Rare actinobacteria: a potential source of bioactive polyketides and peptides

Polyketides and peptides obtained from actinobacteria are important therapeutic compounds which include front line antibiotics and anticancer drugs. Many screening programs are directed towards isolation of bioactive compounds from these organisms but the chances of finding novel antimicrobial leads among common actinobacteria are fast dwindling. As a result, the focus has shifted to the members of less exploited genera of rare actinobacteria. Three isolates, MMS8, MMS16 and KCR3 found to be potent polyketide and peptide producers were identified by 16S rRNA gene sequencing and their sequences deposited in the GenBank under the accession numbers MG407702, MG372012 and MG430204 respectively. MMS8 identified as Micromonospora auratinigra, yielded one potent compound determined to be chloroanthraquinone with an minimum inhibitory concentration (MIC) of 8 µg/ml against Bacillus subtilis and an IC₅₀ value of 10 µg/ml and 4 μg/ml against HeLa and IMR cell lines respectively. This is the first report of the production of chloroanthraquinone by M. auratinigra. MMS16, identified as a member of the family Micromonosporaceae, yielded a potent compound MMS16B analyzed to be a novel bafilomycin analogue. The MIC of the compound was found to be 7 μg/ml against B.subtilis and IC₅₀ value against HeLa and IMR was observed to be 9 μg/ml and 14 μg/ml respectively. MMS16B was also found to exhibit anti-quorum sensing (AQS) activity at sublethal concentrations. KCR3 identified as Kocuria kristinae yielded a novel antimicrobial peptide with antibacterial, antifungal and AQS activity. To the best of our knowledge, no antimicrobial activity has ever been reported from K. kristinae.