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The discovery of an antibiotic that is more active against gram-negative than against gram-positive bacteria and that appears to be related to the polymyxinaerosporin type of antibiotics was recently announced by Murray and Tetrault (1948). The suggestion is made that this substance be named "circulin" since the organism producing it closely resembles BaciUus circulans. This preliminary report summarizes some general information on circulin and will be followed by more detailed papers on the various aspects of our work. EXPERIMENTAL METHODS AND RESULTS Isolation and identification of the organism. The organism, one of about 50

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capsules. Similarly, cells grown in peptone glycerol broth with added phosphate did not contain appreciable amounts of stainable polysaccharide material. Growth was good and pigment and polysaccharide were formed readily when mannose, galactose, sucrose, or maltose was substituted for the glycerol; but when glucose was used, growth and pigmentation were poor and cellular morphology atypical, unless calcium carbonate or another buffer was present to control the high acidity that developed.Aerobacter aerogenes was also grown on the shaker in the peptone glycerol media, with and without added phosphate. The microscopic appearance of the cells in both media was strikingly similar to that of the Hy strain of S. marcescens. The cytoplasm of the cells grown in the low phosphate medium was intensely stained by Schiff's reagent following treatment with periodate, whereas that of the cells grown with added phosphate showed only a trace of pink.Therefore our results with S. marcescens are entirely in agreement with those obtained by Duguid with Aerobacter aerogenes: when growth was checked by low concentrations of nitrogen or of phosphate, the cells elaborated capsular material and assumed a "nuclear" appearance. This was particularly evident when the cultures were grown on a shaker. In addition, it was found that the cytoplasm of these "nuclear" cells, which had lost the ability to retain simple basic dyes, gave a positive reaction for polysaccharides when exposed to Schiff's reagent after treatment with periodate. It is of interest to us that accumulation of the pigment prodigiosin, a pyrrole derivative (Wrede and Rothhaas