Isolation of two new polymyxin group antibiotics. (Studies on antibiotics from the genus Bacillus. XX).

Two new members of polymyxin group antibiotics, polymyxins S1 and T1, were isolated from the culture broths of strains identified as Bacillus polymyxa Rs-6 and Bacillus polymyxa E-12, respectively. These antibiotics are strongly basic substances, their hydrochloric acid salts are soluble in water and methanol. They are primarily active against Gram-negative bacteria in vitro and in vivo though polymyxin T1 exhibits higher activities against Gram-positive bacteria than other polymyxin group antibiotics.