General Microbiology - Schlegel H. 1987

Incomplete oxidations
Acetic acid production and acetic acid bacteria

Most aerobic microorganisms oxidize organic nutrients during Respiration into CO₂ and Water. Since the carbon in the CO₂ molecule reaches its highest oxidation state, this process is referred to as complete oxidation, distinguishing it from INCOMPLETE OXIDATIONS, in which partially oxidized Organic compounds are released as metabolic products.

By "complete oxidation," it is merely meant that no organic substances are released; however, this does not imply that the entire absorbed substrate is oxidized. In each case, a significant portion of the substrate (40-70%) is assimilated—that is, converted into cellular material.

The End products of "incomplete oxidations" can include acetic, gluconic, fumaric, citric, and lactic acids, among several Other Compounds. Because these products are similar to those formed during fermentations (such as propionic, butyric, succinic, and lactic acids), and because industrial Fermentation processes require specialized technical equipment (fermenters), incomplete oxidations are also referred to as "oxidative fermentation" or "aerobic fermentation." In this context, the terms "fermentation" and "aerobic fermentation" reflect the technological aspect rather than the strict biochemical definition.

We also classify the simple abstraction of hydrogen from a substrate, as well as the USE OF MICROORGANISMS to catalyze certain reactions that serve no standard metabolic purpose for them under normal conditions, under the umbrella of "incomplete oxidations." Some Examples of such oxidations are discussed below.

All acetic acid Bacteria share The ability to produce acids through the incomplete oxidation of sugars or alcohols. These acids are released into the medium either as intermediates or as unusable metabolic end products. Acetic acid bacteria are Gram-negative rods with peritrichous (Acetobacter) or polar (Acetomonas = Gluconobacter) flagella, exhibiting varying degrees of motility. They resemble pseudomonads but differ from them by a high tolerance to acids, weak peptolytic activity, low mobility, and the absence of pigments. In nature, acetic acid bacteria typically inhabit plants. Yeasts frequently coexist with acetic acid bacteria in sugary plant exudates.

A simple experiment can determine whether a given bacterium belongs to the peroxydans group—meaning organisms that accumulate acetic acid merely as an intermediate product—or to the suboxydans group, whose representatives do not subject acetic acid to further oxidation. On opaque "chalk" Agar (containing ethanol, Yeast extract, and CaCO₃), a clear halo appears around growing colonies As a result of chalk dissolution by the released acid. In suboxydans cultures, this halo persists, whereas in peroxydans cultures, further oxidation of acetic acid soon leads to turbidity due to the secondary precipitation of CaCO₃. A typical representative of the first group is Gluconobacter oxydans, while the second group includes Acetobacter aceti and A. pasteurianum. Numerous intermediate forms exist between these two extremes. Acetobacter xylinum, A. aceti, and A. acidophilum oxidize acetic acid only very slowly. Most acetic acid bacteria require complex nutrient media.

Acetic acid bacteria oxidize primary alcohols into their corresponding Fatty acids, for example:

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and secondary alcohols into ketones, for example:

Sugar-derived alcohols are oxidized into aldoses and ketoses (e.g., sorbitol to sorbose). This type of oxidation has gained significant industrial importance as a key intermediate step in the preparative synthesis of ascorbic acid from glucose. D-Sorbitol can be produced via the electrolytic reduction of D-glucose. Using Gluconobacter oxydans, sorbose is obtained with a 90% yield from solutions containing up to 30% sorbitol. Acetic acid bacteria also oxidize glycerol, tetritols, pentitols, hexitols, and heptitols (for example, D-mannitol to D-fructose).

Aldehydes, aldoses, and ketoses are converted into their respective acids, for example:

Acetic acid bacteria can convert D-gluconic acid into ketogluconic acids. Individual strains vary in their ability to produce 2- or 5-ketogluconate. Gluconobacter melanogenum produces 2,5-diketogluconic acid (via 2-ketogluconic acid). This acid, unstable at pH 4.5, is responsible for the brownish-black coloration of this species' colonies on glucose agar (hence the species name melanogenum).

Vinegar Production Technology. The production of vinegar from wine or alcohol is primarily associated with a single challenge—aeration technology (Section 6.2). The objective is to ensure intimate contact between the bacteria, the liquid being oxidized, and atmospheric oxygen. Three main cultivation Methods are distinguished: surface culture, immobilized culture, and submerged culture.

If wine is left in shallow dishes or fermentation vats, a continuous surface film of Acetobacter xylinum (= Mycoderma aceti) develops. The oxidation process in such surface fermentation proceeds very slowly. Immobilized cultivation refers to all methods in which bacteria are fixed to a support material (such as grape pomace or crushed grapevine shoots) in well-aerated vats. In the "generator" or quick-vinegar process, the alcohol-containing liquid is repeatedly passed through a vessel filled with beechwood shavings (the vinegar generator), with air blown in from below. Submerged culture methods are also increasingly applied in vinegar production.

Biochemistry of Acetic Acid Formation. Acetobacter and Gluconobacter possess Alcohol dehydrogenase, glucose dehydrogenase, and other polyol dehydrogenases containing a recently discovered prosthetic group known as methoxatin or pyrroloquinoline quinone (PQQ). These Enzymes are located on the outer surface of Cell/30.html">The Plasma Membrane and catalyze The oxidation of ethanol, glycerol, or glucose into their corresponding acids (acetic, glyceric, gluconic). During this process, electrons enter the Electron Transport Chain, while Protons are pumped outward into the periplasmic space. Methoxatin also enters the nutrient medium and dietary vinegar, imparting a yellow color to the final product.