The emergence of antimicrobial-resistant bacteria is an enormous challenge to public health. Aeromonas hydrophila and Aeromonas veronii are opportunistic pathogens in fish. They exert tremendous adverse effects on aquaculture production, owing to their acquired antibiotic resistance. A few Clinical and Laboratory Standards Institute (CLSI) epidemiological cut-off values (ECVs) against Aeromonas spp. are available. We evaluated antimicrobial susceptibility by establishing 8 ECVs using two analytical methods, normalized resistance interpretation and ECOFFinder. We detected antimicrobial resistance genes in two motile Aeromonas spp. isolated from aquatic animals. Results showed that 89.2% of A. hydrophila and 75.8% of A. veronii isolates were non-wild types according to the oxytetracycline ECV_CLSI and ECV_NRI, respectively. The antimicrobial resistance genes included tetA, tetB, tetD, tetE, cat, floR, qnrA, qnrB, qnrS, strA-strB, and aac(6′)-1b. The most common tet gene in Aeromonas spp. isolates was tetE, followed by tetA. Some strains carried more than one tet gene, with tetA–tetD and tetA–tetE found in A. hydrophila; however, tetB was not detected in any of the strains. Furthermore, 18.6% of A. hydrophila and 24.2% of A. veronii isolates showed presumptive multidrug-resistant phenotypes. The emergence of multidrug resistance among aquatic aeromonads suggests the spread of drug resistance and difficult to treat bacterial infections.

Aeromonas salmonicida is a predominant pathogen that infects fish. The pathogen A. salmonicida subsp. masoucida (ASM) was isolated for the first time from diseased starry flounders (Platichthys stellatus). Our study aimed to isolate, characterize, and investigate the pathogenicity of ASM. Bacterial species were identified using 16s rRNA, gyrB, dnaJ, and vapA analyses. Phylogenetic tree analysis revealed that the ASM strains were clustered with the ASM ATCC strain and other strains isolated from black rockfish. In the antimicrobial susceptibility test, the three ASM strains were considered non-wild types for enrofloxacin, florfenicol, flumequine, oxolinic acid, and oxytetracycline susceptibility. Histopathological analysis revealed bacterial colonies in the secondary lamella and heart, indicating that ASM strains are highly virulent in fish. Comparative analysis and annotation via genome sequencing revealed that, among the 1156 factors, adherence factors were the most prevalent putative virulence determinants, followed by the effector delivery system and adherence. ASM was found to possess 43 type III secretion systems, 22 type VI secretion systems, 11 antimicrobial resistance genes, 3 stress genes, and prophage regions. These findings provide new insights into the virulence profile of ASM and highlight the risk posed by emerging pathogenic strains to starry flounders.

Paralichthys olivaceus (olive flounder) is widely cultivated in Korea. However, data on the antibiotic susceptibility of bacterial pathogens that infect olive flounders in Korea are limited. The susceptibility of 84 strains of 3 pathogenic bacteria (Streptococcus spp., Vibrio spp., and Edwardsiella piscicida) to 18 antibiotics was tested using the minimum inhibitory concentration (MIC) panels, and the distribution of the MIC values for each species was confirmed. Among the panel antibiotics, nine commonly used antibiotics were selected, and the multiple antibiotic resistance (MAR) index and antibiotic resistance pattern were indicated using the disk diffusion method. It was confirmed that most of the isolates had a MAR index greater than 0.2, indicating a high-risk source. The distribution patterns of the MIC values and resistance pattern between gram-positive and gram-negative bacteria showed slightly different results. Ampicillin, erythromycin, and clindamycin were more effective against gram-positive bacteria than gram-negative bacteria. However, the MIC values of flumequine for gram-positive bacteria were higher than those of gram-negative bacteria. Through the distribution patterns of the MIC values and resistance patterns presented in this study, the need for monitoring the multidrug-resistant bacteria in aquaculture is emphasised.