Bacteria can be classified in several different ways. One way is based on their need for oxygen—whether they need oxygen to live and grow:
Bacteria are microscopic, single-celled organisms. There are thousands of different kinds, and they live in every conceivable environment all over the world. They live in soil, seawater, and deep within the earth’s crust. Some bacteria have been reported to live in radioactive waste.
Many bacteria live in the bodies of people and animals—on the skin and in the airways, mouth, and digestive, reproductive, and urinary tracts—without causing any harm. Such harmless bacteria are called commensals, resident flora, or the microbiome. Many resident flora are actually helpful to people. For example, they help people digest food or prevent the growth of other, more dangerous bacteria.
Only a few kinds of bacteria always cause disease when present. They are called pathogens. Sometimes bacteria that normally reside harmlessly in the body cause disease when, for example, they leave their normal location in the body. Bacteria can cause disease by producing harmful substances (toxins), invading tissues, or doing both.
Anaerobic bacteria make up a large part of the normal resident flora on mucous membranes, especially in the mouth, lower gastrointestinal tract, and vagina. These anaerobes can cause disease when mucous membranes are broken. Anaerobes from outside the body sometimes cause disease when they enter breaks in the skin or are consumed.
Infections caused by anaerobic bacteria include
Often, several species of anaerobic bacteria are present in infected tissues. Aerobic bacteria are frequently also present. Infections with more than one species of anaerobic bacteria or anaerobic and aerobic bacteria are called mixed anaerobic infections.