Food signal production of Photorhabdus luminescens inducing the recovery of entomopathogenic nematodes Heterorhabditis spp. in liquid culture

Abstract

Photorhabdus luminescens are bacterial symbionts of entomopathogenic nematodes of the genus Heterorhabditis. The bacto-helminthic complexes are used in biocontrol of insect pests in cryptic environments. For in vitro production, liquid media are incubated with P. luminescens for 24 h prior to the inoculation of nematode dauer juveniles. The nematodes develop to self-fertilizing hermaphrodites and produce offspring. The exit from the developmentally arrested dauer stage (recovery) is a response to a yet undescribed food signal. Major process instability is caused by low and unsynchronized recovery of the dauers. In living insects, dauer recovery is approximately 95% within 1 day. In liquid cultures of P. luminescens the recovery is spread over several days and varies between 0 and 81%. In complex culture media no food signal was detected. A food signal is produced by P. luminescens and excreted into the culture medium. The maximum food signal production was recorded during the late exponential growth phase. Compared to the food signal found in insects, the efficacy of the bacterial signal is much lower. The reasons for the variable activity of the bacterial food signal and its function during the nematode life cycle are discussed.