Brucella abortus Ornithine Lipids Are Dispensable Outer Membrane Components Devoid of a Marked Pathogen-Associated Molecular Pattern

Leyre Palacios-Chaves,Edgardo Moreno,Jean-Pierre Gorvel,Amaia Zúñiga-Ripa,Ignacio Moriyón,Elías Barquero-Calvo,Carlos Chacón-Díaz,Vilma Arce-Gorvel,María-Jesús Grilló,Esteban Chaves-Olarte,Maite Iriarte,Yolanda Gil-Ramírez,Raquel Conde-Álvarez,María-Jesús De Miguel

doi:10.1371/journal.pone.0016030

Abstract

The brucellae are α-Proteobacteria facultative intracellular parasites that cause an important zoonosis. These bacteria escape early detection by innate immunity, an ability associated to the absence of marked pathogen-associated molecular patterns in the cell envelope lipopolysaccharide, lipoproteins and flagellin. We show here that, in contrast to the outer membrane ornithine lipids (OL) of other Gram negative bacteria, Brucella abortus OL lack a marked pathogen-associated molecular pattern activity. We identified two OL genes (olsB and olsA) and by generating the corresponding mutants found that olsB deficient B. abortus did not synthesize OL or their lyso-OL precursors. Liposomes constructed with B. abortus OL did not trigger IL-6 or TNF-α release by macrophages whereas those constructed with Bordetella pertussis OL and the olsB mutant lipids as carriers were highly active. The OL deficiency in the olsB mutant did not promote proinflammatory responses or generated attenuation in mice. In addition, OL deficiency did not increase sensitivity to polymyxins, normal serum or complement consumption, or alter the permeability to antibiotics and dyes. Taken together, these observations indicate that OL have become dispensable in the extant brucellae and are consistent within the trend observed in α-Proteobacteria animal pathogens to reduce and eventually eliminate the envelope components susceptible of recognition by innate immunity.

Highlights

The members of the genus Brucella are a-2 Proteobacteria that cause brucellosis, an important disease affecting livestock and wild life as well as human beings
ornithine lipids (OL) are outer membranes (OM) components of B. abortus To determine the cellular localization of OLs, we first examined the free-lipids in virulent S B. abortus 2308 NalR grown in tryptic soy broth to the stationary phase, in the OM fragments released spontaneously during growth [19] and in non-delipidated B. abortus LPS [20]
We searched the B. abortus 2308 genome for orthologs of the genes involved in OL synthesis in other a-2 Proteobacteria

Summary

Introduction

The members of the genus Brucella are a-2 Proteobacteria that cause brucellosis, an important disease affecting livestock and wild life as well as human beings. These bacteria trigger only low proinflammatory responses in the initial stages of infection and, they follow a stealthy behavior that allows them to reach sheltered intracellular niches before effective immunity activation. The Brucella LPS is clearly implicated in these properties and there is evidence that other lipid molecules contribute. Ornithine lipids (OLs) are present in relatively large amounts in Brucella [11] and, they have interesting properties in other bacteria, have not been investigated

Methods

Results

Conclusion