Selective isolation of bioactive soil actinomycetes belonging to the Streptomyces violaceusniger phenotypic cluster

To devise and evaluate a strategy for isolating members of the Streptomyces violaceusniger phenotypic cluster, which are known to be a promising source of bioactive metabolites.

The treatment of four soil samples with 1.5% phenol (30 degrees C, 30 min) prior to inoculation on humic acid-vitamin (HV) agar eliminated most of the streptomycetes and other bacterial populations. The surviving streptomycetes on the HV isolation plates were subcultured, and species-group identification was made according to the probabilistic identification system of Williams et al. (1989). Of the 133 streptomycetes subcultured, 102 (77%), were assigned to the S. violaceusniger cluster. A test with an overlay technique revealed that all of these S. violaceusniger-cluster isolates had broad antimicrobial spectra, as they inhibited the growth of all test Gram-positive bacteria, yeasts and filamentous fungi. Antitumour activity against colon carcinoma cells was found among 68 or 67%, of these S. violaceusniger-cluster isolates, following growth in submerged culture.

Chemical pretreatment of soil samples with phenol reduces the growth of ubiquitous Streptomyces species, thereby facilitating the recovery of less-abundant S. violaceusniger-cluster strains that are characterized by high antimicrobial and antitumour activities.

The development and application of new methodologies with which to selectively isolate rare, bioactive streptomycete groups is important for discovering novel secondary metabolites with bioactive properties.