Fermentable substrates and the ratio of products

Typical for the propionic acid fermentation is the formation of identical fermentation products from Ce-, C5-, C4- and Cs-compounds. The ratio of products may differ, depending to a large extent on the degree of oxidation of the utilized carbon source. The ratio of propionic to acetic acid in glycerol medium was found to be 2 1, in lactate medium it was 1 1.5, and in pyruvate medium it was 1 2 (Vorobjeva, 1958a). Utilization of pyruvate resulted in the production of acetic acid with a constant rate (Vorobjeva, 1958b). The yield of propionic acid was twofold lower, and after four days it was almost unchanged (Fig. 3.6). No equimolarity between the acetate and CO2 

Evidently, Cs-compounds are utilized via the dikinase reaction (at least with lactate and pyruvate as substrates), gluconeogenetic reactions, hexose monophosphate and diphosphate pathways. It is suggested that glycerol degradation proceeds through the formation of glycerol-3-phosphate and dihydroxyacetone (Stjemholm and Wood, 1963). In cells growing on 

Lactate can be oxidized directly to pyruvate by FAD-dependent lactate dehydrogenases, and pyruvate can be further metabolized to acetate and CO2. Flowever, propionate is formed as a result of the reduction of lactate or pyruvate via the methylmalonyl-CoA pathway. 

Quantitatively, the product ratios are similar when glycerol, erythritol, adonitol or mannitol are fermented (Wood and Leaver, 1953). The same type of radioactive labeling (by CO2) is found in products arising from the fermentation of C3- and Ce-substrates. These data show that there are no separate mechanisms for C4- and Cs-compounds. 

As mentioned above, propionic, acetic, succinic acids and CO2 are file main, but not the sole fermentation products in propionic acid bacteria. Propionibacteria contain enzymatic systems responsible for the formation of formate, with variable activity levels in different species. P. shermanii and P. arabinosum produced up to 0.077 and 0.160 pM formic acid, respectively, in glucose media (Mashur et al., 1971). In P. jensenii the formation of formic acid was observed in neutral or weakly alkaline lactate media, but the quantity of formate decreased towards the end of the fermentation. Addition of sodium formate to lactate stimulates the growth of propionibacteria (Vorobjeva, 1958a). A small production of formate was found in glucose fermentations of P. shermanii, P. pentosaceum, P. rubrum andP. petersonii (Vorobjeva, 1972). 

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