The role of hydroponic solution of Ulva fasciata in modulating the antibiotic resistance of Vibrio alginolyticus.

Aims: This study aimed to ascertain the active substances in the hydroponic solution of Ulva (U.) fasciata (HSUF) and their effects on antibiotics resistance of pathogenic bacteria Vibrio alginolyticus (Va.) at environmental levels.

Methods and results: Analytical results of HSUF by solid-phase extraction and gas chromatography-mass spectrometry showed that HSUF contained rich active substances and dominated by 2,4,6-tribromophenol, acetal, mono (2-ethylhexyl) phthalate and 2- (4-hydroxybenzene) ethanol, etc. Thereinto, 2,4,6-tribromophenol had the lowest minimal inhibitory concentrations (MICs) of 64-128 μg mL-1 for Va. strains. The rich antibacterial substances of HSUF inhibited 47%-63% of isolated Va. strains. After 20 generations of subculture for Va. strains in three dilutes (1/2 (HT), 1/20 (MT) and 1/50 (LT)) of HSUF (20 g L-1), the resistance to streptomycin of Va. S1 and Va. M3 changed from intermediate resistance ability to susceptible level. The large promotion of reactive oxygen species was observed in different HSUF levels, but the biofilm formation of Va. S1 did not changed significantly. Transcriptome sequencing of Va. S1 demonstrated that antibiotic resistance gene lpxA was down-regulated at different HSUF levels.

Conclusions: Live U. fasciata excreted a variety of active secondary metabolites (SMs) in HSUF, which exhibited strong inhibitory effects on three isolated Va. strains. HSUF changed the antibiotic resistance of Va. Strain, especially at a higher HSUF concentrations.