Outer membrane vesicles facilitate trafficking of the hydrophobic signaling molecule CAI-1 between Vibrio harveyi cells

Many bacteria use extracellular signaling molecules to coordinate group behavior, a process referred to as quorum sensing (QS). However, some QS molecules are hydrophobic in character and are probably unable to diffuse across the bacterial cell envelope. How these molecules are disseminated between bacterial cells within a population is not yet fully understood. Here we show that the marine pathogenVibrio harveyipackages the hydrophobic QS molecule CAI-1, a long-chain amino ketone, into outer membrane vesicles. Electron micrographs indicate that outer membrane vesicles of variable size are predominantly produced and released into the surroundings during stationary phase ofV. harveyi, which correlates with the timing of CAI-1-dependent signaling. The large vesicles (diameter < 55 nm) can trigger a QS phenotype in CAI-1 non-producingV. harveyiandV. choleraecells. Packaging of CAI-1 into outer membrane vesicles might stabilize the molecule in aqueous environments and facilitate its distribution over distances.IMPORTANCEFormation of membrane vesicles is ubiquitous among bacteria. These vesicles are involved in protein and DNA transfer and offer new approaches for vaccination. Gram-negative bacteria use among others hydrophobic signaling molecules for cell-cell communication, however due to their hydrophobic character it is unclear how these molecules are disseminated between bacterial cells. Here we show that the marineVibrio harveyipackages one of its quorum sensing molecules, the long-chain ketone CAI-1, into outer membrane vesicles (OMVs). Isolated CAI-1-containing vesicles trigger a quorum sensing phenotype in CAI-1 non-producingV. harveyiand also inV. choleraecells. Packaging of CAI-1 into OMVs not only solubilize, stabilize and concentrate this class of molecules, but facilitate their distribution between bacteria that live in aqueous environments

From insect to man: Photorhabdus sheds light on the emergence of human pathogenicity

Photorhabdus are highly effective insect pathogenic bacteria that exist in a mutualistic relationship with Heterorhabditid nematodes. Unlike other members of the genus, Photorhabdus asymbiotica can also infect humans. Most Photorhabdus cannot replicate above 34°C, limiting their host-range to poikilothermic invertebrates. In contrast, P. asymbiotica must necessarily be able to replicate at 37°C or above. Many well-studied mammalian pathogens use the elevated temperature of their host as a signal to regulate the necessary changes in gene expression required for infection. Here we use RNA-seq, proteomics and phenotype microarrays to examine temperature dependent differences in transcription, translation and phenotype of P. asymbiotica at 28°C versus 37°C, relevant to the insect or human hosts respectively. Our findings reveal relatively few temperature dependant differences in gene expression. There is however a striking difference in metabolism at 37°C, with a significant reduction in the range of carbon and nitrogen sources that otherwise support respiration at 28°C. We propose that the key adaptation that enables P. asymbiotica to infect humans is to aggressively acquire amino acids, peptides and other nutrients from the human host, employing a so called “nutritional virulence” strategy. This would simultaneously cripple the host immune response while providing nutrients sufficient for reproduction. This might explain the severity of ulcerated lesions observed in clinical cases of Photorhabdosis. Furthermore, while P. asymbiotica can invade mammalian cells they must also resist immediate killing by humoral immunity components in serum. We observed an increase in the production of the insect Phenol-oxidase inhibitor Rhabduscin normally deployed to inhibit the melanisation immune cascade. Crucially we demonstrated this molecule also facilitates protection against killing by the alternative human complement pathway

Languages and dialects: bacterial communication beyond homoserine lactones

Gram-negative bacteria use N-acyl homoserine lactones (acyl-HSLs) for communication, predominantly mediated by LuxR-type receptors. Recent studies uncovered aryl-HSLs, alpha-pyrones and dialkylresorcinols as further chemical languages of Gram-negative bacteria. These findings extend the number of bacterial signaling molecules and suggest that cell-cell communication goes far beyond acyl-HSL signaling in nature

Dialkylresorcinols as bacterial signaling molecules

It is well recognized that bacteria communicate via small diffusible molecules, a process termed quorum sensing. The best understood quorum sensing systems are those that use acylated homoserine lactones (AHLs) for communication. The prototype of those systems consists of a LuxI-like AHL synthase and a cognate LuxR receptor that detects the signal. However, many proteobacteria possess LuxR receptors, yet lack any LuxI-type synthase, and thus these receptors are referred to as LuxR orphans or solos. In addition to the well-known AHLs, little is known about the signaling molecules that are sensed by LuxR solos. Here, we describe a novel cell-cell communication system in the insect and human pathogen Photorhabdus asymbiotica. We identified the LuxR homolog PauR to sense dialkyl-resorcinols (DARs) and cyclohexanediones (CHDs) instead of AHLs as signals. The DarABC synthesis pathway produces the molecules, and the entire system emerged as important for virulence. Moreover, we have analyzed more than 90 different Photorhabdus strains by HPLC/MS and showed that these DARs and CHDs are specific to the human pathogen P. asymbiotica. On the basis of genomic evidence, 116 other bacterial species are putative DAR producers, among them many human pathogens. Therefore, we discuss the possibility of DARs as novel and widespread bacterial signaling molecules and show that bacterial cell-cell communication goes far beyond AHL signaling in nature

Comparative analysis of LytS/LytTR-type histidine kinase/response regulator systems in &gamma;-proteobacteria.

Bacterial histidine kinase/response regulator systems operate at the interface between environmental cues and physiological states. Escherichia coli contains two LytS/LytTR-type histidine kinase/response regulator systems, BtsS/BtsR (formerly YehU/YehT) and YpdA/ YpdB, which have been identified as pyruvate-responsive two-component systems. Since they exhibit remarkable similarity, we analyzed their phylogenetic distribution within the &gamma;-proteobacteria, and experimentally characterized them in a set of representative species. We found that BtsS/BtsR is the predominant LytS/LytTR-type two-component system among &gamma;-proteobacteria, whereas YpdA/YpdB primarily appears in a supplementary role. Based on our observations in E. coli, we used the highly conserved DNA-binding motifs to test the in vivo functionality of both systems in various genera, including Salmonella, Enterobacter, Citrobacter, Xenorhabdus, Yersinia, Aeromonas and Vibrio. The results suggest that, in all cases tested, BtsS/BtsR and YpdA/YpdB respond to different levels of pyruvate in the environment

New Vocabulary for Bacterial Communication

Quorum sensing (QS) is widely accepted as a procedure that bacteria use to converse. However, prevailing thinking places acyl homoserine lactones (AHLs) at the forefront of this communication pathway in Gram-negative bacteria. With the advent of high-throughput genomics and the subsequent influx of bacterial genomes, bioinformatics analysis has determined that the genes encoding AHL biosynthesis, originally discovered to be indispensable for QS (LuxI-like proteins and homologues), are often absent in QS-capable bacteria. Instead, the sensing protein (LuxR-like proteins) is present with an apparent inability to produce any outgoing AHL signal. Recently, several signals for these LuxR solos have been identified. Herein, advances in the field of QS are discussed, with a particular focus on recent research in the field of bacterial cell-cell communication

Microbiology: A new language for small talk

A new signal–receptor pair involved in regulating biofilm formation and virulence was detected in Vibrio cholerae. Both the signal and the transcription factor belong to common classes of natural products and receptor proteins, suggesting widespread importance of related systems in nature