Development of a dual fluorescent reporter system in Clostridioides difficile reveals a division of labor between virulence and transmission gene expression

Abstract

The bacterial pathogen Clostridioides difficile causes gastroenteritis through its production of toxins and transmits disease through its production of resistant spores. Toxin and spore production are energy-expensive processes that are regulated by multiple transcription factors in response to many nutritional inputs. While toxin and sporulation genes are both heterogeneously expressed in only a subset of C. difficile cells, the relationship between these two sub-populations remains unclear. To address whether C. difficile coordinates the generation of these sub-populations, we developed a dual transcriptional reporter system that allows toxin and sporulation gene expression to be simultaneously visualized at the single-cell level using chromosomally-encoded mScarlet and mNeonGreen fluorescent transcriptional reporters. We then adapted an automated image analysis pipeline to quantify toxin and sporulation gene expression in thousands of individual cells in different media conditions and genetic backgrounds. These analyses revealed that toxin and sporulation gene expression rarely overlap during growth on agar plates, but broth culture increases this overlap in a manner dependent on the multifunctional RstA transcriptional regulator. Our results suggest that certain growth conditions promote a "division of labor" between transmission and virulence gene expression, highlighting how these subpopulations are influenced by environmental inputs. Given that recent work has revealed population-wide heterogeneity for numerous cellular processes in C. difficile, we anticipate that our dual reporter system will be broadly useful for determining the overlap in these subpopulations. IMPORTANCEClostridioides difficile is an important nosocomial pathogen that causes severe diarrhea by producing toxins and is transmitted by producing spores. While both processes are crucial for C. difficile disease, only a subset of cells express toxins and/or undergo sporulation. Whether C. difficile coordinates the relationship between these energy-expensive processes remains unknown. We developed a dual fluorescent reporter system coupled with an automated image analysis pipeline to rapidly characterize expression two genes of interest across thousands of bacterial cells. Using this reporter system, we discovered that toxin and sporulation gene expression appear to undergo a "division of labor" in certain growth conditions, particularly during growth on agar plates. Since C. difficile specializes into subpopulations for numerous vital cellular processes, this novel dual reporter system will enable future studies aimed at understanding how C. difficile coordinates various subpopulations throughout its infectious disease cycle.