Disproof of the Proposed Structures of Bradyoxetin, a Putative Bradyrhizobium japonicum Signaling Molecule, and HMCP, a Putative Ralstonia solanacearum Quorum-Sensing Molecule.

First, we revisited the reported NMR data of bradyoxetin, a putative cell density factor of Bradyrhizobium japonicum, and found some inconsistencies in the proposed structure. To elucidate the correct structure, we synthesized model oxetane compounds and confirmed that the NMR data of the synthetic compounds did not match those of the reported bradyoxetin. After reinterpreting the reported NMR data, we concluded that bradyoxetin must be chloramphenicol. Next, some derivatives of 2-hydroxy-4-((methylamino)(phenyl)methyl)cyclopentanone (HMCP), which is a putative quorum-sensing molecule of Ralstonia solanacearum, were synthesized. The NMR spectra of the synthesized compounds were completely different from those of the reported natural products. Based on theoretical studies, including the estimation of 1H and 13C NMR chemical shifts using density functional theory calculations, we confirmed the correctness of the structure of the synthesized compound. These results strongly suggest that the proposed structure of HMCP could be incorrect.

A preliminary study of bowel rest strategy in the management of Clostridioides difficile infection.

Clostridioides difficile infection (CDI) is an important nosocomial infection and is the leading cause of infectious diarrhea in hospitalized patients. We aimed to assess the effect of bowel rest on the management of CDI. A single-center retrospective cohort study was conducted. The primary outcome was the composite of the all-cause mortality and CDI recurrence within 30 days. The main secondary outcome was switching from metronidazole to vancomycin. Of the 91 patients with CDI enrolled as the full cohort, 63 patients (69%) and 28 patients (31%) constituted the control group and the bowel rest group, respectively. After one-to-one propensity score matching, a total of 46 patients were included as the matched cohort. In the full cohort, the composite outcome occurred in 19.0% and 14.3% of the patients in the control and the bowel rest group, respectively (p = 0.768). In the matched cohort, it was 17.4% in each group. Although there was no statistically significant difference, the trend of switching was lower in the bowel rest group. The bowel rest may not affect the all-cause mortality and CDI recurrence within 30 days. However, in those prescribed bowel rest, switching from metronidazole to vancomycin may reduce.

Quorum Sensing Inhibition Attenuates the Virulence of the Plant Pathogen Ralstonia solanacearum Species Complex.

Strains of Ralstonia solanacearum species complex (RSSC) cause "bacterial wilt" on a wide range of plant species and thus lead to marked economic losses in agriculture. Quorum sensing (QS), a bacterial cell-cell communication mechanism, controls the virulence of RSSC strains by regulating the production of extracellular polysaccharide (EPS) and secondary metabolites, biofilm formation, and cellular motility. R. solanacearum strain OE1-1 employs (R)-methyl 3-hydroxymyristate (3-OH MAME) as a QS signal, which is synthesized by the PhcB methyltransferase and sensed by the PhcS/PhcRQ two-component system. We describe the design, synthesis, and biological evaluation of inhibitors of the phc QS system. Initial screening of a small set of QS signal analogues revealed that methyl 3-hydroxy-8-phenyloctanoate, named, PQI-1 (phc quorum sensing inhibitor-1), inhibited biofilm formation by strain OE1-1. To improve its inhibitory activity, the derivatives of PQI-1 were synthesized, and their QS inhibition activities were evaluated. PQIs-2-5 evolved from PQI-1 more strongly inhibited not only biofilm formation but also the production of ralfuranone and EPS. Furthermore, RNA-Seq analysis revealed that the PQIs effectively inhibited QS-dependent gene expression and repression in strain OE1-1. On the other hand, the PQIs did not affect the canonical QS systems of the representative reporter bacteria. These antagonists, especially PQI-5, reduced wilting symptoms of the tomato plants infected with strain OE1-1. Taken together, we suggest that targeting the phc QS system has potential for the development of chemicals that protect agricultural crops from bacterial wilt disease.

Signal Production and Response Specificity in the phc Quorum Sensing Systems of Ralstonia solanacearum Species Complex.

Ralstonia solanacearum strains are devastating plant pathogens with global distribution, a wide host range, and genetic diversity, and they are now also referred to as the R. solanacearum species complex (RSSC). RSSC strains employ the quorum sensing (QS) system composed of the phcBSR operon to regulate their virulence on plants. The RSSC strains previously examined produce either (R)-methyl 3-hydroxymyristate (3-OH MAME) or (R)-methyl 3-hydroxypalmitate (3-OH PAME) as their QS signals. Analogously, the phylogenetic analyses of the signal synthase PhcB and the signal receptor PhcS from 15 RSSC strains revealed that these proteins have two clades dependent on their QS signal types. However, the biochemical mechanism underlying this selectivity of QS signal production remains to be elucidated. We demonstrated that the PhcB methyltransferases synthesize QS signals from the cognate fatty acids (R)-3-hydroxymyristic acid or (R)-3-hydroxypalmitic acid. The RSSC strains used here produced both fatty acids, and thus the selectivity of QS signal production depends on the activity of PhcB enzymes. On the other hand, the enantioselective supply of the precursors functioned in the production of enantiopure QS signals. The opposite QS signals weakly induced the production of virulence factors in the RSSC strains. Furthermore, the complementation of the phcB gene encoding the 3-OH PAME-type synthase to the phcB-deletion mutant of the 3-OH MAME-producing strain did not rescue its virulence on tomato plants. Taken together, we propose that the specific production of 3-OH MAME/3-OH PAME ensures full virulence of the RSSC strains.