Abstract
Phase variation in Xenorhabdus and Photorhabdus spp. has a significant impact on their symbiotic relationship with entomopathogenic nematodes by altering the metabolic by-products upon which the nematodes feed. The preferential retention of the phase I variant by the infective-stage nematode and its better support for nematode reproduction than phase II indicates its importance in the bacterial-nematode interactions. However, there is no obvious role for phase II in these interactions. This study has revealed differences in the respiratory activity between the two phases of Xenorhabdus nematophilus A24 and Photorhabdus luminescens Hm. After experiencing periods of starvation, phase II cells recommenced growth within 2 to 4 h from the addition of nutrients, compared with 14 h for phase I cells, indicating a more efficient nutrient uptake ability in the former. The levels of activity of major respiratory enzymes were 15 to 100% higher in phase II cells from stationary cultures in complex media than in phase I cells. Transmembrane proton motive force measurements were also higher by 20% in phase II under the same conditions. The increased membrane potentials reflect upon the ability of the phase II variant to respond to nutrients, both through growth and nutrient uptake. It is postulated that while phase I cells are better adapted to conditions in the insect and the nematode, phase II cells may be better adapted to conditions in soil as free-living organisms.