a shortened name for microorganisms, are living organisms too small to see without a microscope. Single-cell microbes were the first living organisms on this planet. They existed alone for 3 billion years. There is more mass in microbes than in all other life forms combined.

Microbes can be plants such as green algae, animals such as dust mites, fungi such as yeast, protists such as protozoa, and the comparably simpler bacteria and archaea. The only microbes commonly associated with beer are yeasts and bacteria, although fungi do contaminate cereals and hops.

Since Roman times, a few rogue scholars believed there must be living things that were smaller than could be seen. Because this hypothesis could not be proven, the existence of these invisible organisms was rarely taken seriously. In 1675 Anton van Leeuwenhoek built his first microscope and was able to see some of these creatures for the first time. Although he was then able to prove microbes existed, it was centuries later before they were proven to make beer and bread or cause disease. In 1860 Louis Pasteur proved more definitively that not only did microbes spoil or ferment foods but also the food could be made free of these spoilage organisms by heating. This process is known today as pasteurization. See pasteur, louis, and pasteurization. After studying anthrax bacteria, in 1876 Robert Koch was able to prove microbes were the source of certain diseases. Although there were still skeptics, the microbes were by then finally accepted into mainstream science and into the brewing profession.