In humans and animals, catecholamines such as epinephrine, norepinephrine, and dopamine are common stress hormones. Stress can increase the body's susceptibility to bacterial infections. In the laboratory, stress hormones stimulate the growth of various pathogens. This had already been observed in Salmonella (Salmonella enterica serovar Typhimurium), and other intestinal bacteria, Escherichia coli and the causative agent of cholera, Vibrio cholerae. Furthermore, epinephrine and norepinephrine make it easier for bacteria to infect the body's cells. And these hormones also influence the biosynthesis of virulence factors, which enable pathogens to adhere to, penetrate, and destroy cells.