Ethanol
Ethanol is produced in some yeasts and bacteria as an end-product to their carbohydrate metabolism. The production of ethanol generates energy for these microbes, and a popular intoxicant for humans. It is for this reason that humans have cultured and cared for various strains of microorganisms for millennia. Recently, we have begun using this alcohol as a partial substitute for petrochemical fuels.
Methanol
Methanol is a highly unpleasant byproduct of simple carbohydrate metabolism in some bacteria and yeasts. Although produced in small amounts in natural fermentation, it is produced preferentially in the metabolism of pectin. Microbes feeding on pectin-rich fermentation substrates (like those made from apples or grapes) produce far more methanol than is typical. Distilled spirits that are contaminated with methanol can cause blindness and death within hours of consumption. Although methanol itself is largely non-toxic, it is metabolized in the human liver into highly toxic formaldehyde, which is in turn metabolized into formic acid. Formic acid, the toxin contained in ants and nettles, causes injury by causing cellular acidosis.
Isopropyl Alcohol
Isopropyl alcohol is another metabolic byproduct of bacterial carbohydrate synthesis. The sugar xanthanose is most frequently decomposed into this alcohol via the butyric acid pathway. Isopropyl alcohol has a great many industrial uses, particularly as an inexpensive non-polar solvent. Ironically, it is most commonly employed as an antiseptic for its anti-microbial properties.
Propanol and Butanol
Propanol and butanol are not generally efficiently produced by most microbes. However, they are attractive alcohols for use in biofuels. They are less corrosive to internal engine components and produce greater caloric energy than ethanol, but are significantly more expensive. Recent genetic engineering of Escherichia coli bacteria, however, has greatly enhanced this species' production of these alcohols. It is hoped that microbes such as this will one day provide the world with low-cost, efficient biofuels made from these alcohols.