Since time immemorial, vinegar has been a fermented foodstuff widely used by mankind as a part of the diet and as a preservative, condiment, and flavoring additive. Because of its bio-healthy properties, vinegar was even traditionally used in ancient medicine [1,2]. Currently, there is a great variety of vinegars around the world, depending on the starter microbial composition, the raw material, and the technical methods used for production [3,4]. From an industrial point of view, the elaboration of vinegar is performed from an alcoholic medium in which a mixed culture of acetic acid bacteria (AAB) is used to carry out a biotransformation process of ethanol into acetic acid, which occurs in specific bioreactors (acetators) [5,6]. Despite the high quality of the final products obtained through traditional systems, including mainly solid-state fermentation and surface culture, their numerous disadvantages, including low efficiency, the slowness of the process, and a lack of control of the operating conditions, have led to the use of the submerged culture system [7,8]. The success of this method, widely implemented in Western and European countries, lies in the high yield and speed of the process under controlled stirring conditions due to the efficiency of mass transfer and continuous vigorous aeration [7,9]. In this sense, the choice of a working mode for the acetators also ensures a suitable environment for the development and activity of AAB, and the control and monitoring of fermentation conditions are some of the fundamental aspects to consider [5,7,10].

On the other hand, vinegar production would not be possible without the activity of the acetic acid bacteria (AAB). These microorganisms, strictly aerobic, can be found in a wide variety of natural and industrial environments. Their versatility and metabolic adaptability make them a taxonomic group of high interest for studying the optimization of obtaining multiple products, with acetic acid as one of the main components, as well as the essential mechanisms that allow them to grow under harsh conditions [11,12,13]. In this sense, the role of their membrane-bound and soluble dehydrogenase system may offer new opportunities in the development of innovative processes based on their capability to carry out the incomplete oxidation of several substrates, including alcohols, sugars, and sugar alcohols, for the production of organic acids [11,14]. Furthermore, the ability of AAB to produce exopolysaccharides is also of great interest for both research and industrial purposes, with some strains considered model organisms for understanding the mechanisms of cellulose synthesis. Moreover, at present, these are the most efficient microorganisms for producing them under controlled conditions [15,16,17]. Finally, the current state of omic technologies and efficient molecular modification methods may be applied to increase the understanding of physiological behavior and the characterization of new strains recovered from these complex media, as well as to exploit the full potential of AAB for producing vinegar and other related bioproducts [6,18,19,20,21,22].

The molecular and biochemical aspects that define the metabolism of acetic acid bacteria are increasingly becoming the target of much research. A general and updated overview of the main AAB metabolic pathways, especially those related to carbon source assimilation including alcohols, sugars, and sugar alcohols for the production of organic acids, has been provided. It is worth noting that many other related metabolic pathways, partially or completely unknown, are presently being studied by several authors [6,154,155,156,157,158].

The overall oxidative biological reaction that defines the biotransformation of ethanol into acetic acid can be represented as follows:

AAB are chemoorganotrophs microorganisms that use ethanol from a medium of alcoholic origin as a carbon source. The genera Acetobacter and Komagataeibacter usually show a higher ethanol preference, although other AAB groups may show a preference for other carbon sources [7,41]. This biotransformation consists of an incomplete oxidation reaction of two steps. First, alcohol dehydrogenase (ADH) binds to pyrroloquinoline quinone (PQQ) to oxidize the ethanol into acetaldehyde. Next, acetaldehyde is oxidized to acetic acid by membrane-bound aldehyde dehydrogenase (ALDH); both enzymes are located on the periplasmic side of the inner cell membrane [159,160]. Oxidized nicotinamide adenine dinucleotide (NAD⁺) and nicotinamide adenine dinucleotide phosphate (NADP⁺), both located in the cytoplasm, may be used as coenzymes by NAD-ADH, NAD-ALDH, and NADP-ALDH [156,161]. The inner acetic acid can be completely oxidized by the acetyl-CoA synthase, which leads to the input of acetyl-CoA into the TCA cycle, and in this case, to CO₂ and H₂O providing energy (ATP) and detoxifying the cell [14]. Other organic acids such as lactic, pyruvic, malic, succinic, citric, and fumaric acids may be similarly metabolized [73]. Because of the strictly aerobic metabolism of AAB, the ADH-PQQ and ALDH complexes are closely linked to the respiratory chain, which transfers reducing equivalents from donor substrates to ubiquinone (UB). Then, electrons from the reduced UB, named ubiquinol (UBH₂), are transferred to the final electron acceptor, oxygen (O₂), by terminal ubiquinol oxidases (UOXs), producing H₂O [14,161]. Some processes related to this central oxidative metabolism may include pathways that aim to obtain biosynthetic precursors of amino acids and nucleic acids, among others, in order to replenish cell material losses throughout the early stages of the acetification process. The TCA cycle also plays a crucial role in the assimilation of internal acetic acid; among the involved enzymes, succinyl-CoA: acetate CoA transferase (SCACT), encoded by aarC, New Orleans, LA, USA, is able to produce acetyl-CoA from inner acetate, being of significant importance in the tolerance to acetic acid [162,163]. Furthermore, membrane mechanisms dependent on proton motive force may be triggered for acetic acid release and cell detoxification at the final moments of the process; here, the importance of outer membrane proteins (OMPA) and efflux pumps (OPRM and ABC-transporters) in the control of the cellular output of acetic acid, as well as MLTA, participating in the maintenance of the peptidoglycan layer under these conditions, are highlighted [6].

The ADH complex of most AAB is composed of three subunits, although it may contain two subunits in some species [164]. Subunit I (72–78 kDa), encoded by the gene adhA, is a catalytic component containing a PQQ and a heme C moiety. Subunit II (44–45 kDa), encoded by the gene adhB, is a membrane-anchoring and ubiquinone-reducing component possessing three heme C moieties; both subunits participate in the intramolecular electron transport to the terminal UB. Subunit III (20 kDa), encoded by the gen adhS, which has no prosthetic group, facilitates the association of subunits I and II to the membrane and acts as a molecular chaperone for the folding and/or maturation of subunit I [73,161]. Several authors have related high ADH stability and activity with a high tolerance and production of acetic acid, mainly in species from the current genus Komagataeibacter [41,82]. The ALDH complex is composed of two or three subunits depending on the AAB species, and it acts as an operon. Although its optimum pH ranges between 4 and 5, the oxidation of acetaldehyde to acetate may be catalyzed at lower pH values. ALDH is highly sensitive to low oxygen concentrations and the presence of ethanol in the medium [73].

AAB can metabolize different carbohydrates as carbon sources, mainly glucose, but also arabinose, fructose, galactose, mannose, ribose, sorbose, and xylose [73]. Most AAB have been characterized by non-functional glycolysis because of the absence of a phosphofructokinase enzyme; therefore, the pentose phosphate pathway (PPP) is the main metabolic route of AAB to oxidize the glucose available in the medium by the catalytic activity of the enzymes glucose-6-P dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGD), providing metabolic precursors such as ribulose-5-phosphate and generating NADPH + H⁺ and energy [165,166,167]. Among AAB, several species from Gluconobacter have a glucose preference, and several Gluconobacter oxydans strains also exhibit the ability to oxidize glucose to gluconic acid via glucono-δ-lactone, forming D-gluconate. This oxidation reaction occurs in the periplasm using a membrane-bound pyrroloquinoline quinone-dependent glucose dehydrogenase (PQQ-GDH) located on the outer side of the cytoplasmic membrane. D-gluconate can be further oxidized rapidly to ketogluconates such as 2-ketogluconate (2-KGA), 5-ketogluconate (5-KGA), and 2,5-diketogluconic acid (2,5-DKGA), both in the periplasm and cytoplasm, by different oxidizing enzymes [166,168,169]. Glucose, gluconic acid, and ketogluconates can be assimilated by these bacteria, thus obtaining biomass and energy and acidifying the medium, possibly as part of their metabolic strategy to prevail over other glucose-like microorganisms [166]. Final products of PPP and the Entner Doudoroff pathway (EDP) may be completely oxidized to CO₂ and H₂O by Acetobacter, Gluconacetobacter, and Komagataeibacter spp. using the TCA cycle when the carbon source of the medium is exhausted; this is not the case for Gluconobacter spp., which show a non-functional TCA cycle [73,169]. Recent studies have proposed a molecular strategy in which K. europaeus, as the predominant species of a complex microbiota involved in the submerged production of vinegar from raw materials with high sugar content, might assimilate, firstly and before the ethanol, the glucose in the medium, draining biosynthetic precursors directly by using enzymes of PPP and the glycolysis to prevail over other species that exhibit high glucose preference [6].

AAB also exhibit the ability to oxidize several sugar alcohols such as glycerol, D-mannitol, and D-sorbitol, among others, with the use of glycerol as a carbon source being especially remarkable in winemaking, producing dihydroxyacetone (DHA) through the activity of some oxidizing enzymes, mainly glycerol dehydrogenase, and providing energy (ATP) via gluconeogenesis. Strains from Acetobacter pasteurianus, Gluconobacter oxydans, and Komagataeibacter xylinus are some of the most studied regarding this oxidative pathway [16,73].