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Which one of the following is a free living obligate anaerobic bacterium ?
A. Clostridium
B. Rhodospirillum
C. Azotobacter
D. Bacillus subtilis

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Hint: Anaerobic bacteria are the bacteria which can survive without oxygen .

Step by step answer:Obligate anaerobes are microorganisms that are killed by the normal atmospheric concentration of oxygen. An anaerobic organism is an organism that does not require oxygen for growth. It may react negatively or even die in the presence of free oxygen.
Clostridium is a free-living, anaerobic obligate bacteria.
Rhodospirillum is an anaerobic and free-living, nitrogen-fixing bacteria.
Azotobacter is a free-living and non-symbiotic, heterotrophic bacteria capable of fixing nitrogen.
Bacillus subtilis and also known as the hay bacillus or grass bacillus is gram positive, catalyst positive bacteria found in the soil, and in the gastrointestinal tract of ruminants and animals.
So the correct answer is (A)
Additional information :
Clostridium is a genus of gram-positive bacteria. There are obligate anaerobes capable of producing endospores. Clostridium contains about 250 species that include common free-living as well as pathogenic bacteria. The normal reproducing cells of clostridium called a vegetative form, are in rod-shaped. Clostridium endospores have a distinct bottle shape. Clostridium species inhabited soils and intestinal tracts of animals including humans. Clostridium is a normal inhabitant of the healthy lower reproductive tract of females.

Note: The endospores of clostridium are as unique than other bacteria as others have ovoid-shaped endospores.
Last updated date: 05th Jun 2023
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