ABSTRACT
In recent years, the insect microbiome has become the focus of many actinomycete researchers in their search for novel bioactive compounds with members of the order Hymenoptera at the forefront of the revolution. Hymenoptera encompasses all bees, wasps, ants, and sawflies and is the third largest insect order by species richness. Additionally, Hymenoptera is the most diverse insect order in terms of ecological roles, behaviors, and social systems, thus making it an ideal starting point in the search for symbiotic actinomycetes. The aim of this review is to summarize current knowledge on hymenopteran associations with actinomycetes including information on interactions between actinomycetes and hymenopterans, isolation, and screening methodologies, as well as novel actinomycete species and natural products discovered between early 2013 and 2023. A total of 19 new species were discovered within this time period, with the genus Streptomyces being represented by 11 species while the remaining 8 belonged to rare actinomycetes genera. In addition, 35 novel compounds were reported from hymenopteran-associated actinomycetes within the same time period with the majority originating from Streptomyces strains. The reported novel compounds exhibit a range of biological activities including antibacterial, antifungal, anticancer, anti-enzymatic, and antiproliferative activity, as well as cytotoxicity.
ABSTRACT
In the search for novel therapeutics to combat the ongoing antimicrobial resistance crisis, scientists are turning to underexplored environments. Defensive mutualisms between hymenopteran insects and actinomycetes represent important reservoirs for bioactive compounds. In this study, we examined the association between actinomycetes and Squamellaria ant-plants spanning three different ant and plant species combinations (Squamellaria imberbis-Philidris nagasau, Squamellaria tenuiflora- Technomyrmex vitiensis, and Squamellaria tenuiflora-Tetramorium insolens). Eight Squamellaria plants were sampled including four containing T. vitiensis, three containing P. nagasau, and a single plant containing T. insolens. A total of 47 actinomycetes were obtained from the sampled material, with 5, 16, and 26 isolates originating from cuticle, tissue, and nest samples, respectively. Cross-streaking tests showed that 12 out of 47 isolates inhibited bacterial pathogens. The most frequently inhibited pathogens in the cross-streaking tests were S. aureus and E. coli while S. enterica was the least inhibited. Among the three primary screening media used, ISP2 agar was the most suitable for secondary metabolism as more isolates exhibited antibacterial activity when grown on the medium. TFS2010 and TFS2003, which matched to Streptomyces gramineus (>99% similarity), were the most bioactive isolates in cross-streaking tests. TFS2010 displayed the strong antibacterial on Nutrient agar, Mueller Hinton agar, and ISP2 agar while TFS2003 only exhibited strong antibacterial activity on Nutrient agar. Furthermore, a difference in potency of extracts based on batch culture medium was noted in TFS2010. DNA was extracted from 19 isolates and followed by 16SrRNA gene sequencing. Analysis of sequence data revealed the presence of six genera, including Amycolatopsis, Asanoa, Jiangella, Nocardia, Nocardiopsis, and Streptomyces, with the latter being the most abundant taxon. Among these, three isolates (PNS3002, PNS3005, and TFS3001) are likely to represent new species while one (TFS2015) is likely to be a member of a novel genus. Our work represents the first attempt to study actinomycetes from Squamellaria-ant mutualisms.
