What is Acetobacter Aceti?

Instructor: Taormina Lepore

Taormina has taught advanced high school biology, is a science museum educator, and has a Master's degree in museum paleontology.

The bacterium Acetobacter aceti is excellent at metabolizing ethanol into acetic acid. How does this little bacterium affect a wide variety of ecological niches, and even drive human economy just by doing what it does every day? This lesson describes what Acetobacter aceti is and why it is so important.

Why Is Acetobacter Important?

The tang of vinegar is familiar to almost everyone, in salads and many other foods. But how does vinegar get to be vinegar? Interestingly enough, it starts out as ethyl alcohol, or ethanol. And the acidic taste of vinegar exists because of a bacterium.

Acetobacter aceti is the bacterium responsible for the metabolic production of acetic acid, or vinegar. Its metabolism has several unique biological factors, and Acetobacter is also important in economic and industrial applications.

Bacterial Classification of Acetobacter

Acetobacter is a Gram-negative bacterium, which means it does not react to the Gram stain test. Gram-negative bacteria do not have the abundant outer layer of peptidoglycan polymer, comprised of amino acids and sugars, that is present in Gram-positive bacteria. Instead, Gram-negative bacteria have a thin peptidoglycan layer that is sandwiched between the cell's wall and membrane. Under Gram staining, Acetobacter stains red instead of purple.

A classic illustration of Acetobacter.
acetobacter

These bacteria have a round-to-elongated shape, and they are obligate aerobes - they need oxygen to survive. They are also nitrogen-fixing, which means they incorporate atmospheric nitrogen into inorganic compounds that plants can use. Because of its nitrogen-fixing capabilities, Acetobacter has symbiotic relationships with the roots of many commercially important plants, such as tobacco and sugarcane.

The phylogenetic classification of Acetobacter places it within the Domain Bacteria, the Phylum Proteobacteria - which includes other gram-negative bacteria (such as E. coli and Salmonella), and the Family Acetobacteraceae - which includes other examples of acid-producing bacteria.

Because Acetobacter produces acetic acid, it and its relatives are known as acidophiles, or acidophilic bacteria. Such bacteria can tolerate and thrive in the presence of acid due to special adaptations in the stability of their outer membranes.

How Does Acetobacter Metabolize?

Acetobacter is actually present in many natural ecological niches, so it has many opportunities to metabolize and ferment sugar in flowers, fruits, honeybee activity, and in the soil and water. It is harmless to humans and cannot grow well on skin, but it is airborne and can be found in canal water, wines, in nectar, and in damaged fruits - anywhere sugar will start to ferment.

How does Acetobacter interact with fermenting sugar, the product of which is ethanol, and turn that ethanol into vinegar? The enzymes of the bacterial Krebs cycle (the same cycle we use to utilize oxygen and produce energy in our cellular mitochondria) breaks down the ethanol produced during sugar fermentation through the oxidation of this alcoholic molecule.

Ethanol is oxidized using NAD+, the hydrogen receiver in this reaction. Two hydrogens are removed, producing the intermediate acetaldehyde and NADH, the receptor.

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