Content area
Abstract
We investigated pellicle (i.e., the bacterial network formed at the air-liquid interface to block the surface of a standing culture) formation governed by the PhoP-PhoQ two-component system (TCS) and its role in susceptibility to harsh environments and virulence of the phytopathogen Dickeya dadantii (formerly Erwinia chrysanthemi) 3937. Pellicle formation was dramatically reduced in phoP and phoQ mutants compared to the wild type in low (10 μM) magnesium, but was greater and more robust in high (10 mM) magnesium. Pellicles formed by the wild type and by the mutants in low magnesium were composed of cellulose, but pellicles formed by these mutants at high magnesium condition were composed of cellulose and a cellulase-resistant polymer. In the wild type at low magnesium, expression of bcsABCD (cellulose biosynthesis operon), adrA (a GGDEF protein) and fliC (a flagellar component called flagellin) increased significantly, whereas expression of these genes was elevated in the mutants at high magnesium condition. Thus, PhoP-PhoQ TCS may regulate pellicle formation by transcriptional control of bcsABCD, adrA and fliC. Overall, pellicle-associated bacteria were more tolerant to adverse environmental cues and more virulent than aerobically grown cells. Furthermore, at low magnesium, wild-type bacteria associated with a pellicle were more resistant to stress environments and more virulent than the pellicle-associated cells of the mutants. Intriguingly, the reverse relationship is found at high magnesium. Thus, formation of pellicle may be required for survival and virulence of D. dadantii 3937.[PUBLICATION ABSTRACT]