In the operations of bacteria under natural conditions it is the general rule to find mixed cultures at work, the variability of the results depending upon many factors. Therefore it has been suggested by Holman (1928) that the general term “bacterial association” be used to cover such processes. When, however, the combined action of two or more micro-organisms effects changes which each by itself is incapable of achieving, the term “synergism” is applied. Synergism appears to be used now in a more restricted sense to describe a particular type of bacterial association, and has been defined by Fiallos (1925) as follows: “two bacilli neither of which causes the production of gas in certain compounds, may do so when artificially mixed together provided one of them is capable of producing acidity (never gas) in these carbon compounds, and the other though inert to these compounds (i.e. produces in them neither acid nor gas) is capable of producing gas from glucose.” In 1911 Penfold observed the production of gas from a glucose medium in which B. typhosus was growing along with a variant non-aerogenic strain of B. coli communis, the latter having been derived from a typical gas-producing culture by selective growth on agar containing sodium monochloracetate. Although this organism had lost the power of gas-fermenting glucose it retained the power of gas-fermenting sodium formate. Castellani (1925, 1926 and 1927) states that he noted the phenomenon, which he calls “symbiotic,” in 1904 when investigating the fermentation reactions of bakers' yeast, which is not a pure culture but consists generally of two or more species of yeasts together with one or more types of Gram-negative bacilli.