Description and Characterization of Indigenous Isolates of the Symbiotic Bacterium Photorhabdus luminescens (Thomas and Poinar)

Abstract This study aimed to identify and characterize Photorhabdus luminescens, a bacterial endosymbiont of the entomopathogenic nematode Heterorhabditis spp., collected from various crop ecosystems in Raichur, Karnataka. Both phenotypic and genotypic analyses were employed to confirm bacterial identity. Three distinct bacterial strains named UASR22PL1, UASR22PL2, and UASR22PL3 were isolated by streaking haemolymph obtained from Galleria mellonella larvae infected with Heterorhabditis spp. All strains exhibited Gram-negative bacillus morphology characteristic of P. luminescens. After 72 hours of incubation, bacterial colonies appeared smooth, small, round, glistening, and convex with entire margins. They developed a red pigmentation with a bluish hue, and over time, transitioned from primary to secondary form due to prolonged static incubation of the culture. The phylogenetic tree constructed further confirmed that the isolated strains belonged to the species P. luminescens, establishing a close group of P. luminescens strain KZ2R1.. Known for their potent insecticidal activity, these three native isolates have potential applications in biocontrol strategies for managing significant agricultural pests.