Acid Fuchsin Methylene Blue Agar: a New Differential Medium for Enteric Bacteria.

Despite the development of successful selective media favoring the more pathogenic members of the Enterobacteriaceae, media of primarily differential nature have not been entirely satisfactory for several reasons, such as the following: (1) Generally only colonies of lactose fermenters are colored, whereas the colorless colonies of nonfermenters tend to be masked by the diffusion of dye from fermenting colonies (Holt-Harris and Teague, 1916). Though two dyes are employed in eosin methylene blue agar, a chemical combination occurs to yield the dye methylene blue eosinate (Wynne et al., 1942), which is absorbed by fermenters only. Since methyl green resembles methylene blue in having a quinoid structure with a pentavalent nitrogen atom, a similar combination is chemically feasible in the recently devised eosin methyl green sulfite agar of Fredericq (1947). Furthermore, the color differentiation obtained with this medium is compatible with such a reaction between the dyes. (2) The medium may be too toxic for the growth of delicate enteric species (Fredericq, 1947). (3) Deterioration may occur on exposure to light (Endo, 1904). (4) Weak fermenters may not be differentiated from nonfermenters (Lederburg, 1948). The recent use of tetrazolium indicator by Lederburg (1948) seems to have largely obviated the first three difficulties, but the problem of weak fermenters is still troublesome. There are, nevertheless, many types of work in which differential media are of vital importance, particularly certain studies on antagonistic coliform strains (Wynne, 1947; Halbert, 1948a,b). Acid fuchsin methylene blue (AFMB) agar was formulated in an attempt to provide a more satisfactory differential medium essentially free from the objections listed above. A mixture of an acid and a basic dye was used on the well-known theory of Stearn and Stearn (1924) that colonies producing acid would tend to be colored by the former and those not producing acid by the latter.