Other Fermentation Pathways

The names of some other fermentation pathways derive from the names of the final products. This is the case for caproic, homoacetic, and methanogenic fermentation. For caproic fermentation, Clostridium kluyveri is the species that can metabolize acetic acid and ethanol under anaerobic conditions, producing butyric and caproic acids, in a specific

Glycolysis

Pyruvic acid

Formic acid

Acetyl-S-CoA

Acetic acid

Ethanol

+Acetyl-S-CoA -CoA-SH

Acetoacetyl-S-CoA

Acetone

Isopropanol

Butyryl-S-CoA

- CoA - SH Butyric acid Butanol

Figure 7 Different metabolic pathways of butyric acid fermentation.

controlled mechanism: if acetic acid is present in excess, a considerable amount of butyric acid is formed, while if ethanol is in excess, caproic acid is the main product. These relationships suggest that butyric acid is an intermediate in the synthesis of caproic acid from acetic acid.

In homoacetic fermentation, the Acetobacterium group converts fructose into acetate, and it appears that the nutritional requirements of this organism are complex. During methanogenesis, methane can be formed via methanogenic fermentation by Methanosaeta and Methanosarcina, which convert acetate and water into CH4 and carbonic acid.

In addition, there are some relatively rare fermentation pathways that are carried out by very restricted anaerobic microorganisms. In Table 4, some examples of the fermented substrates, the microorganisms and the biochemistry of the reactions are summarized. Generally, these fermentation pathways are carried out by specialized bacteria that use substrates that cannot be metabolized by other microbiological groups. Nevertheless, during the catabolic process, all of these rare fermentation pathways produce intermediate compounds that have high residual energies and are commonly CoA derived, from which these microorganisms obtain their ATP.

The first example includes anaerobic microorganisms that can degrade xenobiotic industrial chemical compounds through a combination of co-metabolic steps, which often yield partial degradation, or by serving as growth substrates that are accompanied by mineralization of at least part of the molecule. Indeed, while aerobic microorganisms use oxidative reactions, degradation by anaerobic bacteria takes place by reduction reactions, and they thus degrade aromatic compounds by reductive conversions with the central intermediates that are ready for hydrolytic ring cleavage having a 1,3-dioxo structure. Using an example, including aromatic, chlor-oaromatic, aliphatic, and chloroaliphatic compounds, one case of anaerobic degradation is presented. The recently isolated fermenting bacterium Pelobacter massiliensis is the only strict anaerobe that is known to grow on hydro-xyhydroquinone (1,2,4-trihydroxybenzene) as the sole

Table 3 Summary of fermentation processes with the corresponding energy yield

Fermentation process

Energy yield

Alcoholic fermentation

2 mol ATP/mol glucose

Glycero-pyruvic fermentation

Net ATP production

Propionic acid fermentation (glucose®)

4 mol ATP/mol glucose

Propionic acid fermentation (lactate®)

0.3 mol ATP/ mol lactate

Amino acid fermentation

0.3 mol ATP /mol amino acid

Lactic acid fermentation (homolactic)

2 mol ATP/ mol glucose

Lactic acid fermentation (heterolactic)

1 mol ATP/ mol glucose

Bifidobacterium lactic acid fermentation

2.5mol ATP/mol glucose

Mixed acid fermentation

2.5 mol ATP/mol glucose

Butanediol fermentation

2.5 mol ATP/mol glucose

Butyric fermentation

3 mol ATP/mol glucose

aCarbon source.

aCarbon source.

Table 4 Microorganisms and type of substrates metabolized in rare fermentation processes

Substrate

Microorganism

Reaction

Acetate

Ethanol and acetate Fructose

Hydroxyhydroquinone

Malonate

Oxalate

Putrescine

Methanosaeta Ciostridium kiuyveri Adetobadterium spp.

Peiobacter massiiiensis, Peiobacter acidigaiiici Maionomonas rubra Oxaiobacter formingenes Gram4 bacteria

Ethanol + Acetate + CO2 ! Caproate + Butyrate + H2

Fructose ! Acetate + H2CO2 ! Acetate + H2O

Malonate + H2O ! Acetate + HCO3

Oxalate + H2O ! Formiate + HCO3

C4H12N2 + H2O ! Acetate + +Butyrate + NH4 + H2 + H4

Lactic acid bacteria and yeast

Fermented milk (kefir, koumiss) and sourdoughs

Lactic acid bacteria and yeast

Fermented milk (kefir, koumiss) and sourdoughs

Cheese ripening

Lactic acid bacteria Bifidobacterium Enteric bacteria

Intestinal tract of warmblooded animals

Propionic bacteria and lactic acid bacteria

Cheese ripening

Lactic acid bacteria Bifidobacterium Enteric bacteria

Intestinal tract of warmblooded animals

Lactic acid bacteria Clostridium bacteria Enteric bacteria

Figure 8 Examples of ecological niches where different fermenting microorganisms coexist.

source of carbon and energy, converting it to stoichio-metric amounts of acetate. Another example is seen in Malonomonas rubra, which is a microaerotolerant fermenting bacterium that can maintain its energy metabolism for growth by decarboxylation of malonate to acetate. M. rubra is closely related to the cluster of mesophilic sulfur-reducing bacteria within the delta subclass of Proteobacteria, with the fermenting bacterium Pelobacter acidigallici and the sulfur reducers Desulfuromusa kysingii, D. bakii, and D. succinoxidans as its closest relatives.

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