Mixed-acid fermentation is characteristic of the Enterobacteriaceae ascribed to the genera Citrobacter, Escherichia, Proteus, Salmonella, Shigella, Yersinia, and Vibrio, and to some species of Aeromonas-, it is also carried out by some anaerobic fungi. This metabolic group includes microorganisms that have a different ecology and impact on human activities. Some of them are members of the normal intestinal microflora of mammals and other vertebrates (E. colt) and have a role in the colonization of lignocellulose in the rumen (Neocallimastix). Some others are pathogens that are responsible for human and animal diseases, and they can be abundant in aquatic and terrestrial environments. These microorganisms can ferment monosaccharides, disacchar-ides, polyalcohol, and, less frequently, polysaccharides, via the glycolytic pathway, producing lactic, formic, succinic and acetic acids, and ethanol (Figure 6). The final amounts of each end product vary depending on the microorganism and the growth conditions, however, the ratio of acid to neutral products is 4:1. Mixed-acid fermentation also results in the production of equimolar amounts of CO2 and H2 in those bacteria with the formate-hydrogen-lyase complex. Pyruvate formate-lyase and LDH, the enzymes that control entry into mixed-acid fermentation, are negatively regulated by oxygen, thus, mixed-acid fermentation requires anaerobic conditions to occur. That is why the natural habitat of microorganism carrying out mixed-acid fermentation is the gastro-enteric apparatus. The ATP yield of mixed-acid fermentation is about 2.5 mol of ATP per mole of glucose.
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