GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

14. TYPES OF FERMENTATION

14.5. MIXED-ACID FERMENTATION AND THE FAMILY ENTEROBACTERIACEAE

14.5.2. Fermentation Products and Metabolic Pathways

Hexose Catabolism proceeds via The Glycolytic Pathway, and very rarely through The pentose phosphate cycle. Gluconate catabolism occurs via the Entner-Doudoroff (KDPG) pathway. Depending on the Fermentation products formed, Two Types of mixed-acid fermentation are distinguished: 1) fermentation characteristic of Escherichia coli, where acids (formic, acetic, succinic, and lactic) are predominantly produced, with no 2,3-butanediol formation; 2) fermentation characteristic of Enterobacter, where 2,3-butanediol is the main product, and organic acids play a secondary role. These fermentation types differ in the reactions associated with Pyruvate METABOLISM.

Features of Escherichia coli fermentation. These features include:

1) Cleavage of pyruvate into acetyl-CoA and formate (formic acid); 2) breakdown of formate into СО₂ and Н₂; 3) reduction of acetyl-CoA to ethanol; 4) formation of succinate as a product of fumarate Respiration; 5) inability to form acetoin and 2,3-butanediol from pyruvate (Fig. 14.5).

Class="center">

Fig. 14.5. Mixed-acid fermentation

Features of Enterobacter aerogenes fermentation. Acetoin is formed from two molecules of pyruvate (see Fig. 14.5). This process consists of two decarboxylation reactions accompanied by СО₂ release. It is this intensive gas production that gave this bacterium its name. Acetolactate is an intermediate product. 2,3-Butanediol is produced via the reduction of acetoin mediated by NADH.