To be successful pathogens, bacteria must often restrict the expression of virulence genes to host environments. This requires a physical or chemical marker of the host environment as well as a cognate bacterial system for sensing the presence of a host to appropriately time the activation of virulence. However, there have been remarkably few such signal–sensor pairs identified, and the molecular mechanisms for host-sensing are virtually unknown. By directly applying a reporter strain of Vibrio cholerae , the causative agent of cholera, to a thin layer chromatography (TLC) plate containing mouse intestinal extracts, we found two host signals that activate virulence gene transcription. One of these was revealed to be the bile salt taurocholate. We then show that a set of bile salts cause dimerization of the transmembrane transcription factor TcpP by inducing intermolecular disulfide bonds between cysteine (C)-207 residues in its periplasmic domain. Various genetic and biochemical analyses led us to propose a model in which the other cysteine in the periplasmic domain, C218, forms an inhibitory intramolecular disulfide bond with C207 that must be isomerized to form the active C207–C207 intermolecular bond. We then found bile salt–dependent effects of these cysteine mutations on survival in vivo, correlating to our in vitro model. Our results are a demonstration of a mechanism for direct activation of the V. cholerae virulence cascade by a host signal molecule. They further provide a paradigm for recognition of the host environment in pathogenic bacteria through periplasmic cysteine oxidation.

Significance: Pathogenic or symbiosis islands are mobile genetic elements that can provide recipient bacteria the capacity to establish intimate interactions with eukaryotic hosts. For example, legume symbionts have evolved via horizontal transfer of symbiotic plasmids or genomic islands. Here, we show that the transfer of the symbiosis island of the Sesbania rostrata symbiont, Azorhizobium caulinodans , to other rhizobia is enhanced by plant flavonoids that also serve as signals to initiate the symbiotic process. These data suggest that eukaryotic hosts are involved in bacterial horizontal gene transfer to promote symbiotic interactions between rhizobia and legumes.

The tea plant is an economically important crop in China, but long-term monoculture and substantial chemical nitrogen fertilizer input cause soil acidification, which in turn affects the nutrient supply and tea quality. Intercropping has drawn more attention in tea gardens because this pattern is expected to improve soil fertility and tea quality and change the soil microbial community composition. However, the roles of some key microorganisms in rhizosphere soils have not been well characterized. Here, a “soybean in summer and smooth vetch in winter” strategy was used in a tea garden to investigate the effects of intercropped legumes on soil fertility, tea quality, and potential changes in beneficial bacteria such as Bacillus. Our data showed that when soybeans were turned into the soil, the intercropping system exhibited higher soil organic matter (SOM), total nitrogen (TN), tea quality indices, and expression of the Camellia sinensis glutamine synthetase gene (CsGS). Notably, intercropping significantly affected the bacterial communities, decreasing the relative abundance of Bacillus but increasing its absolute abundance. Bacillus amyloliquefaciens BM1 was isolated from intercropped soil and showed outstanding plant growth-promoting (PGP) properties when co-inoculated with rhizobia. In winter, intercropping with smooth vetch had a beneficial effect on soil properties and tea quality. Co-inoculation with strain BM1 and Rhizobium leguminosarum Vic5 on smooth vetch (Vicia villosa) produced huge improvements in SOM, TN, and tea leaf quality, which were accompanied by the highest level of amino acids and the lowest levels of polyphenols and caffeine (p < 0.05). Our findings demonstrate that intercropping with some legumes in the tea garden is a strategy that increases SOM, TN, and tea quality, and the optional use of some PGP Bacillus species produces an amplification effect.

Oxidative stress is a major challenge faced by bacteria. Many bacteria control oxidative stress resistance pathways through the transcriptional regulator OxyR. The human pathogen Vibrio cholerae is a Gram-negative bacterium that is the causative agent of cholera. V. cholerae lives in both aquatic environments and human small intestines, two environments in which it encounters reactive oxygen species (ROS). To study how V. cholerae responds to oxidative stress, we constructed an in-frame oxyR deletion mutant. We found that this mutant was not only sensitive to H₂O₂, but also displayed a growth defect when diluted in rich medium. Further study showed that two catalases, KatG and KatB, either when expressed in living cells, present in culture supernatants, or added as purified recombinant proteins, could rescue the oxyR growth defect. Furthermore, although it could colonize infant mouse intestines similar to that of wildtype, the oxyR mutant was defective in zebrafish intestinal colonization. Alternatively, co-infection with wildtype, but not katG-katB deletion mutants, greatly enhanced oxyR mutant colonization. Our study suggests that OxyR in V. cholerae is critical for antioxidant defense and that the organism is capable of scavenging environmental ROS to facilitate population growth.

Zinc is an essential trace metal required for numerous cellular processes in all forms of life. In order to maintain zinc homeostasis, bacteria have developed several transport systems to regulate its uptake. In this study, we investigated zinc transport systems in the enteric pathogen Vibrio cholerae , the causative agent of cholera. Bioinformatic analysis predicts that two gene clusters, VC2081 to VC2083 (annotated as zinc utilization genes znuABC ) and VC2551 to VC2555 (annotated as z inc- r egulated g enes zrgABCDE ), are regulated by the putative zinc uptake regulator Zur. Using promoter reporter and biochemical assays, we confirmed that Zur represses znuABC and zrgABCDE promoters in a Zn ²⁺ -dependent manner. Under Zn ²⁺ -limiting conditions, we found that mutations in either the znuABC or zrgABCDE gene cluster affect bacterial growth, with znuABC mutants displaying a more severe growth defect, suggesting that both ZnuABC and ZrgABCDE are involved in Zn ²⁺ uptake and that ZnuABC plays the predominant role. Furthermore, we reveal that ZnuABC and ZrgABCDE are important for V. cholerae colonization in both infant and adult mouse models, particularly in the presence of other intestinal microbiota. Collectively, our studies indicate that these two zinc transporter systems play vital roles in maintaining zinc homeostasis during V. cholerae growth and pathogenesis.