While many of our tissues in the human body are perfused with oxygenated blood, there are regions where anaerobic conditions exist. Naturally, much of animals intestinal tracts and gut microbiomes are made of obligate anaerobes. Pathologically, necrotic tissue and other circulation issues can create transient anaerobic conditions.

Anaerobic Conditions in Humans

All organisms that come in contact with ROS must contain at least one of three essential enzymes to detoxify the reduced oxygen to avoid damage; obligate anaerobes do not contain any of these enzymes:
- Peroxidases
- Superoxide dismutase (SOD)
- Catalase 

Detoxifying Reactive Oxygen Species

Obligate anaerobes cannot grow in the presence of oxygen, which is why they grow densest at the bottom of the Thioglycolate tube. In nature, they can be found in places like deep in the ocean or in anaerobic environments inside of our bodies. 

- Obligate Aerobes 
- Obligate Anaerobes
- Facultative Anaerobes
- Aerotolerant Anaerobes
- Microaerophiles

Categorization of Microbes via Optimum Oxygen Concentrations

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Obligate aerobes require abundant levels of environmental oxygen levels to grow. Therefore they can be observed growing at the top of Thioglycolate tubes where oxygen levels are the highest. 

Obligate Aerobes

Obligate Anaerobes

Facultative anaerobes thrive in aerobic environments with abundant oxygen, but have the means to conduct anaerobic respiration or fermentation so that they can grow it absence of oxygen as well. 

Facultative Anaerobes

Aerotolerant anaerobes utilize anaerobic respiration or fermentation, but are indifferent to the presence of oxygen.

Aerotolerant Anaerobes

Microaerophiles thrive in areas of oxygen concentrations below the typical 21% in the Earth's atmosphere. They grow best in ranges closer to 1%-10% oxygen concentration.

Microaerophiles

Capnophiles thrive in environments of high CO<sub>2</sub> concentrations and oxygen concentrations lower than atmospheric.

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