An anaerobic organism or anaerobe is any organism that does not require oxygen for growth. It could possibly react negatively and may even die if oxygen is present. There are three types:

• obligate anaerobes, which cannot use oxygen for growth and are even harmed by it
• aerotolerant organisms, which cannot use oxygen for growth, but tolerate the presence of it
• facultative anaerobes, which can grow without oxygen but can utilize oxygen if it is present

In humans beings these organisms are usually found in gastrointestinal tract.