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It has now been definitely established that the ability to produce streptomycin is limited to a certain few strains of Streptomyces griseus. Numerous other strains of this organism, either isolated from natural substrates or taken from culture collections, were found to be unable to produce streptomycin or any other antibiotic. A certain few strains, however, were found capable of forming other antibiotics (Waksman, Schatz, and Reynolds, 1946). One such strain produced in the culture medium an antibacterial substance that was designated "grisein"; this antibiotic was characterized by certain chemical and antibacterial properties that were quite distinct from other well-known antibiotics (Reynolds, Schatz, and Waksman, 1947). Although grisein is in some respects similar to streptothricin and to streptomycin, such as in its solubility in water and its selective activity against various gram-positive and gram-negative bacteria, it is strikingly different from these two antibiotics in other respects. Grisein is adsorbed from the medium on activated charcoal; it is not eluted from the adsorbent by alcoholic acid solutions, but by neutral 95 per cent ethanol. Grisein has a much narrower antibacterial spectrum than that of streptothricin and streptomycin. It is not active, for example, upon Mycobacterium tuberculosis or upon other bacteria sensitive to the other two antibiotics. Bacteria develop resistance to grisein much more rapidly than to either of the other antibiotics. EXPERIMENTAL PROCEDURES AND RESULTS Description of organism. The grisein-producing strain of Streptomyces griseus (Krainsky) Waksman and Curtis shows the major characteristic properties of the species, and can scarcely be distinguished from the streptomycin-producing strains of this organism or from the inactive strains. The following are the characteristic properties of S. griseus: (1) cream-colored growth on organic and synthetic media; (2) white to cream-colored aerial mycelium with light-greenish tinge; (3) formation of aerial hyphae in clusters or tufts, the hyphae being straight and without spirals (figure 1); (4) lack of pigment formation on potato, gelatin, and various peptone and protein-containing media; and (5) strong proteolytic activities, as characterized by rapid liquefaction of gelatin.

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