ABSTRACT With a global rise in industrialization and chemical processing units, the introduction of anthropogenic pollutants into the environment has been on a rise. Aquatic pollutants, unlike terrestrial pollutants, are easily diluted in water environments and exist at a sub-inhibitory concentration (sub-IC). At sub-IC, they do not directly inhibit bacterial growth but can modulate gene expression profiles. Aquaculture industry relies heavily on the use of antibiotics for control of fish pathogens, and the indiscriminate use of antibiotic agents increases antibiotic resistance in the aquatic environment. Previous studies have reported that sub-IC of antibiotics are able to regulate biofilm formation in a variety of pathogenic bacteria. To understand the environmental signals in the biofilm formation of E. piscicida , we hypothesized that the biofilm of E. piscicida can be regulated by sub-IC of erythromycin, which is widely used antibiotic in aquaculture. Our results indicate that at sub-IC of erythromycin, biofilm formation was induced due to increased type 1 fimbrial expression. This caused an increased colonization of the host and hyper-virulence in E. piscicida . Our study reveals a new aspect of antibiotic functioning as a signaling molecule that modulates biofilm-related-gene expression and virulence in fish pathogen. This study demonstrates the comprehensive molecular response of the fish pathogen to environmentally discharged erythromycin. Additionally, the study provides scientific evidence for the hypertoxicity of pathogens through the indiscriminate use of antibiotics—ultimately leading to detrimental effects to the fishery industry. IMPORTANCE Antibiotics released into aquatic environments often persist at sub-inhibitory concentrations, where they no longer suppress bacterial growth but instead act as signaling molecules. Here, we show that sub-inhibitory erythromycin enhances biofilm formation and virulence in the fish pathogen Edwardsiella piscicida by upregulating type 1 fimbriae. This response promotes host colonization and hypervirulence, demonstrating that environmentally relevant antibiotic exposure can unintentionally increase pathogenic potential. Our findings provide in vivo evidence that sub-therapeutic antibiotics reshape bacterial behavior and host-pathogen interactions. This study highlights an underappreciated ecological and economic risk of indiscriminate antibiotic use in aquaculture, with direct implications for fish health, disease management, and environmental safety.
Park et al. (2026) studied this question.