Question

What are obligate anaerobes?

Answer 1

Hint: Any organism that does not need oxygen to grow is known as an anaerobic organism or anaerobe. If free oxygen is present, it can react negatively or even die.

Complete answer:
Obligate anaerobes are microorganisms that are destroyed by natural oxygen concentrations in the atmosphere. Oxygen tolerance varies by species, with some able to survive in up to $8\%$ oxygen and others losing viability until oxygen levels are less than 0.5 percent.
The oxygen sensitivity of obligate anaerobes is due to several factors, including:
Since the outer orbital of molecular oxygen comprises two unpaired electrons, it is easily
reduced to superoxide and hydrogen peroxide.
To detoxify these ingredients, aerobic organisms produce superoxide dismutase and
catalase, but obligate anaerobes produce these enzymes in very small amounts, or not at all.
The redox potential of a solution is increased by dissolved oxygen, and a high redox potential
prevents the development of certain obligate anaerobes.
Some enzymes require sulfide, which is oxidized by molecular oxygen to form disulfide,
rendering some enzymes inactive (e.g. nitrogenase). When essential enzymes are deactivated, organisms are unable to develop.
Since electrons are depleted in reducing oxygen, growth may be slowed due to a lack of
reducing equivalents for biosynthesis.

Note:
Actinomyces, Bacteroides, Clostridium, Fusobacterium, Peptostreptococcus,
Porphyromonas, Prevotella, Propionibacterium, and Veillonella are examples of obligately
anaerobic bacterial genera. Clostridium species are endospore-forming bacteria that can live
in this dormant state in atmospheric oxygen concentrations. The rumen fungi Neocallimastix, Piromonas, and Sphaeromonas are examples of obligately anaerobic fungal genera.