Zymomonas is a Gram-negative beer-spoilage bacterium. It has a notorious legacy as a historic contaminant of breweries. Zymomonas bacteria are similar to Gluconobacter and are distinguished by their short rods and very active polar flagellae. These give the cells a strong motility indicated by their rapid movement under a microscope. This bacterium is distinctive in that it can ferment glucose, fructose, and sucrose—as can brewers yeasts—but it cannot ferment maltose. Also like yeast, it produces ethanol and carbon dioxide, but at a more efficient rate. However, its fermentation by-products include acetaldehyde as well as hydrogen sulphide, which is rapidly converted into objectionable vegetal flavors. Zymomonas is also resistant to acidic conditions and may thrive together with acetic acid bacteria. A Zymomonas infection can therefore result in a very sour beer. Because Zymomonas does not flourish in a habitat with high levels of maltose, it does not grow well in wort. As soon as yeast has converted maltose into alcohol, however, Zymomonas can become a problem and spread quickly. This is why in the 1950s and 1960s, when the priming of cask-conditioned ales with glucose or sucrose was fairly common in the UK, outbreaks of Zymomonas in a brewery were common, too—a problem that may have hastened the decline of cask beer and the general shift to filtered kegged beer. Once Zymomonas has colonized brewery equipment, it is difficult to eradicate. Sometimes contaminated equipment even needs to be replaced. Today, Zymomonas has become relatively rare in breweries, mostly because of much improved hygiene management and because priming with sugars is rarely employed nowadays. Zymomonas has one promising modern application, though: Its efficient production of ethanol makes it highly valuable in the manufacture of bioethanol and other chemical products.