The type III secretion system (T3SS) of Edwardsiella tarda plays an important role in infection by translocating effector proteins into host cells. EseB, a component required for effector translocation, is reported to mediate autoaggregation of E. tarda . In this study, we demonstrate that EseB forms filamentous appendages on the surface of E. tarda and is required for biofilm formation by E. tarda in Dulbecco's modified Eagle's medium (DMEM). Biofilm formation by E. tarda in DMEM does not require FlhB, an essential component for assembling flagella. Dynamic analysis of EseB filament formation, autoaggregation, and biofilm formation shows that the formation of EseB filaments occurs prior to autoaggregation and biofilm formation. The addition of an EseB antibody to E. tarda cultures before bacterial autoaggregation prevents autoaggregation and biofilm formation in a dose-dependent manner, whereas the addition of the EseB antibody to E. tarda cultures in which biofilm is already formed does not destroy the biofilm. Therefore, EseB filament-mediated bacterial cell-cell interaction is a prerequisite for autoaggregation and biofilm formation.

This study aims to evaluate the acute toxicity of pymetrozine to juvenile Procambarus clarkii. Two 96-h toxicity tests were conducted to assess the lethal concentration 50 (LC₅₀) values, behaviors, and histopathology (at 50% of the 96 h LC₅₀) after pymetrozine exposure. The results showed high toxicity of pymetrozine to juvenile P. clarkii in a dose and time dependent manner, with a decreasing LC₅₀ from 1.034 mg/L at 24 h to 0.479 mg/L at 96 h. The maximum allowable concentration (MAC) of pymetrozine for P. clarkii was 0.106 mg/L. Behavioral abnormalities were observed in pymetrozine-treated crayfish, such as incunabular hyperexcitability, subsequent disequilibrium, lethargy, and increased defecation. Significant lesions were observed in all pymetrozine-treated tissues, including: (1) in gill, hemocytic infiltration and 33.27% of epithelial cells lesions; (2) in perigastric organs, 64.37%, 29.06%, and 13.99% of tubules with lumen atrophy, vacuolation, and cell lysis, respectively; (3) in heart, 2.5%, 8.55% and 7.74% of hemocytic infiltration, vacuolization, and hyperplasia, respectively; (4) in stomach, 80.82%, 17.77%, 6.98%, 5.24% of cuticula swelling, vacuolization, muscle fragmentation, hemocytic infiltration, respectively; (5) in midgut, 7.45%, 10.98%, 6.74%, and 13.6% of hyperplasia, tissue lysis and vacuolation, hemocytic infiltration, muscle fracture; and (6) in abdominal muscle, 14.09% of myofiber fracture and lysis. This research demonstrates that pymetrozine is highly toxic to juvenile P. clarkii, with significant effects on mortality, behavior and histopathology at concentrations of ≤1.1 mg/L, while the estimated practical concentration of pymetrozine in rice-crayfish culture water was around 20 times lower than the calculated MAC.

The mixture of bensulfuron-methyl and acetochlor (MBA) has been widely applied as a rice herbicide in China, but the mixture toxicity of MBA to aquatic organisms is largely unknown. The current study aims to investigate the acute effects of MBA to juvenile red swamp crayfish, Procambarus clarkii. Firstly, a 96 h semi-static exposure was conducted to determine the Lethal Concentration 50 (LC₅₀) values at 24, 48, 72 and 96 h, as well as to assess the behavioral and morphological effects. A second 96 h exposure was conducted at an MBA concentration of 50% of the 96 h LC₅₀ (72.62 mg/L) to assess the histological changes in the gill, perigastric organ, muscle, heart, stomach, and midgut. The results showed that MBA exhibited low acute toxicity with the 24, 48, 72 and 96 h LC₅₀ values of 191.25 (179.37–215.75), 166.81 (159.49–176.55), 154.30 (148.36–160.59) and 145.24 (138.94–151.27) mg/L, respectively. MBA-exposed crayfish showed body jerk, belly arch, equilibrium loss, body and appendage sway, and lethargy; and the dead crayfish showed dark gray or grayish-white body color and separated cephalothorax and abdomen. At 72.62 mg/L, MBA exposure caused significant histopathological alterations, mainly including the cuticular and epithelial degeneration of all the gills; atrophy of tubule lumina and cellular vacuolation of the perigastric organs (61.15 ± 9.90% of the tubules showed lesions); epithelial hyperplasia (48.40 ± 9.00%), myocardial fibers and epithelial cell lysis (17.30 ± 2.01%), and hemocytic infiltration of the hearts; cuticular swelling (15.82 ± 2.98%) and vacuolate connective tissue (11.30 ± 2.47%) of the stomachs; atrophied bladder cell and fragmented longitudinal muscles (95.23 ± 4.77%) of the midguts; and slight myofibers fragmentation and lysis (7.37 ± 0.53%) of the abdominal muscles. Our results indicate that MBA can cause behavioral, morphological and histopathological effects on juvenile P. clarkii at relatively high concentrations, but its acute toxicity is low compared with many other common herbicides.