For example, Moraxella catarrhalis and Haemophilus influenzae may spread from the nasal region to the middle ear where they may cause otitis media when the Eustachian tube is blocked. Abundant nutrients and moisture support a large population of microflora in the mouth. Unfortunately, many of the microbes that inhabit the mouth cause tooth decay, periodontitis, or halitosis. Good oral hygiene is essential to keep the microbial population of the mouth in check. Low pH of stomach acids prevent normal microflora from living in the stomach.
Some microbes do survive transit through the stomach to the intestines.
The relatively low pH and presence of digestive enzymes in the small intestines keeps the number of normal bacteria fairly low. Conditions in the large intestine favor the growth of normal microflora. Most of the bacteria in the large intestine are anaerobic. The largest population of microflora lives in the large intestines. The vagina is populated with beneficial lactobacilli.