The pks genomic island of Escherichia coli encodes polyketide (PK) and nonribosomal peptide (NRP) synthases that allow assembly of a putative hybrid PK-NRP compound named colibactin that induces DNA double-strand breaks in eukaryotic cells. The pks-encoded machinery harbors an atypical essential protein, ClbP. ClbP crystal structure and mutagenesis experiments revealed a serine-active site and original structural features compatible with peptidase activity, which was detected by biochemical assays. Ten ClbP homologs were identified in silico in NRP genomic islands of closely and distantly related bacterial species. All tested ClbP homologs were able to complement a clbP-deficient E. coli mutant. ClbP is therefore a prototype of a new subfamily of extracytoplasmic peptidases probably involved in the maturation of NRP compounds. Such peptidases will be powerful tools for the manipulation of NRP biosynthetic pathways.

During a 1-year survey of Shiga toxin-producing Escherichia coli (STEC) prevalence in central France, 2,143 samples were investigated by PCR for Shiga toxin-encoding genes. A total of 330 (70%) of 471 fecal samples collected from healthy cattle at the Clermont-Ferrand slaughterhouse, 47 (11%) of 411 beef samples, 60 (10%) of 603 cheese samples, and 19 (3%) of 658 stool specimens from hospitalized children with and without diarrhea were positive for thestx gene(s). A STEC strain was isolated from 34% (162 of 471) of bovine feces, 4% (16 of 411) of beef samples, 1% (5 of 603) of cheese samples, and 1.5% (10 of 658) of stool specimens. Of the 220 STEC strains isolated, 34 (15%) harbored thestx 1 gene, 116 (53%) harbored thestx 2 gene, and 70 (32%) carried both thestx 1 and stx 2 genes. However, 32 (14.5%) were not cytotoxic for Vero cells. Theeae gene, found in 12 (5%) of the 220 strains, was significantly associated with the stx 1 gene and with isolates from children. Sequences homologous to ehxAwere found in 102 (46%) of the 220 strains. Thirteen serotypes, OX3:H2, O113:H21, O113:H4, OX3:H21, O6:H10, OX178:H19, O171:H2, O46:H38, O172:H21, O22:H16, O91:H10, O91:H21, and O22:H8, accounted for 102 (55%) of 186 typeable isolates, and only one strain (0.5% of the 186 STEC isolates from cattle), belonged to the O157:H7 serotype. We showed that the majority of the STEC isolates from cattle, beef, and cheese were not likely to be pathogenic for humans and that the STEC strains isolated from children in this study were probably not responsible for diarrheal disease. Finally, the strains associated with hemolytic-uremic syndrome in the same geographical area were shown to belong to particular subsets of the STEC population found in the bovine reservoir.

The widespread magnetotactic bacteria have the peculiar capacity of navigation along the geomagnetic field. Despite their ubiquitous distribution, only few axenic cultures have been obtained worldwide. In this study, we reported the first axenic culture of magnetotactic bacteria isolated from the Mediterranean Sea. This magneto-ovoid strain MO-1 grew in chemically defined O(2) gradient minimal media at the oxic-anoxic transition zone. It is phylogenetically related to Magnetococcus sp. MC-1 but might represent a novel genus of Proteobacteria. Pulsed-field gel electrophoresis analysis indicated that the genome size of the MO-1 strain is 5 ± 0.5 Mb, with four rRNA operons. Each cell synthesizes about 17 magnetosomes within a single chain, two phosphorous-oxygen-rich globules and one to seven lipid storage granules. The magnetosomes chain seems to divide in the centre during cell division giving rise to two daughter cells with an approximately equal number of magnetosomes. The MO-1 cell possesses two bundles of seven individual flagella that were enveloped in a unique sheath. They swam towards the north pole with a velocity up to 300 μm per second with frequent change from right-hand to left-hand helical trajectory. Using a magneto-spectrophotometry assay we showed that MO-1 flagella were powered by both proton-motive force and sodium ion gradient, which is a rare feature among bacteria.

Magnetosomes comprise a magnetic nanocrystal surrounded by a lipid bilayer membrane. These unique prokaryotic organelles align inside magnetotactic bacterial cells and serve as an intracellular compass allowing the bacteria to navigate along the geomagnetic field in aquatic environments. Cryoelectron tomography of Magnetospirillum strains has revealed that the magnetosome chain is surrounded by a network of filaments that may be composed of MamK given that the filaments are absent in the mamK mutant cells. The process of the MamK filament assembly is unknown. Here we prove the authenticity of the MamK filaments and show that MamK exhibits linear distribution inside Magnetospirillum sp. cells even in the area without magnetosomes. The mamK gene alone is sufficient to direct the synthesis of straight filaments in Escherichia coli, and one extremity of the MamK filaments is located at the cellular pole. By using dual fluorescent labeling of MamK, we found that MamK nucleates at multiple sites and assembles into mosaic filaments. Time-lapse experiments reveal that the assembly of the MamK filaments is a highly dynamic and kinetically asymmetrical process. MamK bundles might initiate the formation of a new filament or associate to one preexistent filament. Our results demonstrate the mechanism of biogenesis of prokaryotic cytoskeletal filaments that are structurally and functionally distinct from the known MreB and ParM filaments. In addition to positioning magnetosomes, other hypothetical functions of the MamK filaments in magnetotaxis might include anchoring magnetosomes and being involved in magnetic reception.assembly ͉ magnetosomes ͉ prokaryote ͉ magnetic reception

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