Symbiosis

Symbiosis insights through metagenomic analysis of a microbial consortium Woyke, T. et al. Nature 17 September 2006 (doi:10.1038/nature05192)

Nicole Dubilier, Edward Rubin and colleagues report in a recent issue of Nature on their metagenomic analysis of the bacterial endosymbionts present under the cuticle of the marine oligochaete worm Olavius algarvensis. Using shotgun sequencing and metabolic pathway reconstruction, Woyke et al. were able to characterize four bacterial cosymbionts. The bacteria are either γ- or δ-proteobacteria, and can generate energy by carbon fixation and either oxidization of sulphides or reduction of sulphates. Other detailed metabolic information obtained allowed the authors to reconstruct the physiology of two of the symbionts and their interactions with the worm.

Bacterial pathogenicity

Virulence factors of Yersinia pestis are overcome by a strong lipopolysaccharide response Montminy, S. W. et al. Nature Immunol. 7, 1066–1073 (2006)

Yersinia pestis can undergo temperature-dependent alterations in its lipopolysaccharide (LPS). At 21–27°C, the average temperature of the flea vector, the lipid A component of LPS is hexa-acylated whereas at 37°C, mammalian body temperature, it is tetra-acylated. It had previously been suggested that this temperature-dependent switch could be involved in immune evasion, as tetra-acylated LPS is a poor stimulator of Toll-like receptor 4 (TLR4). Montminy et al. expressed the gene encoding the Escherichia coli LpxL acyltransferase in Y. pestis. The presence of this gene caused Y. pestis to synthesize hexa-acylated LPS, which was a potent TLR4 stimulator, but this strain of Y. pestis (KIM1001-pLpxL) was avirulent in mice. Resistance to KIM1001-pLpxL was found to require the presence of TLR4 and the TLR4 adaptor MyD88 and co-receptor MD-2. These results indicate that the virulence of Y. pestis is strongly dependent on the evasion of the LPS–TLR4 signalling pathway.

Quorum sensing

Ligand-induced asymmetry in histidine sensor kinase complex regulates quorum sensing Neiditch, M. B. et al. Cell 126, 1095–1108 (2006)

The quorum-sensing signal autoinducer 2 (AI-2) is produced and detected by both Gram-negative and Gram-positive species. In Vibrio harveyi, the AI-2 receptor comprises LuxP and LuxQ, a periplasmic protein and membrane sensor histidine kinase, respectively. Neiditch and colleagues present the crystal structures of the periplasmic domain of LuxQ (LuxQp) and of LuxPQp bound to AI-2. The most notable ligand-induced structural change is a major conformation change in LuxP — AI-2 binding promotes the dimerization of the LuxP periplasmic regions, generating asymmetric LuxPQp dimers. The authors propose a model in which, in the absence of AI-2, the interaction between LuxP and LuxQ forms a 'clasp' between the proteins, and in the presence of AI-2, this clasp is released.