Electron microscopic investigation of symbiotic bacteria Photorhabdus laumondii transmission and retention in the entomopathogenic nematode Heterorhabditis bacteriophora

Summary The transmission and retention of the bacterial symbiont Photorhabdus laumondii during endotokia matricida in Heterorhabditis bacteriophora were examined by transmission electron microscopy of first-stage juveniles (J1), pre-dauer juveniles (J2d), dauer juveniles (DJ), and hermaphrodites. A newly developed method was applied specifically for the fixation and cutting of the J1 stage. Bacteria were present in all developmental stages, except J1. In the J2d and young DJ stages symbiont cells were localised within the cardia, while in fully-developed infective DJ they were observed to divide in the intestinal lumen. In hermaphrodites, bacteria were found in vacuole-like structures in the terminal cells of the intestine near the rectum as well as throughout the intestine. The density of the bacterial cells varied considerably between stages, being lower in J2d than in young DJ. Prolonged storage of DJ resulted in degradation of bacterial cell morphology. These findings on bacterial retention across life stages sheds light on bacterial transmission and retention mechanisms during endotokia matricida in H. bacteriophora .