A novel tool for stable genomic reporter gene integration to analyze heterogeneity in Photorhabdus luminescens at the single-cell level

Determination of reporter gene activity at the single-cell level is a prerequisite for analyzing heterogeneous gene expression in bacteria. The insect pathogenic enteric bacterium Photorhabdus luminescens is an excellent organism in which to study heterogeneity since it exists in two phenotypically different forms, called the primary and secondary variant. A tool for generating stable genomic integrations of reporter genes has been lacking for these bacteria, and this has hampered the acquisition of reliable data sets for promoter activities at the single-cell level. We therefore generated a plasmid tool named pPINT-mCherry for the easy and stable introduction of gene fragments upstream of an mCherry reporter gene followed by stable integration of the plasmid into the P. luminescens genome at the rpmE/glmS intergenic region. We demonstrate that the genomic integration of reporter genes for single-cell analysis is necessary in P. luminescens since plasmid-borne reporter genes mimic heterogeneity and are therefore not applicable in these bacteria, in contrast to their use in single-cell analysis in other bacteria like Escherichia coli.