A Wzx-Wzy-like Polysaccharide Biosynthetic Pathway Contributes to Virulence in Brucella abortus 2308 through an Unknown Mechanism

Abstract

Brucella abortus is an intracellular pathogen that causes spontaneous abortion in cattle and undulant fever in humans. As a member of the a-proteobacteria, B. abortus is closely related to the bacterial species Caulobacter crescentus and Agrobacterium tumefaciens, which have been shown to produce a holdfast and a unipolar polysaccharide (UPP), respectively, at one pole of the bacterial cell. Although little is currently known about exopolysaccharide production and function in Brucella species, recent evidence suggests that these bacteria are capable of producing exopolysaccharides, and with the exception of a flippase gene, B. abortus 2308 carries the genes for a complete UPP biosynthetic pathway. In A. tumefaciens, UPP can be detected by crystal violet, Congo red, or lectin staining; however, UPP production was not detected in B. abortus 2308 using these assays. UPP production can also be increased in A. tumefaciens by overexpressing the diguanylate cyclase PleD, which regulates intracellular cyclic di-GMP levels, but we were unable to demonstrate that UPP is regulated by cyclic di-GMP in B. abortus 2308. Nonetheless, upp homologs in B. abortus 2308 are able to restore biofilm production in A. tumefaciens. In A. tumefaciens, knockout mutants of the outer membrane transporter uppC and the polyisoprenylphosphate hexose-1-phosphate transferase uppE result in strong biofilm phenotypes, wherein the ability of A. tumefaciens to produce a biofilm is nearly abrogated. B. abortus 2308 uppC and uppE homologs restore biofilm production in the respective Agrobacterium upp mutants, indicating that uppC and uppE from B. abortus 2308 produce functional proteins. A uppCE mutant in B. abortus 2308 also exhibits significant attenuation in mice and an altered intracellular replication profile in cultured murine macrophages. Accordingly, uppCE appears to be required for virulence in B. abortus 2308, although by a currently unknown mechanism. Additional studies will be needed to determine whether B. abortus 2308 produces an authentic UPP as well as define the mechanisms by which the putative upp genes contribute to virulence.