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Examples of both of these and a good understanding of the concept is sometimes needed to answer Step 1 questions.


These are microorganisms that can live without oxygen.
Facultative anaerobes can live without oxygen but they use it if it available. Most of the bacteria we know falls in the facultative anaerobes category (e.g E. Coli).
Obligate anaerobes are harmed by the presence of oxygen and these usually utilize fermentation to produce their necessary ATP.
Examples of obligate anaerobes:
Good way to remember these is ABC :)

Yet, there's another sub category of anaerobes and these are called aerotolerant anaerobes. These bugs unlike the obligate ones, are not harmed in the presence of oxygen.
Examples of aerotolerant anaerobes:
Propionibacterium and Lactobacillus (both are normal human flora)


Obligate aerobes require oxygen to in order to oxidize nutrients and they usually have catalase and superoxide mutase. They cannot survive without oxygen.
Examples of obligate aerobes:
Pseudomonoa (remember aerogenosa)
A good mnemonic to remember these is Nagging Pets Must Breath
Microaerophilics are those organisms that may use oxygen but only when it's in low concentrations and the two most commonly tested bugs in this category is Campylobacter and Helicobacter (both of which are GI bugs).
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