Bacteria are microscopic organisms, typically a few micrometers (thousandths of a millimeter) in size, with a simple, single-celled structure. They have diverse physiologies and are widely distributed in the environment from the upper atmosphere to the deep sea and from deserts to ice fields. Many are intimately associated with animals and plants. Genetic evolutionary analyses have revealed three major divisions (or domains) of all life on this planet, of which two are bacterial in nature: Archaea (thought to represent the first life forms) and Bacteria (the “true bacteria”). The third domain, Eukarya, comprises plants (including fungi and yeast) and animals. Beer is an inhospitable environment for bacteria because of its low pH (high acidity), lack of nutrients (the yeast has consumed the easily-assimilated sugars and amino acids), relatively high ethanol concentration, lack of oxygen, and presence of inhibitory hop compounds such as the iso-alpha acids. Consequently, of the many thousands of genera and species of bacteria, few can grow or even survive in beer. Notably, no human pathogenic bacterium has ever been associated with beers. Some bacteria generate their energy by respiration, using oxygen as we do, or perhaps nitrate, which is reduced to nitrite in the process. Alternatively, they can ferment sugars in the absence of oxygen to produce, for example, lactic acid or ethanol. Some bacteria are restricted to respiration or fermentation, whereas others are more versatile and can switch between physiologies. In breweries, bacteria are largely seen as spoilage organisms. Bacteria associated with beer and breweries include acetic acid bacteria, lactic acid bacteria, Obesumbacterium, Pediococcus, Pectinatus, and Zymomonas species. Some beer styles, however, involve bacteria in their normal production, including lambic beers, Berliner weisse, and modern sour beer styles produced by adventurous craft brewers.
