Photorhabdus luminescens subsp. akhurstii SL0708 pathogenicity in Spodoptera frugiperda (Lepidoptera: Noctuidae) and Galleria mellonella (Lepidoptera: Pyralidae)

Entomopathogenic nematodes are key players for insect pest control and constitute an environmentally friendly alternative for crop protection. The insecticidal activity of the family Heterorhabditidae relies on a tight symbiotic relationship with enterobacteria of the genus Photorhabdus, where the bacterial contribution towards the death of the host has been highlighted. In the present work, we report the identification and pathogenic characterization of Photorhabdus luminescens strain SL0708, which is the natural symbiont of Heterorhabditis indica SL0708. We evaluated the pathogenicity of whole bacterial cells and acellular extracts against both Spodoptera frugiperda larvae and Galleria mellonella. Phylogenetic analyses using a polygenic sequencing approach assigned the bacterial strain to Photorhabdus luminescens subsp. akhurstii and bioassays showed it is highly pathogenic for both insects. After 48 h of treatment with 1 × 10³ − 1 × 10⁴ CFU/larva, 100% mortality was attained. Furthermore, when intra- or extracellular bacterial extracts were injected into G. mellonella, a cumulative percent mortality of 63% and 100% was respectively obtained after 72 h. In contrast, a 10% and 93% mortality was achieved for S. frugiperda with intra and extracellular extracts, respectively highlighting the role of extracellular factors in pathogenicity. We detected extracellular activities potentially accounting for the high pathogenicity observed and these included; proteases, esterases, ureases, hemolysins and siderophores. Interestingly, S. frugiperda was more susceptible to P. luminescens SL0708 cells than G. mellonella, which contrasted to its higher resistance to H. indica SL0708 nematodes, which suggests that EPN biological control potential should also be evaluated based on bacterial symbiont pathogenicity.