Cephabacin M1-6, new 7-methoxycephem antibiotics of bacterial origin. I. A producing organism, fermentation, biological activities, and mode of action

New 7-methoxycephem antibiotics were found in culture filtrates of a bacterium isolated from a plant and named cephabacin M1-6. They are the first members of 7-methoxycephem antibiotics of bacterial origin. The producing organism was taxonomically characterized and identified as Xanthomonas lactamgena YK-431; other strains of this species have recently been reported to produce cephabacin F and H group antibiotics. Cephabacin M1-6 exhibited moderate antibacterial activity against Gram-negative and Gram-positive bacteria. Cephabacin M1-6 were as stable as cephamycin C to cephalosporinases. They showed inhibitory activity against a cephalosporinase of Proteus vulgaris GN 4413. The mode of action of cephabacin M1 was examined using Escherichia coli and Bacillus subtilis as test organisms; primary lethal targets of cephabacin M1 are penicillin-binding protein (PBP) 1 in E. coli and PBP 4 in B. subtilis.