Naphthacemycins, novel circumventors of β-lactam resistance in MRSA, produced by Streptomyces sp. KB-3346-5. I. The taxonomy of the producing strain, and the fermentation, isolation and antibacterial activities

Screening for circumventors of β-lactam resistance in methicillin-resistant Staphylococcus aureus (MRSA) led us to find 17 novel antibiotics, naphthacemycins A 1 –A 11 , B 1 –B 4 and C 1 –C 2 . The naphthacemycins were isolated from a cultured broth of Streptomyces sp. KB-3346-5 by repeated silica gel column chromatography and HPLC. Naphthacemycins enhanced imipenem activity 100–500 times against MRSA at 0.5 μg ml −1 , and naphthacemycins A 4 –A 11 themselves showed MIC 50 values of 1–4 μg ml −1 against 22 MRSA strains.