| Summary: | <i>Salmonella</i> Enteritidis is a non-typhoidal serovar of great public health significance worldwide. The RpoE sigma factor and CpxRA two-component system are the major regulators of the extracytoplasmic stress response. In this study, we found that the CpxR has highly significant, but opposite effects on the auto-aggregation and swarming motility of <i>S</i>. Enteritidis. Auto-aggregation was negatively affected in the ∆<i>cpxR</i> mutant, whereas the same mutant significantly out-performed its wild-type counterpart with respect to swarming motility, indicating that the CpxR plays a role in biofilm-associated phenotypes. Indeed, biofilm-related assays showed that the CpxR is of critical importance in biofilm development under both static (microtiter plate) and dynamic (flow cell) media flow conditions. In contrast, the RpoE sigma factor showed no significant role in biofilm development under dynamic conditions. Transcriptomic analysis revealed that the <i>cpxR</i> mutation negatively affected the constitutive expression of the operons critical for biosynthesis of O-antigen and adherence, but positively affected the expression of virulence genes critical for <i>Salmonella</i>-mediated endocytosis. Conversely, CpxR induced the expression of curli <i>csgAB</i> and fimbrial <i>stdAC</i> operons only during biofilm development and flagellar <i>motAB</i> and <i>fliL</i> operons exclusively during the planktonic phase, indicating a responsive biofilm-associated loop of the CpxR regulator.
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