Phenol and formalin are major water pollutants that are frequently discharged into the aquatic milieu. These chemicals can affect broad domains of life, including microorganisms. Aquatic pollutants, unlike terrestrial pollutants, are easily diluted in water environments and exist at a sub-inhibitory concentration (sub-IC), thus not directly inhibiting bacterial growth. However, they can modulate gene expression profiles. The sub-IC values of phenol and formalin were measured by minimal inhibitory concentration (MIC) assay to be 0.146% (1.3 mM) and 0.0039% (0.38 mM), respectively, in Edwardsiella piscicida CK108, a Gram-negative fish pathogen. We investigated the differentially expressed genes (DEG) by RNA-seq when the cells were exposed to the sub-ICs of phenol and formalin. DEG analyses revealed that genes involved in major virulence factors (type I fimbriae, flagella, type III and type VI secretion system) and various cellular pathways (energy production, amino acid synthesis, carbohydrate metabolism and two-component regulatory systems) were up- or downregulated by both chemicals. The genome-wide gene expression data corresponded to the results of a quantitative reverse complementary-PCR and motility assay. This study not only provides insight into how a representative fish pathogen, E. piscicida CK108, responds to the sub-ICs of phenol and formalin but also shows the importance of controlling chemical pollutants in aquatic environments.

Antibiotics have been widely used to inhibit microbial growth and to control bacterial infection; however, they can trigger an imbalance in the gut flora of the host and dysregulate the host gene regulatory system when discharged into the aquatic environment. We investigated the effects of chronic exposure to a low concentration of erythromycin and ampicillin, focusing on gut microbiome and global gene expression profiles from Korea native ricefish (Oryzias latipes). The proportion of Proteobacteria (especially the opportunistic pathogen Aeromonas veronii) was significantly increased in the ricefish under the chronic exposure to erythromycin and ampicillin, whereas that of other bacterial phyla (i.e., Fusobacteria) decreased. In addition, the expression of genes involved in immune responses such as chemokines and immunocyte chemotaxis was significantly influenced in ricefish in the aquatic environment with antibiotics present. These results show that the internal microbial flora and the host gene expression are susceptible even at a low concentration of chronic antibiotics in the environment, supporting the importance of the appropriate use of antibiotic dose to maintain the sustainable and healthy aquaculture industry and water ecosystem.

Edwardsiella piscicida is a fish pathogen that belongs to the Enterobacteriaceae family. Infection of E. piscicida causes severe economic loss to the fishery industry (Baek et al., 2020;Wang, Yang, et al., 2009;Yoon et al., 2020). E. piscicida is the causative pathogen for edwardsiellosis, a systematic water-borne fish disease (Mohanty & Sahoo, 2007). E. piscicida, which can infect fish through the skin, anus and intestine, penetrates various cells, such as epithelial cells and macrophages (Ling et al., 2000;. Various pathogen virulence factors are involved in the pathogenesis of edwardsiellosis, such as effector proteins

Antibiotics have been used in various fields such as livestock farm and fish farm as well as hospital in order to treat diseases caused by bacteria. However, the antibiotics that are not completely decomposed, but remains as residue and discharge to aquatic environment, can cause an imbalance in the gut flora of host, as well as regulate abnormal host gene regulatory system. We investigated the effects of chronic exposure with the low concentrations of erythromycin and ampicillin on gut microbiome and immune and stress-related gene expression using Korea native ricefish (Oryzias latipes). As a result of microbiome analysis, the proportion of Proteobacteria was increased in the ricefish when exposed to erythromycin and ampicillin chronically, whereas the proportion of other bacterial phyla decreased. In addition, the immune and stress-related genes were significantly influenced in the ricefish under the chronic antibiotics exposure. These results show that the internal microbial flora and the host gene expression are susceptible even in the low concentration of chronic antibiotic existing environments. This study provides the importance of the appropriate use of antibiotics dose to maintain the sustainable and healthy aquaculture industry and water ecosystem.