Escalating resistance of pathogens, especially Gram-negative bacteria, to antibiotics has become a public health problem arousing worldwide concern because of the abuse of antibiotics. The "Trojan Horse" strategy emerges as a promising approach to the development of new antibacterial agents. This strategy improves the antibacterial activity or broadens the antibacterial spectrum of antibiotics by using the siderophore-mediated bacterial iron transport system. In 2019, cefiderocol as the first siderophore-antibiotic conjugate was approved for marketing, which has garnered wide attention of scientists in this field. Currently, researchers mainly focus on siderophores or utilizing antibiotics with different mechanisms and ignoring linkers in the design of agents based on the "Trojan Horse" strategy. This review will summarize the impact of different linkers of conjugates on antibacterial activity, which could provide reference for the development of new antibacterial drugs and combating bacterial resistance.

[Objective] To study the sulfur oxidation characteristics of Halothiobacillus diazotrophicus LS2 under different oxygen levels and to decipher the mechanism of strain LS2 adapting to low-oxygen environments. [Methods] The concentrations of S₂O₃^2- and SO₄^2- were measured by ion chromatography. Bacterial growth was determined by plate dilution coating method. The differentially expressed genes and related metabolic pathways were identified and analyzed by transcriptome sequencing and bioinformatics technology. [Results] Strain LS2 oxidized reduced sulfur compounds and grew under 0.2%–21.0% oxygen, and it maintained high sulfur oxidation activity under the oxygen level above 1.6%. Comparative transcriptomic analysis screened out 851 differentially expressed genes that might be related to the adaptation to low oxygen, including 464 up-regulated genes and 387 down-regulated genes. In sulfur metabolism, thiosulfate sulfurtransferase, sulfur oxidase/reductase, and sulfide: quinone oxidoreductase were up-regulated, while the Sox enzyme system was down-regulated, which indicated that strain LS2 might change the sulfur oxidation pathway to adapt to low-oxygen environment. In the low-oxygen group, the cbb₃-type cytochrome c oxidase was up-regulated to increase the O₂-binding efficiency. Meanwhile, since less electron could be received by O₂, the nitrogenase genes nifDKH and Fix complex genes fixA, fixB, fixC, fixX were up-regulated, making N₂ and CO₂ the alternative electron accepters to maintain redox balance, which explained the higher maximum bacterial growth in low-oxygen environments. [Conclusion] Strain LS2 is a sulfur-oxidizing bacterium that can maintain high sulfur oxidation activity in the low-oxygen environment. Sulfide: quinone oxidoreductase, high-oxygen-affinity terminal oxidases, and nitrogenase play a role in the adaptation to the low-oxygen environment. This study is of positive significance for deciphering the mechanism of sulfur oxidation under low oxygen and provides a theoretical basis for optimizing the treatment process of sulfur-containing wastewater.

[Objective] In view of the enhanced cell-to-cell spread ability of the dsbA-deleted strain (ΔdsbA) of Listeria monocytogenes, this study aims to elucidate the mechanism that how the disulfide bond formation protein DsbA mediates this biological process. [Methods] The mRNA and protein levels of virulence factors in the wild type and ΔdsbA were compared by RT-qPCR and Western blotting, respectively. The immunofluorescence co-localization analysis method was employed to observe the impact of DsbA deficiency on the actin recruitment by the virulence factor ActA in the cell-to-cell spread of L. monocytogenes (analyzing the length and quantity of the comet tails formed on one side of the bacteria by co-localization of ActA and actin). The presence or absence of interaction between DsbA and ActA was determined by isothermal titration calorimetry (ITC). [Results] Compared with the wild type, ΔdsbA showed no significant changes in the mRNA levels of virulence factors, downregulated protein levels of InlA, InlB, PlcA, and PlcB, and upregulated protein levels of ActA and LLO. In addition, ΔdsbA showed increased number and average length of comet tails, which indicated that the actin recruitment of ΔdsbA was enhanced. The ITC results revealed that DsbA bound to ActA, which gradually showed endothermic reactions, suggesting the presence of interaction between DsbA and ActA. [Conclusion] This study proved for that DsbA attenuated the recruitment ability of actin by regulating virulence proteins, thus affecting the cell-to-cell spread of L. monocytogenes. The findings help to further dissect the virulence regulatory mechanisms of L. monocytogenes during host infection, which is of great importance for controlling the contamination of zoonotic intracellular pathogens threatening public health.

Airborne microorganisms (AMs), including bacteria, fungi, and viruses, are ubiquitous. They are indispensable components of the atmospheric ecosystem, play an essential role in maintaining the stability and functions of the atmospheric ecosystem. AMs include not only beneficial microorganisms but also pathogenic microorganisms, which pose a threat to human health. Therefore, comprehensively revealing the distribution characteristics and succession patterns of AMs is of great significance for improving air quality, safeguarding human health, and ensuring national biosafety. This review systematically elucidates the sources, distribution characteristics, influencing factors, and categories of pathogenic microorganisms as well as the health risks associated with AMs. It contributes to the in-depth understanding of AMs pollution and its health risks, providing a scientific basis for preventing and controlling the pollution and protecting human health and eco-environment.

[Objective] The Yellow River estuary located at the confluence of the Yellow River, land, and ocean is an area with mixed freshwater and seawater and a diverse and productive estuary ecosystem. This study aims to characterize the bacterial communities in freshwater and seawater of the Yellow River estuary. [Methods] High-throughput absolute abundance quantification was adopted to measure the absolute abundance of bacterial communities. The dominant taxa, α and β diversity, co-occurrence network, assembly mechanisms, and potential functions were compared between the bacterial communities in freshwater and seawater. The correlations between dominant taxa and environmental factors were explored. [Results] The absolute abundance of bacteria in freshwater was 2.61×10⁶ copies/mL, which was 1.8 times of that in seawater. The common dominant phyla in freshwater and seawater were Actinomycetota, Pseudomonadota, Cyanobacteriota, and Bacteroidota, with significant differences in absolute abundance. The abundance of Actinomycetota ranked first in freshwater, which was approximately equal to the sum of all dominant phyla in seawater. The abundance of Pseudomonadota was the highest in seawater. The alpha diversity of bacteria in freshwater was higher than that in seawater. There were significant differences in the bacterial community structure between freshwater and seawater, mainly due to the differences in the abundance of the dominant taxa. The bacterial co-occurrence network in freshwater was more complex and stable than that in seawater, and stochastic processes dominated the bacterial community assembly in both freshwater and seawater. The bacterial communities in freshwater and seawater presented different functions, while they shared some common functions. Metabolism was the most abundant function, with higher relative abundance in freshwater than in seawater. Five environmental factors ((pH, oxidation-reduction potential (ORP), electrical conductivity (EC), total organic carbon (TOC), and total nitrogen (TN)) correlated with the dominant bacterial taxa to different extent. There were collinear relationships among the four environmental factors except EC. The dominant genera showing positive correlations with pH, TOC and TN were all negatively correlated with ORP, and vice versa. Actinomycetota and Pseudomonadota were positively and negatively correlated with pH, respectively. [Conclusion] The bacterial communities showed great differences between freshwater and seawater in the Yellow River estuary. The differences were mainly reflected in the abundance, diversity, functional structure, and co-occurrence network. The bacterial communities in freshwater and seawater had similar dominant taxa and assembly mechanisms. The results provide data support for studying the microbial ecology and exploiting microbial resources in the Yellow River estuary.

[Objective] To obtain a stable microbial consortium with a high yield of caproate and achieve high-value carbon recovery from Baijiu-making wastewater (Huangshui). [Methods] We used the plate screening approach to obtain a simplified caproate-producing microbial consortium, evaluated the preferred carbon source of the consortium, and optimized the substrate concentration, pH, and feeding strategy. The metagenomics based on nanopore sequencing was employed to determine the composition and stability characteristics of the simplified caproate-producing microbial consortium. [Results] SimpCom1, a simplified caproate-producing microbial consortium, demonstrated significantly higher conversion rate of lactate than glucose to caproate. When SimpCom1 was used to ferment unsterilized Huangshui, we controlled the working concentration of Huangshui between 30% and 50% and initial pH 5.50 to achieve stable growth and metabolism of the consortium for caproate production. The fermentation was carried out in a fed-batch manner with 50% Huangshui, initial pH 5.50, and pH 6.50–7.00 after 48 h. Within four fed-batch fermentation cycles, the average caproate titer, productivity, proportion of caproate in total acids, and conversion rate of lactate to caproate reached 16.83 g/L, 3.05 g/(L·d), 67.27%, and 0.42 g/g, respectively. The metagenomic analysis showed that Caproicibacterium lactatifermentans, Ligilactobacillus acidipiscis, Clostridium tyrobutyricum, and 'Butyriproducens baijiuensis BJN0003' were the core species of SimpCom1. C. lactatifermentans and 'B. baijiuensis BJN0003' remained stable growth and metabolism in the unsterilized Huangshui, with the relative abundance of 45.3% and 6.7%, respectively, at the end of fermentation with 50%-diluted Huangshui. [Conclusion] We successfully established an efficient and low-cost approach for producing caproate by fermentation of unsterilized Baijiu-making wastewater with a simplified microbial consortium containing C. lactatifermentans and 'B. baijiuensis BJN0003'.

[Objective] To study the effects of endoplasmic reticulum stress on the regulation of the lifespan and autophagy of Saccharomyces cerevisiae by the protein O-mannosyltransferase 1 (PMT1). [Methods] The double deletion strain (pho8Δ60 pmt1Δ) was constructed based on the genetic homologous recombination. The daughter cells produced by the mother cell of the PMT1-deleted yeast strain (pmt1Δ) treated with tunicamycin (inducing endoplasmic reticulum stress) were counted under a light microscope, and the replicative lifespan of the strain was examined. A microplate reader was used to measure the alkaline phosphatase activity of the pho8Δ60 pmt1Δ strain in the SD-N medium (for inducing autophagy). Western blotting was employed to determine the expression level of the autophagy marker Atg8 in the presence of tunicamycin. The transcript levels of autophagy-related genes ATG1 and ATG8 in the pmt1Δ strain treated with tunicamycin were determined by RT-qPCR. [Results] The replicative lifespan of the pmt1Δ strain was shortened by 38.7%, while the alkaline phosphatase activity of pmt1Δ strain was increased compared with those of the wild type in the presence of tunicamycin. The expression levels of GFP-Atg8 fusion protein and free GFP in the pmt1Δ strain were up-regulated with the increase in the concentration of tunicamycin. The transcript levels of ATG1 and ATG8 were up-regulated in the pmt1Δ strain treated with tunicamycin. [Conclusion] Endoplasmic reticulum stress impairs the replicative lifespan and enhances the autophagy of PMT1-deleted yeast cells.

In Pseudomonas aeruginosa, tryptophan can be converted into anthranilate via the KynABU pathway, and anthranilate as a substrate is further converted into alkyl quinolones (AQs), including 2-heptyl-3-hydroxy-4-quinolone (PQS) and 2-heptyl-4-quinolone (HHQ), under the action of pqsABCDE, a synthetic gene cluster of AQs. At the same time, anthranilate can be degraded into the tricarboxylic acid cycle under the catalysis of the anthranilate dioxygenase complex AntABC, while the biological effect of AntABC on P. aeruginosa remains unclear. [Objective] To construct and characterize the phenotype of the antABC-deleted mutant of P. aeruginosa. [Methods] With P. aeruginosa PAO1 as the starting strain, we constructed the antABC-deleted mutant by homologous recombination to study the effects of the operon on tryptophan degradation, biofilm formation, pyocyanin synthesis, motility, and virulence of P. aeruginosa. [Results] The deletion of antABC or kynU completely inhibited the growth of P. aeruginosa with tryptophan as the sole carbon source, while ΔpqsA did not present this phenotype, indicating that antABC was essential for the degradation of tryptophan by P. aeruginosa, and KynABU-AntABC pathway was the only way for the degradation of tryptophan by the bacterium under the culture conditions in this study. In addition to affecting tryptophan degradation in P. aeruginosa, the deletion of antABC promoted the biofilm formation of P. aeruginosa by inducing the expression of the extracellular polysaccharide synthesis operon pel, and it promoted the synthesis of pyocyanin by inducing the expression of the pyocyanin synthesis operons phz1 and phz2. In addition, the deletion of antABC enhanced the swarming motility and twitching motility of P. aeruginosa. Interestingly, further deletion of pqsA completely reversed the physiological phenotypes of ΔantABC. Therefore, the regulation of antABC on these physiological phenotypes depended on AQs. The deletion of antABC increased the HHQ accumulation while inhibiting the synthesis of PQS in P. aeruginosa. These results indicated that the regulation of these physiological phenotypes by antABC mainly depended on HHQ. In addition, the deletion of antABC enhanced the virulence of P. aeruginosa to Chinese cabbage and Galleria mellonella larvae, while further deletion of pqsA only partially reversed this virulence phenotype. Moreover, the deletion of antABC caused increased accumulation of anthranilate in P. aeruginosa. Therefore, the enhancement of antABC deletion on the virulence of P. aeruginosa was mediated by HHQ and anthranilate together. Finally, bioinformatics analysis revealed that the missense mutations of antABC operon occurred in more than 90% of clinical isolates of P. aeruginosa. Therefore, antABC was expected to be used as a biomarker to determine whether clinical isolates of P. aeruginosa were highly virulent. [Conclusion] AntABC plays an important role in the tryptophan degradation, biofilm formation, pyocyanin synthesis, motility, and virulence of P. aeruginosa. This finding lays a foundation for the clinical diagnosis and antimicrobial development of P. aeruginosa infection.

Lignin, the most abundant aromatic biopolymer resource in the nature, is difficult to be degraded by common microorganisms due to its complexity and highly aggregated aromatic structure. Lignin-degrading microorganisms from extreme environments are considered as suitable candidates for lignin bioprocessing. This review summarizes several types of extremophiles capable of degrading lignin and the extremozymes produced by them and elucidates the properties, catalytic mechanisms, and metabolic pathways of the extremozymes. Furthermore, this article discusses the prospects for the identification of novel extremophiles and extremozymes by multi-omics and makes an outlook on the development and utilization methods of extremophiles, with a view to providing a reference for the subsequent screening and development of more efficient lignin-degrading strains.

[Objective] Riemerella anatipestifer (RA) is a Gram-negative bacterium that mainly infects domesticated birds such as ducks and geese. The clinical isolates of RA are multi-drug resistant and increasing year by year. However, the transmission ways of antibiotic resistance genes in RA have not been identified. This study aims to identify the transmission ways and distribution of antibiotic resistance genes in the clinical isolates of RA. [Methods] The drug resistance phenotypes of the reference strain RA ATCC 11845 and the clinical isolates RA CH-1 and RA CH-2 to 28 antibiotics belonging to 10 categories were determined. The antibiotic resistance genes were identified by genome analysis and construction of gene deletion strains. The transmission ways of antibiotic resistance genes were identified by natural transformation. The distribution of these resistance genes in different clinical isolates was detected by PCR. [Results] RA CH-1 and RA CH-2 were resistant to β-lactams, tetracyclines, macrolides, lincosamides, and amide alcohols, while RA ATCC 11845 was sensitive to the above antibiotics. The resistant strains became sensitive to the corresponding antibiotics after the deletion of 13 resistance genes, respectively, indicating that these genes were involved in antibiotic resistance. All the resistant genes can be transferred to the sensitive strain RA ATCC 11845 by natural transformation. The detection rates of resistance genes in 100 clinical isolates from 2017 to 2023 varied within the range of 3% to 89%. [Conclusion] Antibiotic resistance genes can be transmitted in RA through natural transformation, and different antibiotic resistance genes presented varied distribution in clinical isolates, among which tetX (B739_0030) and bla_OXA (G148_1768) were carried by the most and fewest strains, respectively.