Comparison of the bioluminescence ofPhotorhabdus species and subspecies type strains

Photorhabdus lux-operon heat shock-like regulation

For decades, transcription of Photorhabdus luminescens lux-operon was considered being constitutive. Therefore, this lux-operon has been used for measurements in non-specific bacterial luminescent biosensors. Here, the expression of Photorhabdus lux-operon under high temperature was studied. The expression was researched in the natural strain Photorhabdus temperata and in the heterologous system of Escherichia coli. P. temperata FV2201 bacterium was isolated from soil in the Moscow region (growth optimum 28 °C). We showed that its luminescence significantly increases when the temperature rises to 34 °C. The increase in luminescence is associated with an increase in the transcription of luxCDABE genes, which was confirmed by RT-PCR. The promoter of the lux-operon of the related bacterium P. luminescens ZM1 from the forests of Moldova, being cloned in the heterologous system of E. coli, is activated when the temperature rises from room temperature to 42 °C. When heat shock is caused by ethanol addition, transcription of lux-operon increases only in the natural strain of P. temperata, but not in the heterologous system of E. coli cells. In addition, the activation of the lux-operon of P. luminescens persists in E. coli strains deficient in both the rpoH and rpoE genes. These results indicate the presence of sigma 32 and sigma 24 independent heat-shock-like mechanism of regulation of the lux-operon of P. luminescens in the heterologous E. coli system.

Competition and Co-existence of Two Photorhabdus Symbionts with a Nematode Host

Photorhabdus spp. (Enterobacteriales: Morganellaceae) occur exclusively as symbionts of Heterorhabditis nematodes for which they provide numerous services, including killing insects and providing nutrition and defence within the cadavers. Unusually, two species (Photorhabdus cinerea and Photorhabdus temperata) associate with a single population of Heterorhabditis downesi at a dune grassland site. Building on previous work, we investigated competition between these two Photorhabdus species both at the regional (between insects) and local (within insect) level by trait comparison and co-culture experiments. There was no difference between the species with respect to supporting nematode reproduction and protection of cadavers against invertebrate scavengers, but P. cinerea was superior to P. temperata in several traits: faster growth rate, greater antibacterial and antifungal activity and colonisation of a higher proportion of nematodes in co-culture. Moreover, where both bacterial symbionts colonised single nematode infective juveniles, P. cinerea tended to dominate in numbers. Differences between Photorhabdus species were detected in the suite of secondary metabolites produced: P. temperata produced several compounds not produced by P. cinerea including anthraquinone pigments. Bioluminescence emitted by P. temperata also tended to be brighter than that from P. cinerea. Bioluminescence and pigmentation may protect cadavers against scavengers that rely on sight. We conclude that while P. cinerea may show greater local level (within-cadaver) competitive success, co-existence of the two Photorhabdus species in the spatially heterogeneous environment of the dunes is favoured by differing specialisations in defence of the cadaver against differing locally important threats.

Escherichia coli K-12 (pEGFPluxABCDEamp): a tool for analysis of bacterial killing by antibacterial agents and human complement activities on a real-time basis

Photorhabdus luminescens luxCDABE genes were integrated into E. coli K-12 using a high copy number plasmid containing modified luxABCDE genes under the control of the powerful Lac promoter. This strain emitted 10 times higher bioluminescence (BL) than P. luminescens. BL production under different growth conditions was studied. In both bacterial strains, the increase in BL signal correlated with the increase in optical density (OD) in a rich growth medium. However, at the logarithmic growth phase, the BL signal was roughly constant. By contrast, in minimal growth media, there was no substantial growth and the BL/cell was approximately five times higher than in the rich medium. The dynamic measurement range of BL was 10(2) -10(7) colony-forming units (CFU) in E. coli and 10(3) -10(7)  CFU in P. luminescens. Because the decrease in the BL signal correlated with the decrease in CFU and OD, i.e. the number of bacterial cells killed, it proved to be very suitable for assessing the antibacterial effects of different antimicrobial agents. Unlike with plate counting, the kinetics of killing can be monitored on a real-time basis using BL measurements. Complement activities in different samples can be estimated using only one serum dilution. The transformed E. coli strain appeared to be superior to P. luminescens in these applications because E. coli was complement sensitive, the detection limit of E. coli was one order lower and the BL-producing system of P. luminescens appeared to be quite unstable.