New Approaches to Drug Discovery for Combating MRSA.

Methicillin-resistant Staphylococuss aureus (MRSA) is a major nosocomial pathogen that has developed resistance to many antibiotics. New anti-infective drugs to prevent and treat MRSA infection are required. Four assay systems were conducted to screen microbial cultures for new anti-infective compounds active against MRSA. Nosokomycins, new members of the phosphoglycolipid family, were discovered from a culture of Streptomyces cyslabdanicus K04-0144 in an MRSA-silkworm infection assay. The target molecule of nosokomycins was suggested to be the transglycosylase of penicillin binding protein (PBP) involved in MRSA peptidoglycan biosynthesis. Spirohexaline, with a hexacycline structure, was isolated from a fungal culture of Penicillium brasilianum FKI-3368 in an enzyme assay of undecaprenyl pyrophosphate (UPP) synthase, which is needed for the synthesis and transport of GlcNAc-MurNAc-pentapeptides from the cytoplasmic membrane site to the external membrane site for peptidoglycan synthesis. Spirohexaline inhibited MRSA growth by the blockade of UPP synthase activity. Cyslabdan, with a cysteine-carrying labdan skeleton, was also discovered from the nosokomycin-producing actinomycete as a potentiator of imipenem activity against MRSA. The molecular target of cyslabdan was identified as FemA, which is involved in the synthesis of a pentaglycine interpeptide bridge in MRSA peptidoglycan. Citridone A with a unique 6-6/5/5-ring system containing a rare phenyl-R-furopyridone skeleton, originally isolated as a potentiator of antifungal miconazole activity, was found to inhibit MRSA yellow pigment production. These new microbial products will serve as lead compounds for developing new anti-infective drugs for combating MRSA.