Characterization of Luciferase from Photorhabdus kayaii and its Application for In vivo Imaging Studies in Mice.

Background: Bioluminescence, or the production of light by luciferases, is the basis of a well-known reporter technology. A quick way to study the efficacy of antimicrobial drugs and vaccines is in vivo bioluminescence imaging (BLI). Photorhabdus spp. represent the only terrestrial group of bioluminescent bacteria. The luciferase obtained from Photorhabdus luminescence has been widely used in BLI studies. However, little information is available about the functions of luciferases obtained from other members of this genera.

Objective: This study aimed to evaluate the applicability of the luciferase obtained from Photorhabdus kayaii for BLI studies.

Methods: P. kayaii starE, an Iranian isolate of P. kayaii, was cultivated on NBTA agar plates. The resulting colonies were cultured on McConkey agar to determine the bacterial phase. Bioluminescence emission was measured using a multimode reader. The luciferase genes of this bacterium were sequenced following the PCR amplification, and the corresponding amino acid sequences were determined. The luciferase tertiary structure was then obtained from the TACOS web server and compared to that of P. luminescence in CE software. The lux operon encoding the luciferase (luxA and luxB genes) and substrate synthesis complex was cloned and expressed in Escherichia coli BL21 (DE3) using the pBBR1MCS2_START vector. The luminescence emission during the growth was examined. Moreover, the effects of pH and sodium deoxycholate (bile salt) on bioluminescence emission were investigated. Appropriate conditions for the use of bioluminescent E. coli for BLI studies in mice were demonstrated in terms of cell numbers and injection routes.

Results: The bacterium was luminescent and in phase I. Its luciferase monomers (α and β) shared 100% amino acid homology with P. kayaii M-HU2 and more than 92% with P. luminescence. Tertiary structures of the luciferase monomers were 93%- 95% identical to those of P. luminescence. The lux operon was expressed in E. coli, and the maximum bioluminescence signal was observed during the decelerating phase of growth. The bioluminescence at different pH values correlated with the cell survival. The luminescence was emitted by cells exposed to the bile salt. A strong bioluminescent signal was emitted from mice after subcutaneous injection of bioluminescent E. coli at 10⁷ CFU. However, no signals were emitted from mice that were administered the same cell number via intraperitoneal injection. A 2.5-fold increase in the cell number resulted in bioluminescence detection in the abdomen of mice after intraperitoneal injection and a 3.22-fold increase in signal intensity after subcutaneous injection.

Conclusion: These results demonstrated the usefulness of P. kayaii luciferase for BLI studies.