Chemotaxis is essential for infection by plant pathogenic bacteria. The causal agent of tobacco wildfire disease, Pseudomonas syringae pv. tabaci 6605 (Pta6605), is known to cause severe leaf disease and is highly motile. The requirement of chemotaxis for infection has been demonstrated through the inoculation of mutant strains lacking chemotaxis sensory component proteins. Pta6605 possesses 54 genes that encode chemoreceptors (known as methyl-accepting chemotaxis proteins, MCPs). Chemoreceptors are classified into several groups based on the type and localization of ligand-binding domains (LBD). Cache LBD-type chemoreceptors have been reported to recognize formate in several bacterial species. In the present study, we identified Cache_3 Cache_2 LBD-type Mcp26 encoded by Pta6605_RS00335 as a chemoreceptor for formate using a quantitative capillary assay, and named it McpF. Although the deletion mutant of mcpF (ΔmcpF) retained attraction to 1% yeast extract, its chemotactic response to formate was markedly reduced. Swimming and swarming motilities were also impaired in the mutant. To investigate the effects of McpF on bacterial virulence, we conducted inoculations on tobacco plants using several methods. The ΔmcpF mutant exhibited weaker virulence in flood and spray assays than wild-type and complemented strains, highlighting not only the involvement of McpF in formate recognition, but also its critical role in leaf entry during the early stages of infection.

Circumneutral iron-rich hot springs may represent analogues of Neoarchean to Paleoproterozoic oceans of early Earth, potentially providing windows into ancient microbial ecology. Here we sampled five Japanese hot springs to gain insights into functional processes and taxonomic diversity in these analog environments. Amplicon and metagenomic sequencing confirm a hypothesis where taxonomy is distinct between sites and linked to the geochemical setting. Metabolic functions shared among the springs include carbon fixation via the reductive pentose phosphate cycle, nitrogen fixation, and dissimilatory iron oxidation/reduction. Among the sites, Kowakubi was unique in that it was dominated by Hydrogenophilaceae, a group known for performing hydrogen oxidation, motivating a hypothesis that H₂ as an electron donor may shape community composition even in the presence of abundant ferrous iron. Evidence for nitrogen cycling across the springs included N₂ fixation, dissimilatory nitrate reduction to ammonia (DNRA), and denitrification. The low-salinity springs Furutobe and OHK lacked evidence for ammonium oxidation by ammonia monooxygenase, but evidence for complete nitrification existed at Kowakubi, Jinata, and Tsubakiyama. In most sites, the microaerophilic iron-oxidizing bacteria from the Zetaproteobacteria or Gammaproteobacteria classes had higher relative abundances than Cyanobacteria. Microaerophilic iron oxidizers may outcompete abiotic Fe oxidation, while being fueled by oxy-phototrophic Cyanobacteria. Our data provide a foundation for considering which factors may have controlled productivity and elemental cycling as Earth’s oceans became oxygenated at the onset of the Great Oxidation Event.

Many plant pathogenic bacteria regulate the expression of virulence factors via N-acylhomoserine lactone (AHL), a quorum-sensing signaling compound. When numerous spore-forming bacteria were isolated from a natural environment, Priestia megaterium was the dominant species, and some P. megaterium strains exhibited AHL-degrading activity. The results of a HPLC ana­lysis of AHL degradation products demonstrated that P. megaterium degraded AHL by AHL lactonase, which hydrolyzes the lactone ring of AHL. The novel AHL lactonase gene, aiiB, was found in the whole genome sequence of AHL-degrading P. megaterium. The relationship between the presence of aiiB and AHL-degrading activity in P. megaterium strains revealed that P. megaterium may be classified into three AHL degradation groups: Group 1 (with AHL-degrading activity and aiiB), Group 2 (with neither AHL-degrading activity nor aiiB), and Group 3 (without AHL-degrading activity, but with aiiB). A comparative genome ana­lysis suggested that aiiB was obtained or missed by a non-transpositional event during the process of evolution in P. megaterium. The amino acid sequences of AiiB in Group 1 and 3 strains were almost identical, and Escherichia coli harboring aiiB from Groups 1 and 3 exhibited high AHL-degrading activity. Although the AHL-degrading activity of Group 3 strains was markedly weaker than that of Group 1 strains, they degraded AHL in a long-term incubation. Based on the present results, Group 1 and 3 strains, the genomes of which contain aiiB, may reduce potato maceration activity under the control of AHL-mediated quorum sensing in P. carotovorum subsp. carotovorum NBRC 12380.

In traditional indigo dyeing, water-insoluble indigo is anaerobically converted into soluble leuco-indigo via microbial reduction in alkaline dye suspensions, allowing its use as a fabric dye. Although various indigo-reducing bacteria have been isolated to date, culture-independent microbial community ana­lyses have suggested that bacteria belonging to uncultured clades also contribute to indigo reduction. Therefore, we aimed to isolate previously overlooked indigo-reducing bacteria using an unconventional culture method. We conducted enrichment cultures and single-colony isolation using a medium supplemented with sukumo, an indigo dye source derived from the composted leaves of indigo-containing plants, as the sole energy, carbon, and nitrogen sources. We isolated a previously uncultured bacterium belonging to the family Tissierellaceae, which had been predicted as a major indigo reducer in various indigo dyeing processes solely based on microbial community ana­lyses. The insoluble indigo-reducing activity of the Tissierellaceae isolate, strain TU-1 was significantly higher than that of known indigo-reducing bacteria. The addition of the culture supernatant of strain TU-1 enhanced the reduction of indigo powder by other indigo-reducing bacteria, with similar stimulatory effects to those of the insoluble electron mediator, anthraquinone. These results indicate that strain TU-1 possesses a high capacity for secreting electron mediators, conferring a significant reduction capacity for insoluble indigo. Further investigations, including the discovery of additional unknown indigo-reducing bacteria and the identification of the mediators they produce, will provide a more detailed understanding of the mechanisms underlying indigo reduction in practical dyeing processes.

A sulfate-reducing bacterium was isolated from the anode surface of a microbial fuel cell (MFC) producing a high current density. 16S rRNA gene ana­lyses showed that the isolate was affiliated with the genus Nitratidesulfovibrio, and the strain was named HK-II. When Nitratidesulfovibrio sp. strain HK-II was incubated anaerobically under sulfate-reducing conditions with Fe(III) citrate, a black precipitate formed. The resulting black precipitate was investigated using multidisciplinary methods. An X-ray diffraction (XRD) ana­lysis revealed that the black precipitate was mainly composed of mackinawite. A cyclic voltammetry ana­lysis showed clear redox peaks, and biogenic mackinawite possessed rechargeable properties. The XRD ana­lysis also showed that the form of the rechargeable biogenic mineral induced by strain HK-II (RBM-II) was changed by discharge and recharge treatments. Field-emission transmission electron microscopy revealed that lepidocrocite and amorphous iron oxide formed from mackinawite under discharged conditions, and the three mineral types were intermingled via charge and discharge cycles. Physicochemical parameters regularly changed under the treatments, suggesting that discharge occurred via iron oxidation followed by sulfur reduction and vice versa. These results indicate that sulfur dynamics are important key processes in charge and discharge mechanisms. MFCs equipped with lactate, strain HK-II, and an anode containing RBM-II consumed lactate under open-circuit conditions, after which MFCs generated a higher current density under reclosed-circuit conditions. These results demonstrate that RBM-II is a rechargeable material that enables the capture of electrons produced by bacterial cells and is useful for enhancing the performance of MFCs.