PHASE VARIATION IN XENORHABDUS-NEMATOPHILUS AND PHOTORHABDUS-LUMINESCENS - DIFFERENCES IN RESPIRATORY ACTIVITY AND MEMBRANE ENERGIZATION

Phase variation in Xenorhabdus and Photorhabdus spp. has a significant impact on their symbiotic relationship with entomopathogenic nematode s by altering the metabolic by-products upon which the nematodes feed. The preferential retention of the phase I variant by the infective-st age nematode and its better support for nematode reproduction than pha se 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 betwe en the two phases of Xenorhabdus nematophilus A24 and Photorhabdus lum inescens Hm. After experiencing periods of starvation, phase II cells recommenced growth within 2 to 4 h from the addition of nutrients, com pared with 14 h for phase I cells, indicating a more efficient nutrien t uptake ability in the former. The levels of activity of major respir atory 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 t he 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, phas e II cells may be better adapted to conditions in soil as free-living organisms.