An efficient method to screen for the soil bacteria producing therapeutically effective antibiotics.

The discovery of novel therapeutic antimicrobials has become an urgent issue in response to the global crisis of the spread of multi-drug-resistant bacteria. In this report, we propose an efficient screening method for antimicrobial agents with therapeutic potential from soil bacteria. With this method, colonies of the soil bacteria were formed first on agar plates containing only an extract of soil, followed by an overlay of soft agar containing the pathogens, an antibiotic target. Then, we selected the colonies that formed the inhibitory zones on soft agar and evaluated the therapeutic efficacy of their culture supernatants using a silkworm bacterial infection model. Using Staphylococcus aureus as an indicator strain to obtain bacteria that produce therapeutically effective antimicrobials, we succeeded in reducing the screening size by 20-fold compared to the conventional method. An analysis of 86 antibiotics producers identified in this study indicated that the majority belonged to Streptomyces sp. and Lysobacter sp., well-known producers of secondary metabolites. Besides, the presence of eight genera and 37 species among the identified species indicated the diversity of antibiotic producers. Based on the finding of our study, we propose this method as an efficient way to discover novel antimicrobial agents that are therapeutically effective.