Fungal pathogens pose unique threats to modern healthcare: current treatment is limited to four antifungal drug classes, and many produce adverse side effects in patients¹. Exploration of new environments could lead to the isolation of novel antifungal-producing microorganisms.

We report the isolationed and characterization ofed a mixed isolate of two Streptomyces species from dried moth larvae (Cirina forda) that exhibited antifungal synergy against filamentous fungi. Antimicrobial profiles were obtained through overlay assays with mature colonies. The observed antibacterial and antifungal activity was further characterized in regards to cellular targets of produced compounds using a panel of whole-cell biosensors containing the luciferase cassette under the control of an antibiotic-specific inducible promotor. Variation in colony morphology led to the subculturing of two morphologically distinct isolates, both of which displayed reduced activity against filamentous fungi. Whole genome sequencing identified these sub-strains as Streptomyces albidoflavus. Nucleotide variations in regulatory, metabolic genes, and biosynthetic gene clusters (BGCs) were found that may explain the observed differences in strain morphology, physiology, and antimicrobial activity. Biosensor tests indicated the production of antifungal polyene macrolide-like compounds in the strains and mixed isolate, and genome mining confirmed the presence of the BGC for the polyene macrolide candicidin. Further analysis revealed discrepancies between the antimicrobial profiles of candicidin and the mixed isolate. Determination of produced antimicrobial compounds via HPLC-MS is currently ongoing.

This work demonstrates the co-isolation of antifungal compound-producing organisms from a niche habitat. Differences in antimicrobial activity between the sub-strains and the observed synergistic relationship in the mixed isolate indicate cross-regulation and activation of antifungal BGC expression in one or both strains. Further research towards these microbial interactions could aid the discovery and development of new antifungals as well as further our understanding of antifungal BGC regulation and the effects and mechanisms of strain interaction on secondary metabolite production in Streptomyces species.

¹WHO fungal priority pathogens list to guide research, development and public health action. Geneva: World Health Organization 2022; Licence: CC BY-NC-SA 3.0 IGO.