Escherichia coli, or E. coli, is a rod-shaped, gram-negative, facultatively anaerobic bacteria occurring naturally in the intestines of humans and most other warm-blooded animals, explains the Centers for Disease Control and Prevention. While some E. coli strains do cause illness, most are harmless.
Todar's Online Textbook of Bacteriology describes E. coli as a highly adaptable bacterium capable of growing in a variety of environmental conditions. As a facultative anaerobe, it can survive and grow in both the presence and absence of oxygen. When oxygen is available, E. coli can utilize glucose through the process of aerobic respiration, but when oxygen is not available, it can survive and grow using either fermentation or anaerobic respiration. This ability to use multiple metabolic pathways allows E. coli to survive in a variety of intestinal and other conditions.
In addition to utilizing different metabolic pathways depending upon the presence or absence of oxygen, E. coli is able to sense temperature, pH, chemicals, and other signals in its environment and respond appropriately. For example, Todar's Online Textbook explains that, depending on environmental conditions, E. coli can swim toward or away from something, or it can attach itself to a cell to become stationary. Additionally, E. coli is able to sense the presence or absence of various chemicals and nutrients in its environment and adjust enzyme production depending on what is or is not available.
While most E. coli are harmless and naturally occurring, there are some that cause disease and are said to be pathogenic, according to the CDC. Pathogenic E. coli are usually transmitted through contaminated food or water, or between people by contact. Pathogenic E. coli can cause symptoms that include diarrhea, respiratory tract infections and urinary tract infections.