Culturable rare Actinomycetes: diversity, isolation and marine natural product discovery

Rare Actinomycetes from underexplored marine environments are targeted in drug discovery studies due to the Actinomycetes' potentially huge resource of structurally diverse natural products with unusual biological activity. Of all marine bacteria, 10 % are Actinomycetes, which have proven an outstanding and fascinating resource for new and potent bioactive molecules. Past and present efforts in the isolation of rare Actinomycetes from underexplored diverse natural habitats have resulted in the isolation of about 220 rare Actinomycete genera of which more than 50 taxa have been reported to be the producers of 2,500 bioactive compounds. That amount represents greater than 25 % of the total Actinomycetes metabolites, demonstrating that selective isolation methods are being developed and extensively applied. Due to the high rediscovery rate of known compounds from Actinomycetes, a renewed interest in the development of new antimicrobial agents from rare and novel Actinomycetes is urgently required to combat the increasing number of multidrug-resistant human pathogens. To facilitate that discovery, this review updates all selective isolation media including pretreatment and enrichment methods for the isolation of marine rare Actinomycetes. In addition, this review demonstrates that discovering new compounds with novel scaffolds can be increased by intensive efforts in isolating and screening rare marine genera of Actinomycetes. Between 2007 and mid-2013, 80 new rare Actinomycete species were reported from marine habitats. They belong to 23 rare families, of which three are novel, and 20 novel genera. Of them, the family Micromonosporaceae is dominant as a producer of promising chemical diversity.