Do acetic bacteria use the electron transport chain when converting ethanol to acetic acid?

And is wikipedia inconsistent here in its definition of fermentation. It says fermentation

Fermentation takes place when the electron transport chain is unusable

(and this is consistent with (academic) microbiology books searchable on google books). Wikipedia defines acetic acid bacteria as

produce acetic acid during fermentation.


Some genera, such as Acetobacter, can oxidize ethanol to carbon dioxide and water using Krebs cycle enzymes. Other genera, such as Gluconobacter, do not oxidize ethanol, as they do not have a full set of Krebs cycle enzymes.

I read in microbiology books on google books, that acetic acid bacteria use oxygen as the terminal electron acceptor, and have a respiratory mechanism.

Brewing Microbiology edited by Fergus Priest p165

Acetobactor spp. possess a respiratory mechanism...Acetobactor spp. are obligately aerobic with a respiratory metabolism (O2 as the terminal electron acceptor)

So is wikipedia wrong to describe acetic acid bacteria as using fermentation?

I see mention of krebs cycle though, which I understand to be associated with respiration rather than fermentation. Also in this reaction/process :

$C_2H_6O\hspace{1mm}(ethanol) + O_2 \rightarrow C_2H_4O_2\hspace{1mm}(acetic\hspace{1mm}acid) + H_2O$

I see water mentioned but not carbon dioxide(if it had both I'd say that really looks like respiration, though it's being called oxidative fermentation). So I still can't really see whether it's respiration or fermentation, though it's being called fermentation (oxidative fermentation), yet there is the mention of krebs cycle enzymes (krebs cycle being associated with respiration), so it makes me unsure whether the electron transport chain is used or not.

  • $\begingroup$ also aside from ethanol->acetic acid, apparently they might also do glucose straight to acetic acid, skipping ethanol.. not sure whether that uses the ETC or not. the additional part to this question is related web.archive.org/web/20170312205200/http://… $\endgroup$
    – barlop
    Mar 12, 2017 at 20:56

2 Answers 2



I find it somewhat ironic that in a response a recent post from the poster concerning itself with the precise definition of ‘fermentation’ I argued that this was a semantic question because of the use of the term in the English language before any biochemistry was known. I now discover that the term is used in an even looser sense than I was aware of — in an aerobic context. The conversion of ethanol to acetic acid by acetic acid bacteria is apparently referred to as oxidative fermentation as explained in this extract from a paper in The Journal of Bacteriology:

Acetic acid bacteria are obligate aerobes that belong to the α-Proteobacteria and have a strong ability to oxidize ethanol, sugar alcohols, and sugars into their corresponding organic acids. Such oxidation reactions are traditionally called oxidative fermentation, since they involve incomplete oxidation of these compounds. These bacteria accumulate the corresponding incomplete oxidation products in large quantities in their surrounding environment.

The biochemistry is summarized by Gómez-Manzo et al.:

Ethanol fermentation by acetic acid bacteria is carried out by two sequential reactions catalyzed by pyrroloquinolinequinone (PQQ)-dependent alcohol dehydrogenase enzymes (ADH) and aldehyde dehydrogenase (ALDH), which are located in the cytoplasmic membrane [6] and transfer electrons to ubiquinone Q10 [7]. PQQ-ADH is a periplasmic quinohemoprotein-cytochrome c complex and catalyzes the first step of ethanol oxidation by transferring electrons to Q10 and producing acetaldehyde which usually is the substrate for another enzyme (ALDH), and converted to acetic acid during the second step of ethanol fermentation.

The fact that the reduced pyrroloquinolinequinone is re-oxidised by ubiquinone, indicates that the conversion of ethanol to acetic acid requires the electron transport chain in the bacterial membrane to continue, and that the ultimate electron acceptor is oxygen.


I do not know about the prokaryotic mechanism for ethanol metabolism. However, in eukaryotes, the metabolism of ethanol does indeed eventually show an effect in the ETC.

When ethanol is metabolised in eukaryotes, alcohol dehydrogenase will remove a proton with the help of NAD+ to create acetate and NADH. Of course, NADH is used within the ETC. So basically, the metabolism of ethanol serves as one of many ways a cell can balance its redox potential.

Some organisms fermentative pathways are inhibited by the presence of oxygen, and some are not. I can not tell you with certainty whether or not this is the case for you in particular. But, I would define a fermentative process as, a process which does not result in the complete oxidation of a carbon-substrate.

  • $\begingroup$ Apparently bacteria are related to mitochondria. And some bacteria have an ETC in their cell membrane, rather like all mitochondria have an ETC in their membrane. $\endgroup$
    – barlop
    Mar 5, 2017 at 22:24
  • $\begingroup$ This is true! But, my point still stands. The ETC would work to provide a means to convert the NADH produced from ethanol oxidation, back to NAD+, provided that there is an electron acceptor present. $\endgroup$
    – Bob
    Mar 5, 2017 at 22:52
  • $\begingroup$ Ah thank you for clarifying my answer @tomd. The acetaldehyde step slipped my mind. But also, in regards to yeast, they are capable of both ethanol production, and ethanol metabolism - so ADH will work in both directions. $\endgroup$
    – Bob
    Mar 5, 2017 at 23:27
  • $\begingroup$ It appears that the 'main' acetic acid bacteria — Acetobactor sp — do not use NAD to oxidize the ethanol to acetic acid, but pyrroloquinolinequinone. The situation is not the same is in mammalian liver. (See my answer.) $\endgroup$
    – David
    Mar 5, 2017 at 23:51
  • $\begingroup$ @xusr Yes this had already been noted, nearly 7 months ago.. thanks $\endgroup$
    – Bob
    Aug 29, 2017 at 16:32

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