Elucidation of lipopeptide biosurfactants responsible for the larvicidal activity of Bacillus velezensis PHP1601 towards Lucilia cuprina larvae

Abstract

ABSTRACT Lipopeptide biosurfactant compounds derived from cultures of Bacillus velezensis PHP1601 (Bacillales: Bacillaceae) show antagonism towards the larval stage of Lucilia cuprina (Diptera: Calliphoridae), a blowfly pest of agricultural significance. A study was undertaken to characterise and elucidate the lipopeptide biosurfactant compounds contributing to this effect. Lipopeptide extracts were obtained from cultures grown in Landy medium by acid precipitation and methanol extraction. Thin layer chromatography and UPLC ESI-TOF MS were used to partially purify and characterise the lipopeptides present in the extract. Lipopeptide fractions contained homologues of surfactin (C13–C17), fengycin (C14–C17) and iturin (C14–C17). Each lipopeptide fraction (20 µg g−1) displayed larvicidal activity against second-instar L. cuprina larvae, with a highly polar surfactin fraction (Rf: 0.90) being the most effective. The potency of surfactin was confirmed with bioassays incorporating a surfactin standard whereby a LT50 of 179.97 h and LC50 of 9.87 µg g−1 was determined. Interestingly, larvae cadavers recovered from the bioassays displayed significant physiological discolouration and stunting; this was attributed to the biosurfactant nature of the lipopeptide compounds. These findings corroborate the role of lipopeptide compounds, specifically surfactin, in the fly biocontrol mechanism of PHP1601 and constitute the first report of these compounds being insecticidal towards blowfly larvae.