Calcium transcriptionally regulates movement, recombination and other functions of Xylella fastidiosa under constant...

Xylella fastidiosa is a xylem-limited bacterial pathogen causing devastating diseases in many economically important crops. Calcium (Ca) is a major inorganic nutrient in xylem sap that influences virulence-related traits of this pathogen, including biofilm formation and twitching motility. This study aimed to adapt a microfluidic system, which mimics the natural habitat of X. fastidiosa, for whole transcriptome analysis under flow conditions. A microfluidic chamber with two parallel channels was used, and RNA isolated from cells grown inside the system was analysed by RNA-Seq. Ca transcriptionally regulated the machinery of type IV pili and other genes related to pathogenicity and host adaptation. Results were compared to our previous RNA-Seq study in biofilm cells in batch cultures (Parker et al., 2016, Environ Microbiol 18, 1620). Ca-regulated genes in both studies belonged to similar functional categories, but the number and tendencies (up-/downregulation) of regulated genes were different. Recombination-related genes were upregulated by Ca, and we proved experimentally that 2 mM Ca enhances natural transformation frequency. Taken together, our results suggest that the regulatory role of Ca in X. fastidiosa acts differently during growth in flow or batch conditions, and this can correlate to the different phases of growth (planktonic and biofilm) during the infection process.