Regulatory Protein OmpR Influences the Serum Resistance of <i>Yersinia enterocolitica</i> O:9 by Modifying the Structure of the Outer Membrane

<div><p>The EnvZ/OmpR two-component system constitutes a regulatory pathway involved in bacterial adaptive responses to environmental cues. Our previous findings indicated that the OmpR regulator in <i>Yersinia enterocolitica</i> O:9 positively regulates the expression of FlhDC, the master flagellar activator, which influences adhesion/invasion properties and biofilm formation. Here we show that a strain lacking OmpR grown at 37°C exhibits extremely high resistance to the bactericidal activity of normal human serum (NHS) compared with the wild-type strain. Analysis of OMP expression in the <i>ompR</i> mutant revealed that OmpR reciprocally regulates Ail and OmpX, two homologous OMPs of <i>Y. enterocolitica</i>, without causing significant changes in the level of YadA, the major serum resistance factor. Analysis of mutants in individual genes belonging to the OmpR regulon (<i>ail</i>, <i>ompX</i>, <i>ompC</i> and <i>flhDC</i>) and strains lacking plasmid pYV, expressing YadA, demonstrated the contribution of the respective proteins to serum resistance. We show that Ail and OmpC act in an opposite way to the OmpX protein to confer serum resistance to the wild-type strain, but are not responsible for the high resistance of the <i>ompR</i> mutant. The serum resistance phenotype of <i>ompR</i> seems to be multifactorial and mainly attributable to alterations that potentiate the function of YadA. Our results indicate that a decreased level of FlhDC in the <i>ompR</i> mutant cells is partly responsible for the serum resistance and this effect can be suppressed by overexpression of <i>flhDC</i> in <i>trans</i>. The observation that the loss of FlhDC enhances the survival of wild-type cells in NHS supports the involvement of FlhDC regulator in this phenotype. In addition, the <i>ompR</i> mutant exhibited a lower level of LPS, but this was not correlated with changes in the level of FlhDC. We propose that OmpR might alter the susceptibility of <i>Y. enterocolitica</i> O:9 to complement-mediated killing through remodeling of the outer membrane.</p>