Practical observations on the use of fluorescent reporter systems in C. difficile

Objectives Fluorescence microscopy is a valuable tool to study a broad variety of bacterial cell components and dynamics thereof. For Clostridioides difficile , the fluorescent proteins CFP opt , mCherry Opt and phiLOV2.1, and the self-labelling tags SNAP Cd and HaloTag, hereafter collectively referred as fluorescent systems, have been described to explore different cellular pathways. In this study, we sought to characterize previously used fluorescent systems in C. difficile cells. Methods We performed single cell analyses using fluorescence microscopy of exponentially growing C. difficile cells harbouring different fluorescent systems, either expressing these separately in the cytosol or fused to the C-terminus of HupA, under defined conditions. Results We show that the intrinsic fluorescence of C. difficile cells increases during growth, independent from sigB or spo0A . However, when C. difficile cells are exposed to environmental oxygen autofluorescence is enhanced. Cytosolic overexpression of the different fluorescent systems alone, using the same expression signals, showed heterogeneous expression of the fluorescent systems. High levels of mCherry Opt were toxic for C. difficile cells limiting the applicability of this fluorophore as a transcriptional reporter. When fused to HupA, C. difficile histone-like protein, the fluorescent systems behaved similarly and did not affect the HupA overproduction phenotype. Conclusions The present study compares several commonly used fluorescent systems for application as transcriptional or translational reporters in microscopy and summarizes the limitations and key challenges for live-cell imaging of C. difficile . Due to independence of molecular oxygen and fluorescent signal, SNAP Cd appears the most suitable candidate for live-cell imaging in C. difficile to date. Abstract Figure