CpxR negatively regulates the production of xenocoumacin 1, a dihydroisocoumarin derivative produced by Xenorhabdus nematophila

Regulatory role of the Cpx ESR in bacterial behaviours

The envelope demarcates the boundary between bacterial cell and its environment, providing a place for bacteria to transport nutrients and excrete metabolic waste, while buffering external environmental stress. Envelope stress responses (ESRs) are important tools for bacteria to sense and repair envelope damage. In this review, we discussed evidence that indicates the important role of the Cpx ESR in pathogen-host interactions, including environmental stress sensing and responses, modulation of bacterial virulence, antimicrobial resistance, and inter-kingdom signaling.

Enhancing the yield of Xenocoumacin 1 in Xenorhabdus nematophila YL001 by optimizing the fermentation process

Xenocoumacin 1 (Xcn 1), antibiotic discovered from secondary metabolites of Xenorhabdus nematophila, had the potential to develop into a new pesticide due to its excellent activity against bacteria, oomycetes and fungi. However, the current low yield of Xcn1 limits its development and utilization. To improve the yield of Xcn1, response surface methodology was used to determine the optimal composition of fermentation medium and one factor at a time approach was utilized to optimize the fermentation process. The optimal medium composed of in g/L: proteose peptone 20.8; maltose 12.74; K2HPO4 3.77. The optimal fermentation conditions were that 25 °C, initial pH 7.0, inoculum size 10%, culture medium 75 mL in a 250 mL shake flask with an agitation rate of 150 rpm for 48 h. Xenorhabdus nematophila YL001 was produced the highest Xcn1 yield (173.99 mg/L) when arginine was added to the broth with 3 mmol/L at the 12th h. Compared with Tryptic Soy Broth medium, the optimized fermentation process resulted in a 243.38% increase in Xcn1 production. The obtained results confirmed that optimizing fermentation technology led to an increase in Xcn1 yield. This work would be helpful for efficient Xcn1 production and lay a foundation for its industrial production.

Discovery of an antitumor compound from xenorhabdus stockiae HN_xs01

Xenorhabdus, known for its symbiotic relationship with Entomopathogenic nematodes (EPNs), belongs to the Enterobacteriaceae family. This dual-host symbiotic nematode exhibits pathogenic traits, rendering it a promising biocontrol agent against insects. Our prior investigations revealed that Xenorhabdus stockiae HN_xs01, isolated in our laboratory, demonstrates exceptional potential in halting bacterial growth and displaying anti-tumor activity. Subsequently, we separated and purified the supernatant of the HN_xs01 strain and obtained a new compound with significant inhibitory activity on tumor cells, which we named XNAE. Through LC-MS analysis, the mass-to-nucleus ratio of XNAE was determined to be 254.24. Our findings indicated that XNAE exerts a time- and dose-dependent inhibition on B16 and HeLa cells. After 24 h, its IC50 for B16 and HeLa cells was 30.178 µg/mL and 33.015 µg/mL, respectively. Electron microscopy revealed conspicuous damage to subcellular structures, notably mitochondria and the cytoskeleton, resulting in a notable reduction in cell numbers among treated tumor cells. Interestingly, while XNAE exerted a more pronounced inhibitory effect on B16 cells compared to HeLa cells, it showed no discernible impact on HUVEC cells. Treatment of B16 cells with XNAE induced early apoptosis and led to cell cycle arrest in the G2 phase, as evidenced by flow cytometry analysis. The impressive capability of X. stockiae HN_xs01 in synthesizing bioactive secondary metabolites promises to significantly expand the reservoir of natural products. Further exploration to identify the bioactivity of these compounds holds the potential to shed light on their roles in bacteria-host interaction. Overall, these outcomes underscore the promising potential of XNAE as a bioactive compound for tumor treatment.

Antimicrobial Activity and Identification of the Biosynthetic Gene Cluster of X-14952B From Streptomyces sp. 135

The bacterial genus Streptomyces is an important source of antibiotics, and genome mining is a valuable tool to explore the potential of microbial biosynthesis in members of this genus. This study reports an actinomycete strain 135, which was isolated from Qinghai-Tibet Plateau in China and displayed broad antimicrobial activity. The fermentation broth of strain 135 displayed strong antifungal activity (>70%) against Sclerotinia sclerotiorum, Botrytis cinerea, Valsa mali, Phytophthora capsici, Glomerella cingulata, Magnaporthe grisea, Bipolaris maydis, Exserohilum turcicum in vitro, meanwhile possessed significant preventive and curative efficacy against S. sclerotiorum, Gaeumannomyces graminis, and P. capsici on rape leaves (54.04 and 74.18%), wheat (90.66 and 67.99%), and pepper plants (79.33 and 66.67%). X-14952B showed the greatest antifungal activity against S. sclerotiorum and Fusarium graminearum which the 50% inhibition concentration (EC₅₀) were up to 0.049 and 0.04 μg/mL, respectively. Characterization of strain 135 using a polyphasic approach revealed that the strain displayed typical features of the genus Streptomyces. 16S rRNA gene sequencing and phylogenetic analysis demonstrated that the isolate was most closely related to and formed a clade with Streptomyces huasconensis HST28^T (98.96% 16S rRNA gene sequence similarity). Average nucleotide identity (ANI) and DNA-DNA hybridization (DDH) values in strain 135 and related type strains were both below the threshold of species determination (91.39 and 56.5%, respectively). OrthoANI values between strain 135 and related type strains are under the cutoff of determining species (<95%). The biosynthetic gene cluster (BGC) designated to X-14952B biosynthesis was identified through genome mining and the possible biosynthesis process was deduced.

Cell-Free Supernatants of Plant Growth-Promoting Bacteria: A Review of Their Use as Biostimulant and Microbial Biocontrol Agents in Sustainable Agriculture

Plant growth-promoting bacteria (PGPB) afford plants several advantages (i.e., improvement of nutrient acquisition, growth, and development; induction of abiotic and biotic stress tolerance). Numerous PGPB strains have been isolated and studied over the years. However, only a few of them are available on the market, mainly due to the failed bacterial survival within the formulations and after application inside agroecosystems. PGPB strains with these challenging limitations can be used for the formulation of cell-free supernatants (CFSs), broth cultures processed through several mechanical and physical processes for cell removal. In the scientific literature there are diverse reviews and updates on PGPB in agriculture. However, no review deals with CFSs and the CFS metabolites obtainable by PGPB. The main objective of this review is to provide useful information for future research on CFSs as biostimulant and biocontrol agents in sustainable agriculture. Studies on CFS agricultural applications, both for biostimulant and biocontrol applications, have been reviewed, presenting limitations and advantages. Among the 109 articles selected and examined, the Bacillus genus seems to be the most promising due to the numerous articles that support its biostimulant and biocontrol potentialities. The present review underlines that research about this topic needs to be encouraged; evidence so far obtained has demonstrated that PGPB could be a valid source of secondary metabolites useful in sustainable agriculture.

Nematode endosymbiont competition: Fortune favors the fittest

Endosymbiotic bacteria that obligately associate with entomopathogenic nematodes as a complex are a unique model system to study competition. These nematodes seek an insect host and provide entry for their endosymbionts. Through their natural products, the endosymbionts nurture their nematodes by eliminating secondary infection, providing nutrients through bioconversion of the insect cadaver, and facilitating reproduction. On one hand, they cooperatively colonize the insect host and neutralize other opportunistic biotic threats. On the other hand, inside the insect cadaver as a fighting pit, they fiercely compete for the fittest partnership that will grant them the reproductive dominance. Here, we review the protective and nurturing nature of endosymbiotic bacteria for their nematodes and how their selective preference shapes the superior nematode-endosymbiont pairs as we know today.

Nematophin, an Antimicrobial Dipeptide Compound From Xenorhabdus nematophila YL001 as a Potent Biopesticide for Rhizoctonia solani Control

This study was conducted to purify and identify metabolites of antimicrobial activity against phytopathogens from Xenorhabdus nematophila YL001. Three dipeptide compounds were purified from its cell-free cultural broth and identified as (±)-nematophin, cyclo (L-Pro-Gly), and N, N'-dimethyl-cyclo (L-Phe-L-Leu). Nematophin demonstrated a wider antifungal spectrum than the other two compounds. It also exhibited strong inhibitory effects on mycelial growth of Rhizoctonia solani and Phytophthora infestans with EC50 values of 40.00 and 51.25 μg/ml, respectively. Its (S)-configuration structure [(+)-nematophin] was also synthesized and exhibited higher antimicrobial activity than the enantiomeric mixture. The detached leaf assay revealed that nematophin possessed significant preventive and curative efficacy against R. solani on broad bean leaves showing corresponding control efficacies of 93.01 and 94.93% at 1,000 μg/ml, comparable to those of a chemical fungicide (carbendazim) at 500 μg/ml. Additionally, the pot experiments indicated that nematophin could effectively inhibit the disease extension on rice and broad bean plants caused by R. solani. Nematophin also exerted some adverse influences on the sclerotial development of R. solani by dramatically suppressing their formation and maturation at 40.00 μg/ml, as well as their germination at 15.00 μg/ml. Morphological and ultrastructural observations showed that the hyphae of R. solani became twisted, shriveled, and deformed at the growing points after exposure to nematophin at 40.00 μg/ml, and that the subcellular fractions also became abnormal concurrently, especially the mitochondrial structure. These results indicate that nematophin has great potential to be used as a bio-pesticide in agricultural production.

ngrA-dependent natural products are required for interspecies competition and virulence in the insect pathogenic bacterium Xenorhabdus szentirmaii

Xenorhabdus species are symbionts of entomopathogenic nematodes and pathogens of susceptible insects. Nematodes enter insect hosts and perforate the midgut to invade the haemocoel where Xenorhabdus bacteria are released transitioning to their pathogenic stage. During nematode invasion microbes from the insect gut translocate into the haemocoel. Different species of nematodes carrying specific strains of Xenorhabdus can also invade the same insect. Xenorhabdus species thereby compete for nutrients and space with both related strains and non-related gut microbes. While Xenorhabdus species produce diverse antimicrobial compounds in complex media, their functions in insect hosts are not well understood. We show that Xenorhabdus szentirmaii produced ngrA-dependent antibiotics that were active against both gut-derived microbes and Xenorhabdus nematophila whereas antibiotics of X. nematophila were not active against X. szentirmaii. X. nematophila growth was inhibited in co-cultures with wild-type X. szentirmaii in medium that mimics insect haemolymph. An antibiotic-deficient strain of X. szentirmaii was created by inactivating the ngrA gene that encodes the enzyme that attaches the 4' phosphopantetheinyl moiety to non-ribosomal peptide synthetases involved in antibiotic biosynthesis. X. nematophila growth was not inhibited in co-cultures with the ngrA strain. The growth of X. nematophila was suppressed in Manduca sexta co-injected with wild-type X. szentirmaii and X. nematophila. In contrast, growth of X. nematophila was not suppressed in M. sexta co-injected with the ngrA strain. Two unique compounds were detected by MALDI-TOF MS analysis in haemolymph infected with the wild-type but not with the ngrA strain. Finally, killing of M. sexta was delayed in insects infected with the ngrA strain. These findings indicate that in the insect host X. szentirmaii produces ngrA-dependent products involved in both interspecies competition and virulence.

CpxR negatively regulates the production of xenocoumacin 1, a dihydroisocoumarin derivative produced by Xenorhabdus nematophila

Xenocoumacin 1 (Xcn1), a major antimicrobial compound produced by Xenorhabdus nematophila, has great potential for use in agricultural productions. In this study, we evaluated the effects of CpxR, a global response regulator associated with the mutualism and pathogenesis of X. nematophila, on the antimicrobial activity and Xcn1 production. The mutation of cpxR could promote the production of Xcn1 significantly with its level in ΔcpxR mutant being 3.07 times higher than that in the wild type. Additionally, the expression levels of xcnA‐L genes, which are responsible for the production of Xcn1, were increased in ΔcpxR mutant while the expression levels of xcnMN, which are required for the conversion of Xcn1 into Xcn2 was reduced. Noticeably, Xcn2 was also enhanced on account of the conversion of excessive Xcn1 in spite of low expression levels of xcnM and xcnN in ΔcpxR mutant. The transcriptional levels of ompR and lrp, encoding the global response regulators OmpR and Lrp which negatively and positively regulate the production of Xcn1 were concurrently decreased and increased, respectively. Correspondingly, ΔcpxR mutant also exhibited increased antimicrobial activities in vitro and in vivo. Together, these findings suggest that CpxR negatively regulates xcnA‐L genes expression while positively regulating xcnMN expression in X. nematophila YL001, which led to a high yield of Xcn1 in ΔcpxR mutant.