An aerotolerant anaerobe is an organism that tolerates the presence of oxygen but does not require it for growth. Instead, aerotolerant anaerobes use fermentation to survive. An example organism is lactobacillus, a bacteria that lives in the gut of humans and is also used in the production of yogurt.
The process of fermentation involves converting sugar into acids, sugar and/or ethanol, as a way of forming energy. There are many fermentation pathways that are used, most commonly through lactic acid. Fungi such as yeasts tend to use ethanol fermentation. This process releases less energy than aerobic methods, as it relies on regenerating ATP from ADP.