Fermentative metabolism is widespread among living organisms, and the ecology of fermentative processes is particularly complicated due to the ability of different organisms to ferment a plethora of substrates under different environmental conditions. Fermentation is carried out in anoxic environments by strict anaerobic or facultative anaerobic microorganisms, although some microaerophilic or facultative anaerobic microorganisms are also able to carry out fermentation in the presence of oxygen.
Table 1 Metabolic biodiversity among living organisms Metabolic diversity Characteristics
Phototrophy When radiant energy is absorbed by chlorophyll or other similar pigments, resulting in an excitation of the electrons present in the complex and providing oxidation that produces oxygen (oxygenic photosynthesis) or does not produce oxygen (nonoxygenic photosynthesis) Aerobic respiration Molecular oxygen is the final acceptor of electrons in a redox reaction and appears in reduced form as water
Anaerobic respiration Under anaerobic conditions where molecular oxygen is absent or limited, inorganic ions (nitrate, sulphate, carbonate) serve as final acceptors of electrons Fermentation An organic compound that is often a metabolic intermediate coming from oxidation of an organic compound serves as terminal oxidant, producing a more reduced organic molecule as the metabolic end product
2 - 3 Butanediol
Figure 1 Main end products of the various fermentation processes.
Microaerophilic fermenting microorganisms, such as 1
lactic acid bacteria, colonize habitats that are character- s ized by low oxygen concentrations or the absence of t oxygen (often c. 10% of atmospheric levels). These i microaerophilic microorganisms show a wide ecological e distribution in well-defined habitats, such as the animal and human oral cavity, the gastro-intestinal tract, and m feces, as well as in fermented meat, beverages, vegetables T
(silage, olive brine, sauerkraut), and dairy products. Lactic g acid bacteria have limited biosynthetic ability and require (
pre-formed amino acids, group B vitamins, purines, and s pyrimidines. These multiple requirements restrict their w growth to habitats where the required compounds are t abundant. The fermentation activity of lactic acid bacteria i produces large amounts of lactic acid, which can cause a v drop in pH to about 4.0, and thus inhibit the growth of :
most other bacteria and exert an antagonistic effect on c spoilage microorganisms and the most common human v pathogens. For these reasons, the transformation of food :
and beverages by fermentation is one of the main mod- t alities for the conservation of food products and the s enhancement of their quality. t
Besides microaerophilic lactic acid bacteria, anaerobic t facultative microorganisms such as yeasts are the main t group of microorganisms that are involved in the transformation of fermented food. In contrast to the facultative f anaerobic bacteria, which always preferentially carry out h respiration as a metabolic pathway in the presence of s oxygen, facultative anaerobic yeast can also adopt a fer- p mentative metabolism in the presence of oxygen. This is n the case for Saccharomyces cerevisae, the most representative t yeast used in a wide varieties of industrial fermentative p processes. S. cerevisiae exhibits a specific mechanism of t respiro-fermentative regulation that is known as the w 'Crabtree effect'. At high sugar concentrations, and even 1 in the presence of oxygen, fermentation overrides i respiration. This mechanism results in a high rate of sugar consumption and therefore the colonization of habitats with high sugar content. Consequently, the ecological niche of fermentative yeasts includes sugary fruits, flowers, lymph, and tree exudates.
Anaerobic facultative bacteria represent a group of microorganisms that is broadly diffuse in several habitats. These microorganisms can grow in the presence of oxygen, and they can use fermentative metabolism when oxygen is not available. Anaerobic facultative bacteria such as Enterobactericeae colonize the intestinal tract of warmblooded animals, and some species belonging to the genera Yersinia, Salmonella, and Shigella can cause infectious diseases. The habitat of enteric bacteria is very specific. Escherichia coli, the most well-known species among the enteric bacteria, is used as indicator of fecal contamination of water and food because of its low survival in other environments. Since enteric bacteria are anaerobic and facultative, they have important roles in the ecology of the gut of warmblooded animals. By consuming the oxygen available in this habitat they maintain the right conditions for the proliferation of other bacteria that constitute the intestinal microflora (lactic acid bacteria, bifidobacteria).
Other facultative or obligate anaerobic bacteria can be found in both soil and water environments, where they have fundamental roles in the transformation of organic substances under anoxygenic conditions. An example is provided by methanogenesis, a bioprocess that occurs in natural environments, such as the rumen, marshes, and the industrial sites of anaerobic wastewater treatment plants. During this process, a first group of fermenta-tive/hydrolytic microorganisms produces low molecular weight organic acids, alcohols, CO2, and H2. Another bacterial group, known as the 'syntrophic microorganisms' is very important for the conversion of organic compounds to CH4 during methanogenesis. In this process, called syntrophy, methanogenic bacteria cooperate with other fermenting microorganisms to produce the substrates that are necessary to realize their specific ecological interactions. The H2 derived from organic molecules and produced after energetically unfavorable fermentation is consumed by methanogenic bacteria, and the overall reaction produces energy that is used by the syntrophic couple.
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