Measuring virulence factor expression by the pathogenic bacterium Photorhabdus luminescens in culture and during insect infection

During insect infection Photorhabdus luminescens emits light and expresses virulence factors, including insecticidal toxin complexes (Tcs) and an RTX- like metalloprotease (Prt). Using quantitative PCR and protein assays, we d escribe the expression patterns of these factors both in culture and during insect infection and compare them to the associated bacterial growth curve s. In culture, light and active Prt protease are produced in stationary pha se. Tca also appears in stationary phase, whereas Tcd is expressed earlier. These patterns seen in a culture flask are strikingly similar to those obs erved during insect infection. Thus, in an infected insect, bacteria grow e xponentially until the time of insect death at approximate to 48 h, when bo th light and the virulence factors Prt protease and Tca are produced. In co ntrast, Tcd appears much earlier in insect infection. However, at present, the biological significance of this difference in timing of the production of the two toxins in unclear. This is the first documentation of the expres sion of Tcs and Prt in an insect and highlights the malleability of Photorh abdus as a model system for bacterial infection.