Streptomyces distallicus, a Potential Microbial Biolarvicide

Infected mosquitos from the genus Aedes have become one of the world's most influential contributors to human morbidity and death. To explore new biopesticides with activity against Aedes aegypti, Streptomyces distallicus, a species related to the subspecies group of Streptomyces netropsis, was investigated. Six metabolites, aureothin, allo-aureothin, deoxyaureothin, 4',7-dihydroxy isoflavone, 2-methyl-5-(3-indolyl)oxazole, and 2-ethyl-5-(3-indolyl)oxazole were isolated, and chemical structures, were elucidated based on one- and two-dimensional NMR spectroscopy analyses and HRMS. The A. aegypti larvicidal activity of these compounds was evaluated. Only two isomeric compounds, aureothin and allo-aureothin, showed larvicidal activity against A. aegypti with LC₅₀ values of 1.5 and 3.1 ppm for 24 h post-treatment, respectively, and 3.8 and 7.4 ppm for 48 h post-treatment, respectively. The crude extract of S. distallicus also demonstrated potent larvicidal activity with LC₅₀ values of 1.46 and 1.2 ppm for 24 and 48 h post-treatment, respectively. Deoxyaureothin, a furan ring reduced form of aureothin, showed no activity against A. aegypti. The hybrid imported fire ants activity of aureothin was also evaluated, but it did not show any activity at the highest dose of 62.5 μg/g. Described here is the first report on a bioassay-directed investigation of the secondary metabolites of S. distallicus and biological evaluation of isolated compounds aureothin and its isomer and intermediates as potential microbial larvicides. S. distallicus and crude extracts thereof are a promising source of potential microbial biolarvicides.