Biological characterization of Photorhabdus spp. and evaluation of the insecticidal and antibiotic action of cell suspension and extracts

This study identified and investigated the insecticidal and antimicrobial properties of symbiotic bacterial strains isolated from entomopathogenic nematode (EPN). The bacterial strains were identified by 16S rDNA amplification confirmed three strains as Photorhabdus luminescens (P05, P06 and L08) and one as Photorhabdus asymbiotica (P04). The insecticidal activity of the strains was assessed by direct inoculation into the hemolymph of three species of lepidopteran insects, and then the two most effective strains were inoculated at different concentrations over time against Galleria mellonella (Lepidoptera: Noctuidae) larvae. All strains were highly pathogenic and virulent to the insect species, resulting in mortality rates ranging from 85% to 100. About inoculum concentrations P04 and L08 exhibited different virulence patterns, with P04 causing more rapid mortality in G. mellonella larvae. All the Photorhabdus spp. strains in cell suspension showed positive antibiotic activity against bacterial targets and have an antifungal effect. The antibiotic and insecticidal activity of extracts (secondary metabolites) has also been the subject of investigation, but insecticidal properties of the extracts were not observed. Interestingly, the L08 extracts exhibited antimicrobial activity against Staphylococcus aureus, while P04 extracts showed fungicidal properties. These results highlight the biotechnological potential of these strains for future studies.