Ear infections, also known as otitis media, are caused by a variety of bacteria, but the four most common are Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, and Staphylococcus aureus. Streptococcus pneumoniae and Haemophilus influenzae are the most common causes, followed by Moraxella catarrhalis, and then Staphylococcus aureus. Each of these bacteria can cause an infection in the outer, middle, or inner ear, depending on the strain and its location. All four of these bacteria can be found in the environment, and they can enter the ear through the Eustachian tube, which connects the middle ear to the throat. The best way to prevent an ear infection is to practice good hygiene habits, such as regular hand-washing and avoiding contact with other people who are sick.