Outer Membrane Protein X (Ail) Contributes toYersinia pestisVirulence in Pneumonic Plague and Its Activity Is Dependent on the Lipopolysaccharide Core Length

Abstract

ABSTRACTYersinia pestis, the causative agent of plague, is one of the most virulent microorganisms known. The outer membrane protein X (OmpX) inY. pestisKIM is required for efficient bacterial adherence to and internalization by cultured HEp-2 cells and confers resistance to human serum. Here, we tested the contribution of OmpX to disease progression in the fully virulentY. pestisCO92 strain by engineering a deletion mutant and comparing its ability in mediating pneumonic plague to that of the wild type in two animal models. The deletion of OmpX delayed the time to death up to 48 h in a mouse model and completely attenuated virulence in a rat model of disease. All rats challenged with 1 × 108CFU of theompXmutant survived, compared to the 50% lethal dose (LD50) of 1.2 × 103CFU for the wild-type strain. Because murine serum is not bactericidal for theompXmutant, the mechanism underlying the delay in time to death in mice was attributed to loss of adhesion/internalization properties but not serum resistance. The rat model, which is most similar to humans, highlighted the critical role of serum resistance in disease. To resolve conflicting evidence for the role ofY. pestislipopolysaccharide (LPS) and OmpX in serum resistance,ompXwas cloned intoEscherichia coliD21 and three isogenic derivatives engineered to have progressively truncated LPS core saccharides. OmpX-mediated serum resistance, adhesiveness, and invasiveness, although dependent on LPS core length, displayed these functions inE. coli, independently of otherYersiniaproteins and/or LPS. Also, autoaggregation was required for efficient OmpX-mediated adhesiveness and internalization but not serum resistance.