Growing resistance of pathogenic bacteria and shortage of antibiotic discovery platforms challenge the use of antibiotics in the clinic. This threat calls for exploration of unconventional sources of antibiotics and identification of inhibitors able to eradicate resistant bacteria. Here we describe a different class of antibiotics, odilorhabdins (ODLs), produced by the enzymes of the non-ribosomal peptide synthetase gene cluster of the nematode-symbiotic bacterium Xenorhabdus nematophila. ODLs show activity against Gram-positive and Gram-negative pathogens, including carbapenem-resistant Enterobacteriaceae, and can eradicate infections in animal models. We demonstrate that the bactericidal ODLs interfere with protein synthesis. Genetic and structural analyses reveal that ODLs bind to the small ribosomal subunit at a site not exploited by current antibiotics. ODLs induce miscoding and promote hungry codon readthrough, amino acid misincorporation, and premature stop codon bypass. We propose that ODLs' miscoding activity reflects their ability to increase the affinity of non-cognate aminoacyl-tRNAs to the ribosome.

Background: In sequencing the genomes of two Xenorhabdus species, we encountered a large number of sequence repeats and assembly anomalies that stalled finishing efforts. This included a stretch of about 12 Kb that is over 99.9% identical between the plasmid and chromosome of X. nematophila.

An in vivo approach was taken to assess whether the phosphorylated state of the transcription activator OmpR was affected by changes in the osmolarity of the growth medium or by mutations in envZ, the gene encoding the inner membrane histidine kinase that phosphorylates OmpR. We present results that support the view that increased phosphorylation of OmpR is correlated with enhanced expression of ompC. The in vivo phosphorylation approach was also used to show that OmpR can be phosphorylated in an envZ null strain. This result indicates that phosphorylation cross talk can occur in vivo between OmpR and a kinase(s) that is functionally homologous to envZ.

BackgroundFlexible genomes facilitate bacterial evolution and are classically organized into polymorphic strain-specific segments called regions of genomic plasticity (RGPs). Using a new web tool, RGPFinder, we investigated plasticity units in bacterial genomes, by exhaustive description of the RGPs in two Photorhabdus and two Xenorhabdus strains, belonging to the Enterobacteriaceae and interacting with invertebrates (insects and nematodes).ResultsRGPs account for about 60% of the genome in each of the four genomes studied. We classified RGPs into genomic islands (GIs), prophages and two new classes of RGP without the features of classical mobile genetic elements (MGEs) but harboring genes encoding enzymes catalyzing DNA recombination (RGPmob), or with no remarkable feature (RGPnone). These new classes accounted for most of the RGPs and are probably hypervariable regions, ancient MGEs with degraded mobilization machinery or non canonical MGEs for which the mobility mechanism has yet to be described. We provide evidence that not only the GIs and the prophages, but also RGPmob and RGPnone, have a mosaic structure consisting of modules. A module is a block of genes, 0.5 to 60 kb in length, displaying a conserved genomic organization among the different Enterobacteriaceae. Modules are functional units involved in host/environment interactions (22-31%), metabolism (22-27%), intracellular or intercellular DNA mobility (13-30%), drug resistance (4-5%) and antibiotic synthesis (3-6%). Finally, in silico comparisons and PCR multiplex analysis indicated that these modules served as plasticity units within the bacterial genome during genome speciation and as deletion units in clonal variants of Photorhabdus.ConclusionsThis led us to consider the modules, rather than the entire RGP, as the true unit of plasticity in bacterial genomes, during both short-term and long-term genome evolution.

The expression of the genes encoding the major outer membrane porin proteins OmpF and OmpC in Escherichia coli is regulated by ompR, which encodes the transcriptional activator protein OmpR, and envZ, which encodes a receptorlike protein located in the inner membrane. To examine the role of EnvZ in the expression of the osmoregulated porin genes, we analyzed the production of OmpF and OmpC in cells that lack envZ function. We show that EnvZ is required for the maximal production of OmpC in cells grown in minimal medium but is not essential for the efficient induction of OmpC that occurs during a shift to a high-osmolarity medium. In contrast, the production of OmpF in cells that lack envZ function was similar to that of the parent strain, whereas OmpF repression during a shift to a high-osmolarity medium was incomplete in the absence of EnvZ. These results are discussed in the context of the putative role of EnvZ in the expression of ompF and ompC.