The Erwinia amylovora PhoPQ system is involved in resistance to antimicrobial peptide and suppresses gene expression of two novel type III secretion systems.

The PhoPQ system is a pleiotropic two-component signal transduction system that controls many pathogenic properties in several mammalian and plant pathogens. Three different cues have been demonstrated to activate the PhoPQ system including a mild acidic pH, antimicrobial peptides, and low Mg(2+). In this study, our results showed that phoPQ mutants were more resistant to strong acidic conditions (pH 4.5 or 5) than that of the wild-type (WT) strain, suggesting that this system in Erwinia amylovora may negatively regulate acid resistance gene expression. Furthermore, the PhoPQ system negatively regulated gene expression of two novel type III secretion systems in E. amylovora. These results are in contrast to those reported for the PhoPQ system in Salmonella and Xanthomonas, where it positively regulates type III secretion system and acid resistance. In addition, survival of phoPQ mutants was about 10-fold lower than that of WT when treated with cecropin A at pH 5.5, suggesting that the PhoPQ system renders the pathogen more resistant to cecropin A.