Aerotolerant Anaerobes: Definition & Examples

Instructor: Bridgett Payseur

Bridgett has a PhD in microbiology and immunology and teaches college biology.

Aerotolerant anaerobic bacteria do not use oxygen to make energy, but they can survive in the presence of oxygen. This lesson will discuss why oxygen is both important and dangerous, and how aerotolerant bacteria survive.


As humans, it's hard to imagine life without oxygen. Every living thing we see needs oxygen--trees, flowers, fish, cows. But the microscopic world of living organisms too small to see is full of life forms that don't need oxygen to live. In fact, in the early earth, billions of years ago, the atmosphere had no oxygen. It wasn't until photosynthetic bacteria, which could convert the energy from sunlight into food, that oxygen started to appear.

While you might take a deep breath of relief at the thought of oxygen, it was actually a problem for early cells. Oxygen can be toxic to cells if they don't have the machinery to process it. Oxygen can combine with water to make hydrogen peroxide, which you might have used at some point to clean a scratch and get rid of any possible germs. The same concept happened on early earth: oxygen could kill cells unless they had evolved some protection.

Today, some bacteria species called aerotolerant anaerobes do not use oxygen at all, but they have evolved to survive in oxygen-rich environments. 'Aero' refers to oxygen, so aerotolerant means the cells can tolerate the oxygen. Putting 'a-' or 'an-' in front of word means 'without,' so an anaerobe is a cell that does not use energy.

Cellular Energy

Human cells use oxygen to make a form of energy called adenosine triphosphate (ATP). ATP is the energy source your cells use to do all of their work. You make ATP from food using a process called aerobic cellular respiration in which your cells basically convert the energy in the food you eat into ATP. This is sort of like converting currency from euros to dollars. You can't spend euros in America, and your cells can't spend that cheeseburger you just ate, either. They need a more directly usable form of energy.

Aerotolerant Anaerobic Bacteria

If oxygen is important for making ATP, how can other cells, like anaerobic bacteria, live without it? Instead of aerobic cellular respiration, these cells perform a process called anaerobic respiration or fermentation. Anaerobic respiration is less efficient than aerobic respiration, but it can still produce plenty of energy for single-celled bacteria.

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