Merck Manual

Please confirm that you are not located inside the Russian Federation

honeypot link

Overview of Anaerobic Bacteria


Larry M. Bush

, MD, FACP, Charles E. Schmidt College of Medicine, Florida Atlantic University

Last full review/revision May 2021| Content last modified May 2021
Click here for the Professional Version
  • Aerobes: Those that need oxygen

  • Anaerobes: Those that have trouble living or growing when oxygen is present

  • Facultative bacteria: Those that can live and grow with or without oxygen

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 Resident Flora Healthy people live in harmony with most of the microorganisms that establish themselves on or in (colonize) nonsterile parts of the body, such as the skin, nose, mouth, throat, large intestine... read more , 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.

NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
Click here for the Professional Version
Others also read

Test your knowledge

Human Immunodeficiency Virus (HIV) Infection
Human immunodeficiency virus (HIV) infection is a viral infection that progressively destroys certain white blood cells. Untreated, it usually leads to acquired immunodeficiency syndrome (AIDS). Transmission of HIV requires contact with bodily fluid that contains the virus or cells infected with the virus. Which of the following is not a bodily fluid that commonly transmits HIV? 
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID

Also of Interest

Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID