Quorum sensing

A new class of homoserine lactone quorum-sensing signals Schaefer, A. L. et al. Nature 18 June 2008 (doi: 10.1038/nature07088)

Most quorum-sensing systems in Gram-negative bacteria use fatty-acid-based acyl homoserine lactones (AHLs) as signalling molecules. AHLs are typically synthesized by AHL synthases that incorporate a fatty acyl group from the metabolic pools that are used for fatty-acid biosynthesis. Now, a new class of non-fatty-acid-based AHLs has been discovered, and is reported in Nature by Caroline Harwood and colleagues. AHL-based signalling involves two main proteins, the LuxI-type AHL synthase, which generates the signal, and the LuxR-type transcriptional regulator, which responds. The phototrophic soil bacterium Rhodopseudomonas palustris possesses a pair of luxIR-type genes (rpaI and rpaR). However, Harwood et al. had noticed that rpaI expression was specifically induced by growth on p-coumarate and that in bioassays designed to detect AHLs, the RpaR receptor failed to respond to culture supernatant from R. palustris that had been grown on p-coumarate. Further investigations of the quorum-sensing signal in R. palustris revealed that rather than using an AHL, R. palustris uses p-coumaroyl homoserine lactone, which is an aryl homoserine lactone, and that this signal is synthesized by RpaI from exogenous p-coumaric acid, which is thought to be obtained from the plant host. Screening for p-coumaroyl homoserine lactone activity in culture extracts from other bacteria that are known to use p-coumarate as a carbon source revealed that this signal is also synthesized by Bradyrhizhobium sp. BTAi1 and Silicibacter pomeroyi DSS-3.

Techniques and applications

Real-time high resolution 3D imaging of the lyme disease spirochete adhering to and escaping from the vasculature of a living host Moriarty, T. J. et al. PLoS Pathog. 6, e1000090 (2008)

Pathogenic spirochaetes are responsible for several emerging and re-emerging diseases, including Lyme disease. In a recent paper in PLoS Pathogens, Tara Moriarty, Ursula Norman and colleagues report the development of a new system to allow real-time, high-resolution 3D and 4D visualization of the in vivo dissemination of the causative agent of Lyme disease, Borrelia burgdorferi. Intravital imaging of spirochaetes has previously been extremely difficult owing to their small size. Moreover, it is technically challenging to modify B. burgdorferi genetically. Moriarty, Norman et al. began by developing a system to engineer infectious and non-infectious B. burgdorferi to express green fluorescent protein under the control of the B. burgdorferi flaB promoter. They then used conventional and spinning-disc confocal intravital microscopy to follow the haematogenous dissemination of B. burgdorferi in mice. It is known that B. burgdorferi is transmitted from host to host through the bite of an infected tick, but the mechanism of dissemination within a host was unknown. Using their labelled B. burgdorferi, the authors were able to follow the progress of the pathogen through the mouse microvasculature and characterize the dissemination process. The authors comment that the methodology they describe could be applied to other bacterial pathogens.