Polymeric quaternary ammonium salt activity against Fusarium oxysporum f. sp. cubense race 4: Synthesis, structure-activity relationship and mode of action

Recent Research on Hybrid Hydrogels for Infection Treatment and Bone Repair

The repair of infected bone defects (IBDs) is still a great challenge in clinic. A successful treatment for IBDs should simultaneously resolve both infection control and bone defect repair. Hydrogels are water-swollen hydrophilic materials that maintain a distinct three-dimensional structure, helping load various antibacterial drugs and biomolecules. Hybrid hydrogels may potentially possess antibacterial ability and osteogenic activity. This review summarizes the recent progress of different kinds of antibacterial agents (including inorganic, organic, and natural) encapsulated in hydrogels. Several representative hydrogels of each category and their antibacterial mechanism and effect on bone repair are presented. Moreover, the advantages and disadvantages of antibacterial agent hybrid hydrogels are discussed. The challenge and future research directions are further prospected.

Aureoverticillactam, a Potent Antifungal Macrocyclic Lactam from Streptomyces aureoverticillatus HN6, Generates Calcium Dyshomeostasis-Induced Cell Apoptosis via the Phospholipase C Pathway in Fusarium oxysporum f. sp. cubense Race 4

Extensive efforts have been made to discover new biofungicides of high efficiency for control of Fusarium oxysporum f. sp. cubense race 4, a catastrophic soilborne phytopathogen causing banana Fusarium wilt worldwide. We confirmed for the first time that aureoverticillactam (YY3) has potent antifungal activity against F. oxysporum f. sp. cubense race 4, with effective dose for 50% inhibition (EC₅₀) of 20.80 μg/ml against hyphal growth and 12.62 μg/ml against spore germination. To investigate its mechanism of action, we observed the cellular ultrastructures of F. oxysporum f. sp. cubense race 4 with YY3 treatment and found that YY3 led to cell wall thinning, mitochondrial deformities, apoptotic degradation of the subcellular fractions, and entocyte leakage. Consistent with these variations, increased permeability of cell membrane and mitochondrial membrane also occurred after YY3 treatment. On the enzymatic level, the activity of mitochondrial complex III, as well as the ATP synthase, was significantly suppressed by YY3 at a concentration >12.50 μg/ml. Moreover, YY3 elevated the cytosolic Ca²⁺ level to promote mitochondrial reactive oxygen species (ROS) production. Cell apoptosis also occurred as expected. On the transcriptome level, key genes involved in the phosphatidylinositol signaling pathway were significantly affected, with the expression level of Plc1 increased approximately fourfold. The expression levels of two apoptotic genes, casA1 and casA2, were also significantly increased by YY3. Of note, phospholipase C activation was observed with YY3 treatment in F. oxysporum f. sp. cubense race 4. These findings indicate that YY3 exerts its antifungal activity by activating the phospholipase C calcium-dependent ROS signaling pathway, which makes it a promising biofungicide.

Biological Control of Fusarium oxysporum f. sp. cubense Tropical Race 4 Using Natively Isolated Bacillus spp. YN0904 and YN1419

Fusarium wilt of banana (FWB) is the main threatening factor for banana production worldwide. To explore bacterial biocontrol resources for FWB, the antagonistic effective strains were isolated from banana-producing areas in Yunnan Province, China. Two isolates (YN0904 and YN1419) displaying strong antagonism against Tropical Race 4 (TR4) were identified from a total of 813 strains of endophytic bacteria. TR4 inhibition rates of YN0904 and YN1419 were 79.6% and 81.3%, respectively. By looking at morphological, molecular, physiological and biochemical characteristics, YN0904 was identified as Bacillus amyloliquefaciens, while YN1419 was identified as B. subtillis. The control effects of YN0904 and YN1419 on TR4 in greenhouse experiments were 82.6% and 85.6%, respectively. Furthermore, YN0904 obviously promoted the growth of banana plantlets. In addition, biocontrol marker genes related to the biosynthesis of antibiotics synthesized and auxin key synthetase genes could be detected in YN0904. Surprisingly, the marker gene sboA could be exclusively detected in YN1419, while other marker genes were all absent. Molecular characterization results could provide a theoretical basis for expounding the biocontrol mechanisms of these two strains. We concluded that natively antagonistic strains derived from local banana plantations could provide new biological control resources for FWB.

Functionalized Silver Nanocapsules with Improved Antibacterial Activity Using Silica Shells Modified with Quaternary Ammonium Polyethyleneimine as a Bacterial Cell-Targeting Agent

Silver nanoparticles (AgNPs) have remarkable and broad-spectrum antibacterial activities against Gram-positive (G+) and Gram-negative bacteria (G-). However, the negative surface potential of AgNPs limits their antibacterial activities due to the electrostatic repulsion with the negatively charged bacterial cell membrane. To address the limitation, AgNPs were loaded in the mesoporous silica nanoparticles by preparing silver core-mesoporous silica shell nanocapsules (Ag@MSNs), and then, a cationic antibacterial polymer, quaternary ammonium polyethyleneimine (QPEI), was used to modify Ag@MSNs for improving their surface potential and antibacterial activities. The results showed that the obtained Ag@MSN-QPEI exhibited a high positive surface potential (+39.6 mV) and a strong electrostatic attraction with Pseudomonas syringae pv. lachrymans cells in coculture, resulting in an excellent bacterial cell-targeting effect. At the same concentration, Ag@MSN-QPEI exhibited less silver content (reducing the silver content of Ag@MSNs by 19%), higher antibacterial activities, and longer effective duration against Clavibacter michiganensis subsp. michiganensis (G+) and P. syringae pv. lachrymans (G-) than Ag@MSNs and QPEI alone. The excellent bacterial cell-targeting effect and synergistic antibacterial action combined with QPEI accounted for the significantly enhanced antibacterial activities of Ag@MSN-QPEI. Therefore, using a cationic antibacterial polymer to confer the bacterial cell-targeting effect and synergistic antibacterial action would be extended to other antimicrobial materials.