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Abstract
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This study analyzes two isolates of entomopathogenic nematodes and their symbiotic bacteria, recovered from soil samples
in north-western Iran. Molecular techniques, including phylogenetic analysis of ITS-rDNA, 28 S-rDNA, 16 S-rDNA, and
gyrB sequences, identified the nematodes as Heterorhabditis bacteriophora and Steinernema carpocapsae, along with their
symbiotic bacteria Photorhabdus thracensis and Xenorhabdus nematophila, respectively. The growth dynamics of these
bacteria were also investigated, revealing similar growth curves but distinct patterns. X. nematophila showed a rapid transi-
tion into the log and stationary phases, contrasting with P. thracensis. Within 32 h post-inoculation, both bacteria exhibited
reduced growth, with P. thracensis showing a sharper decline. Additionally, the study assessed bacterial virulence against
Galleria mellonella larvae, demonstrating higher virulence for X. nematophila (LC 50 = 294.36 × 10 6 ). The inhibitory effects
of antibiotics were also examined, with Ampicillin and Chloramphenicol identified as the most effective against P. thracen-
sis and X. nematophila, respectively. The antimicrobial effects were concentration-dependent, emphasizing the importance
of selecting appropriate concentrations to effectively control bacterial infections while minimizing disruption to beneficial
symbiotic bacteria during mass production of the bacteria or nematodes. Among the antibiotics with the lowest inhibitory
effects, Erfamicin caused the least mortality in the infective juveniles of both nematode species
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