All bacteria have a relatively simple cell structure lacking a cell nucleus and organelles such as mitochondria and chloroplasts. Most bacteria are relatively small and possess distinctive cell and colony morphologies (shapes) as described below. The most important bacterial structural characteristic is the cell wall. Some bacteria are capable of forming endospores which allows them to survive extreme environmental and chemical stresses.

Bacteria are both harmful and useful to the environment and animals, including humans. The role of bacteria in disease and infection is important. Some bacteria act as pathogens and cause tetanus, typhoid fever, pneumonia, syphilis, cholera, food-borne illness, leprosy, and tuberculosis(TB). Sepsis, a systemic infectious syndrome characterized by shock and massive vasodilation, or localized infection, can be caused by bacteria such as Streptococcus, Staphylococcus, or many gram-negative bacteria. Some bacterial infections can spread throughout the host's body and become systemic.