Of all the results of pyruvate metabolism, probably the most varied is fermentation. Technically speaking, fermentation is a process that involves (1) the incomplete oxidation of glucose or other carbohydrates in the absence of oxygen; (2) the oxidation of NADH to NAD+, which is essential to the continuation of the glycolytic pathway; (3) the use of organic compounds as the terminal electron acceptors; and (4) the release of a small amount of ATP.
Over time, the term fermentation has acquired several looser meanings. Originally Pasteur called the microbial action of yeast during wine production ferments, and to this day, biochemists use the term in reference to the production of ethyl alcohol by yeasts acting on glucose and other carbohydrates. Fermentation is traditionally what bacteriologists call the formation of acid, gas, and other products by the action of various bacteria on pyruvic acid. Industrial processes that produce chemicals on a massive scale through the actions of microbes are also called fermentations. Each of these usages is acceptable for one application or another.
It may seem that fermentation yielding only meager amounts of energy (2 ATPs maximum per glucose) would slow down growth. And yet the process is a common metabolic strategy among bacteria. What actually happens is that many bacteria can grow as fast as they would in the presence of oxygen. This rapid growth is made possible by an increase in the rate of glycolysis. From another standpoint, fermentation permits survival and growth in the absence of molecular oxygen and allows colonization of anaerobic environments. It also enables microorganisms with a versatile metabolism to adapt to variations in the availability of oxygen. For such facultative microbes, fermentation provides a means to function even when oxygen levels are too low for aerobic respiration or the usual electron acceptor is unavailable. An additional advantage of fermentation is that it generates a number of intermediates that can be used in anabolic pathways and shared within a population of microbes.
Bacteria that digest cellulose in the rumens of cattle and other ruminants are largely fermentative. After initially hydrolyzing cellulose to glucose, they ferment the glucose to organic acids, which are then absorbed as the bovine’s principal energy source. Even human muscle cells can undergo a form of fermentation that permits short periods of activity after the oxygen supply in the muscle has been exhausted. Muscle cells convert pyruvic acid into lactic acid, which allows anaerobic production of ATP to proceed for a time. But this cannot go on indefinitely, and after a few minutes, the accumulated lactic acid causes muscle fatigue.
Products of Fermentation in Microorganisms
Alcoholic beverages (wine, beer, whiskey) are perhaps the most prominent among fermentation products; others are solvents (ace tone, butanol), organic acids (lactic, acetic), dairy products, and many other foods. Derivatives of proteins, nucleic acids, and other organic compounds are fermented to produce vitamins, antibiotics, and even hormones such as hydrocortisone.
Fermentation products can be grouped into two general categories: alcoholic fermentation products and acidic fermentation products (figure 1). Alcoholic fermentation occurs in yeast or bacterial species that have metabolic pathways for converting pyruvic acid to ethanol. This process involves a decarboxylation of pyruvic acid to acetaldehyde, followed by a reduction of the acetaldehyde to ethanol. In oxidizing the NADH formed during glycolysis, NAD+ is regenerated, thereby feeding back to and maintaining the glycolytic pathway. These processes are crucial in the production of beer and wine, though the actual techniques for arriving at the desired amount of ethanol and the prevention of unwanted side reactions are important tricks of the brewer’s trade (8.5 Making Connections). Note that the products of alcoholic fermentation include both ethanol and CO2, a gas that accounts for the bubbles in champagne and beer and the rising of bread dough.
Fig1. The fermentation systems that produce acid and alcohol. In both cases, the final electron acceptor is an organic compound. In yeasts, pyruvic acid is decarboxylated to acetaldehyde and CO2, and the NADH given off in the glycolytic pathway reduces acetaldehyde to ethyl alcohol. In homolactic fermentative bacteria, pyruvic acid is reduced by NADH to lactic acid. Both systems regenerate NAD+ to feed back into glycolysis or other cycles (pink dotted lines).
Alcohols other than ethanol can be produced during bacterial fermentation pathways. Certain clostridia produce butanol and iso propanol through a complex series of reactions. Although this process was once an important source of alcohols for industrial use, it has been largely replaced by a nonmicrobial petroleum process.
The pathways of acidic fermentation are extremely varied. Lactic acid bacteria ferment pyruvate in the same way that humans do—by reducing it to lactic acid. If the product of this fermentation is mainly lactic acid, as in certain species of Streptococcus and Lactobacillus, it is termed homolactic. The souring of milk is due largely to the production of this acid by bacteria. When glucose is fermented to a mixture of lactic acid, acetic acid, and carbon dioxide, as is the case with Leuconostoc and other species of Lactobacillus, the process is termed heterolactic fermentation.
Many members of the family Enterobacteriaceae (Escherichia, Shigella, and Salmonella) possess enzyme systems for converting pyruvic acid to several acids simultaneously (figure 2). Mixed acid fermentation produces a combination of acetic, lactic, succinic, and formic acids, and it lowers the pH of a medium to about 4.0. Propionibacterium produces primarily propionic acid, which gives the characteristic flavor to Swiss cheese; gas (CO2) from the same fermentation reaction produces the holes. Some members also further decompose formic acid completely to carbon dioxide and hydrogen gases. Because enteric bacteria commonly occupy the intestine, this fermentative activity accounts for the accumulation of some types of gas—primarily CO2 and H2—in the intestine. Some bacteria reduce the organic acids and produce the neutral end product 2,3-butanediol.
Fig2. Miscellaneous products of pyruvate fermentation and some of the bacteria involved in their production.
We have provided only a brief survey of fermentation products, but it is worth noting that microbes can be harnessed to synthesize a variety of other substances by varying the raw materials provided to them. In fact, so broad is the meaning of the word fermentation that the large-scale industrial syntheses by microorganisms are not technically fermentations and often utilize entirely different mechanisms from those described here. They even occur aerobically, particularly in antibiotic, hormone, vitamin, and amino acid production, but they still produce a valuable commercial product.
