Comparative pathogenicity of Photorhabdus entomopathogens against a stored grain pest, Tenebrio molitor and secondary metabolite profiles

Photorhabdus (Entomopathogenic bacteria; EPB) is the symbiont of entomopathogenic nematodes (EPNs) in the genus Heterorhabditis . EPB are virulent and kill host insects. This study aimed to explain the virulence of Photorhabdus against a stored grain pest, Tenebrio molitor (Coleoptera: Tenebrionidae). The Photorhabdus strains were Photorhabdus temperata subsp. temperata ANU101 (‘Ptt’), P. luminescens subsp. thracensis (‘Pl thra’), P. luminescens 193 (‘Pl 193’), and P. luminescens subsp. laumondii (‘Pl laum’). Glycerine stock of these strains was cultured in tryptic soy broth and different dose (0, 10 1 ,10 2 ,10 3 ,10 4 ,10 5, and 10 6 CFU/larva) were injected into the hemocoel of fifth instar larvae to observe their pathogenicity against T. molitor . Among these, ‘Ptt’ showed the highest pathogenicity (LD 50 : 484.56 CFU/larva and LT 50 : 14.01 h) but ‘Pl laum’ showed the lowest pathogenicity (LD50: 1604.53 CFU/larva and LT50: 20.33 h) against T, molitor . GC-MS analysis of the organic extracts predicted the maximum number (166) of metabolites from ‘Ptt’ while the lowest number (111) of metabolites from ‘Pl laum’. However, eight metabolites were common in all Photorhabdus strains and from these, two metabolites, namely o-Cyanobenzoic acid (CBA) and Indole (IND), were synthesized and checked for their insecticidal activity. Mortality bioassays showed that both metabolites had toxicity against the third instar larvae of T. molitor , but CBA was more toxic (LC 50 : 17.45 ppm) than IND (LC 50 : 647.08 ppm). This study first reported the comparative study among four Photorhabdus strains for metabolite profiles, pathogenicity and toxicity against the stored product pest, T. molitor . • Photorhabdus temperata subsp. temperata ANU101 (‘Ptt’) found most virulent against Tenebrio molitor larvae. • GC-MS analysis predicts maximum number of metabolites from ‘Ptt’ culture broth. • Eight high quality (≥90) metabolites including o-Cyanobenzoic acid (CBA) and Indole (IND) were common in Photorhabdus strains. • Bioassay with T. molitor larvae showed that both CBA and IND are toxic; however, CBA was more toxic than IND.