The difference between strict, facultative and aerotolerant anaerobes

Anaerobic organisms are those that don't need oxygen to live, grow and multiply. For these organisms, oxygen can be harmless, but it can also be toxic and harmful, even lethal.

Although anaerobic bacteria are the most popular, there are many other types of anaerobic organisms, including fungi, archaea, and even some aquatic invertebrates. Of course, when talking about strict or obligate anaerobes, facultative and aerotolerant anaerobes, we are usually talking about microorganisms, almost always bacteria.

More precisely, an anaerobic organism is one that does not use molecular oxygen (Otwo) in their energy metabolism. To synthesize ATP, they use various types of fermentation and anaerobic respiration in which oxygen does not intervene.

A classification of the most common for anaerobic organisms distinguishes three groups:

  1. Strict or obligate anaerobes: Molecular oxygen is toxic to them. There are those that carry out fermentation and there are those that carry out some type of anaerobic respiration. For example, Acitomyces, Clostridium, Porphyromonas either Propionibacterium.
  2. aerotolerant anaerobes: they do not need oxygen but it is not harmful to them either. All known aerotolerants are fermenting organisms. For example, Streptococcus mutans.
  3. facultative anaerobes: they preferably use aerobic respiration in the presence of oxygen, but they have the capacity or faculty, hence the name, to carry out fermentation or some type of anaerobic respiration if they do not have oxygen. For example, Escherichia coli, Salmonella, listeria either Staphylococcus.

Strict or obligate anaerobes

Strictly anaerobic organisms and bacteria are those that cannot live in the presence of oxygen. It's not just that they don't use it, it's that molecular oxygen is toxic to them.

More precisely, strict anaerobes are usually defined as those organisms that die if exposed to normal atmospheric concentrations of oxygen (20.95%), which is why they are also called obligate anaerobesor live in places without oxygen or die.

There are strict anaerobes that perform fermentation and others who do anaerobic respiration.

The strict fermentative anaerobic organisms They mainly use lactic acid fermentation. Yeasts generally use ethanolic fermentation. Among the archaea, a wide variety of fermentations occur: propionic, butyric, butadienolic, Stikland fermentation, acetogenesis, methanogenesis, etc.

Anaerobic respiration is a type of cellular respiration that does not use molecular oxygen as the final electron acceptor in the respiratory chain, but other inorganic molecules, such as nitrates, sulfates, iron, manganese, mercury or carbon dioxide, there are even organisms that use a organic electron acceptor, for example fumarate.

Being a strict anaerobe does not mean that these organisms cannot spread through environments where there is oxygen, albeit under some form of resistance. For example, bacteria of the genus Clostridium form endospores to survive under unfavorable conditions, and in the form of endospores they can survive atmospheric air without problems.

Examples of obligate anaerobes can also be found among fungi. For example, genres Neocallimastix, Pyromones, Sphaeromonas and others rumen fungithe first stomach chamber of cows, goats, and other ruminants.

aerotolerant anaerobes

Aerotolerant anaerobes do not use any type of anaerobic respiration. all are fermenting organisms. Unlike strict anaerobes, aerotolerant organisms have protection systems against the oxidizing power of molecular oxygen.

Between these antioxidant mechanisms the enzymes peroxidase and superoxide dismutase stand out, the latter catalyzing the dismutation of the superoxide ion (Otwo).

However, aerotolerant anaerobes do not have catalase, the enzyme that catalyzes the breakdown of hydrogen peroxide, or hydrogen peroxide (HtwoEITHERtwo), into water and molecular oxygen. Catalase is one of the most important antioxidant enzymes at the cellular level of virtually any other organism exposed to oxygen.

facultative anaerobes

Facultative anaerobes use the aerobic respiration if oxygen is availablebut if there is not, they can go to anaerobic metabolism, some carry out fermentation and others anaerobic respiration.

They are also called facultative aerobes, as opposed to obligate aerobewhich would be the organisms incapable of synthesizing ATP if they do not have oxygen.

Among the facultative anaerobes there are many bacteria, but also many eukaryotes, for example the fungus Saccharomyces cerevisiae. Many species of bacteria are also considered facultative anaerobes. aquatic invertebratesfor example many types of polychaetes or marine worms.

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