Photorhabdus spp. (Enterobacteriaceae) are bacterial symbionts of soil-living entomopathogenic nematodes, Heterorhabditis spp. (Nematoda: Heterorhabditidae) except for Photorhabdus asymbiotica, which was reported in clinical samples of human wounds . The infective juveniles (IJs) of the nematode carry the bacterial symbiont in their intestine, and after entry into an insect host the IJs release the bacterial symbiont into the insect hemocoel. The insect dies within 48 h. The bacterium and nematode symbionts continue to grow and multiply within the infected insect until emergence of a new generation of IJs carrying the bacterial symbionts to initiate a new infection cycle in another insect.
A characteristic feature of this nematode–bacterium–insect interaction is that the infected insect cadavers do not putrefy , which is unlike the normal multiple microbial degradation of cadavers of insects that have died from other causes. This caused Dutky  to speculate that the symbiotic bacteria produce antimicrobial metabolites that prevent the growth of competing microorganisms and putrefaction of the nematode-infected insect cadavers. It is now known that antibiotic production is common to Photorhabdus spp. when cultured in vitro, and several antibiotics, such as stilbene derivatives, anthraquinone derivatives, genistine [3–5], a furan derivative and a phenol derivative, have been identified (Hu et al., unpublished). Although this hypothesis was generally accepted when describing the natural situation in the tripartite, nematode–bacterium–insect interaction, it is based mainly on in vitro antibiotic bioassays [2,3,5–7], and there is little in vivo experimental evidence to support the hypothesis [8,9]. Jarosz  questioned this hypothesis and reported that only a low antibiotic potency of limited spectrum of antibacterial activity occurred throughout the entire development of the nematode in Galleria mellonella infected with Steinernema carpocapsae or Heterorhabditis bacteriophora. Consequently, Jarosz  proposed that the lack of putrefaction of the infected insect was due rather to rapid growth of the bacterial symbiont preventing or minimizing competition by secondary invaders of the insect cadaver. However, Hu et al.  reported that the antibiotic, 3,5-dihydroxy-4-isopropylstilbene (ST), was present at about 1500 and 4000 μg g−1 wet insect, respectively, in larval G. mellonella cadavers infected by the nematode, Heterorhabditis megidis 90, 2 and 5 days post nematode infection. These concentrations of ST are many times higher than that needed to inhibit the growth of several species of soil bacteria and fungi under in vitro experimental conditions . Hu et al.  studied the metabolic composition of the Photorhabdus–Heterorhabditis–Galleria interaction and found that the antimicrobials, ST, 3,5-dihydroxy-4-ethylstilbene and several anthraquinone derivatives, were major metabolic components of larval G. mellonella cadavers infected by H. megidis 90. In the same study the authors reported also small amounts of an unidentified antibiotic, AT, which since has been identified as a novel antibiotic, apoxide (Hu et al., unpublished). More recently, Hu et al.  reported that ST was commonly produced in larval cadavers of G. mellonella infected by each of five different Heterorhabditis–Photorhabdus associations. This series of in vivo studies provides evidence of the presence of relatively large quantities of antibiotics in the entomopathogenic nematode-infected cadavers and tends to support Dutky's  hypothesis of antibiotic inhibition.
Despite the accumulating and sometimes conflicting information about this tripartite nematode–bacterium–insect interaction, the relationship between antibiotic production, the nematode and bacterial growth and the effect of the antibiotics on non-symbiotic bacteria within the infected cadaver is unclear. Consequently, experiments were done to investigate the time course of ST production during the growth and development of P. luminescens C9 and H. megidis 90 in larval G. mellonella.