توصیف دو جدایه جدید از باکتری‌های همزیست نماتدهای بیمارگر حشرات Heterorhabditis bacteriophora و Steinernema carpocapsae

The current research investigates two novel isolates of entomopathogenic nematodes from families Heterorhabditidae and Steinernematidae and their accompanying symbionts retrieved from soil specimens originating from northwestern Iran. By employing molecular techniques encompassing phylogenetic analyses of ITS-rDNA, 28S-rDNA, 16S-rDNA, and gyrB gene sequences based on maximum Parsimony criteria, we determined the nematodes' identities as Heterorhabditis bacteriophora and Steinernema carpocapsae. Additionally, based on reconstructed phylogenetic trees our findings revealed the corresponding bacterial associates as Photorhabdus thracensis and Xenorhabdus nematophila, respectively. To visualize the growth dynamics of these bacteria, we generated time-series graphs depicting population expansion over a 72-hour period following initial inoculation. Both bacterial species exhibited sigmoidal growth curves, characterized by an initial lag phase followed by exponential multiplication leading up to a plateau stage. However, striking differences emerged upon closer inspection. Xenorhabdus nematophila underwent a swift progression into its logarithmic and stationary phases before gradually tapering off. Conversely, P. thracensis manifested a slower rate of proliferation, remaining predominantly with the lag phase. Within 32 hours post-inoculation, both bacteria experienced growth reduction, but P. thracensis demonstrated a steeper decrease. Moreover, the study assessed the virulence of the bacteria against Galleria mellonella larvae, highlighting higher virulence of X. nematophila (LC50 = 294.36 × 106). Additionally, we explored the inhibitory effect of antibiotics, with Ampicillin and Chloramphenicol identified as the most effective against P. thracensis and X. nematophila, respectively. The inhibitory effect of antimicrobial agents on the growth of P. thracensis and X. nematophila was concentration-dependent, highlighting the importance of selecting appropriate concentrations for effective control of bacterial infections while minimizing disruption to beneficial symbiotic bacteria during mass production process of bacteria or entomopathogenic nematodes. Among the antibiotics with the lowest inhibitory effects on bacteria, Erfamicin caused lowest mortality on infective juveniles of both nematodes.