells. Most of these bacteria are located in the intestines and colon, where they break down carbohydrates and aid in the absorption and digestion of nutrients. Though these bacteria are not usually harmful to humans, they are capable of acting as opportunistic pathogens during times of compromised immune function. Most bacteria living in the human gut are anaerobes, which means that they thrive in environments that do not offer access to oxygen.
In order to reproduce, bacteria go through a process called binary fission, in which a single cell divides into two identical daughter cells. This process replicates the DNA of bacteria, and in certain species, can occur as frequently as every 20 minutes. The capacity for such rapid reproduction is the reason humans become ill so quickly when infected with an opportunistic bacterial pathogen.