Characterization of Two Novel Isolates of Photorhabdus and Xenorhabdus: Molecular Identification, Growth Rate, Virulence, and Antibacterial Susceptibility

This study provides an analysis of two isolates of entomopathogenic nematodes and their symbiotic bacteria recovered from soil samples of north-west of Iran. Molecular techniques, including phylogenetic analysis of ITS-rDNA, 28s-rDNA, 16s-rDNA and gyrB sequences identified 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 explored, revealing similar growth curves and distinctive patterns. Xenorhabdus nematophila displayed a rapid transition into the log and stationary phases, differing from P. thracensis . 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 × 10 ⁶ ). 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.