Fermentative processes are carried out by different micro-bial groups that can interact in well-defined habitats for the transformation of the substrate. The balance between the different microbial groups in each specific ecosystem depends on their initial concentrations and on environmental conditions (e.g., temperature, O2 concentration, pH, and nutrients), and the predominance of one or the other determines the type of fermentation and the characteristics of the final product. Some fermented milks (Kefir, Koumiss) and sourdoughs are ecological niches colonized by different fermenting microorganisms, such as lactic acid bacteria and yeasts, which carry out contemporary lactic acid and alcoholic fermentation. Lactic acid and pro-pionic bacteria intervene sequentially during 'eyed' cheese production. Lactic acid bacteria ferment lactose and produce lactic acid during the first stages of cheese making. Lactic acid is then used by propionic bacteria during cheese ripening. Another example of an ecosystem is the intestinal tract of warmblooded animals, where enteric bacteria, lactic acid bacteria, bifidobacteria, and clostridia coexist, all of which are involved in the transformation of the substrate via different fermentation pathways. In this ecological niche, lactic acid and bifidobacteria are involved in the maintenance of the balanced composition of the intestinal microflora. Different fermenting microorganisms, such as lactic acid bacteria and clostridia, are involved in silage fermentation and maturation. Also in this case, lactic acid bacteria should predominate to counteract the development of clostridia and other spoilage microorganisms (Figure 8).
See also: Biological Nitrogen Fixation; Excretion.
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