Antibiotic-Producing Properties of Streptomyces 3560, A Member of the S. Flavus Group.

STArION, RUTGERS UNIVERSITY Communicated March 20, 1952 It has often been observed that when the potency of certain antibiotics is determined by the agar-streak dilution method, an increase in the time of incubation of the plates will result in an apparent decrease in antibiotic potency. This is true particularly of aureomycin and terramycin.' This points to the fact that either the test organisms are becoming rapidly resistant to these antibiotics or that the antibiotics are unstable under the conditions of the assay. Since it has been shown that the development of resistance among bacteria to aureomycin and terramycin follows the penicillin and neomycin pattern, rather than that of streptomycin, one may be justified in concluding that the lability of antibiotics of the AT2 type or their rapid inactivation by some of the constituents in the medium or by the products of the test organisms is the correct explanation of the observed phenomenon. Similar results were obtained when aureomycinor terramycin-producing cultures were tested for their effects upon different batceria by the agar cross-streak method: the longer the incubation period of the plates, after cross-streaking with the test bacteria, the smaller were the zones of their growth inhibition.-.. Waksman and Lechevalierl suggested that by taking into-consideration this fact, namely, the diminishing rather than the absolute width of the zones of inhibition of growth of test organism in the cross-streak tests. it