Aeromonas veronii outbreaks in tilapia farming caused relatively high mortalities, and the bacteria was resistant to many kinds of antimicrobials used in Thailand aquaculture. According to the CLSI standard, the determination of antimicrobials efficacy has been limited to phenotypic analyses, and a genomics study is required. This research aimed to analyze the resistome of A. veronii isolated from diseased tilapia in Chainat, Nong Khai, and Uttaradit provinces in Thailand. A total of 12 isolates of A. veronii were identified based on the gyrB sequencing and then, the MIC values to eight antimicrobials (AMP, AML, GEN, ENR, OXO, OTC, SXT, and FFC) were determined. According to the MIC patterns, whole genome sequencing (WGS) of five representatives and resistome analysis were performed, including 15 genomes of A. veronii isolated from freshwater fish available in the NCBI. All tilapia isolates were susceptible to FFC but resistant to AML and AMP while OTC resistance was the most dominant. In addition to the WGS analysis, 4.5 Mbp of A. veronii was characterized. A total of 20 ARGs were detected by resistome analysis and 16 genes were shared among the A. veronii population. In conclusion, A. veronii strains isolated from tilapia exhibited a resistance to several antimicrobials and multidrug resistance (MDR) which was related to the presence of multiple ARGs. Aeromonas veronii shared the ARGs in their population worldwide with a possibility of a plasmid-mediated acquisition due to the presence of resistance islands.

Motile Aeromonas septicemia (MAS), a disease caused by Aeromonas spp., is recognized as a major disease in freshwater aquaculture. This study aimed to investigate the distribution and diversity of Aeromonas spp. and their antimicrobial susceptibility patterns. A total of 86 isolates of Aeromonas spp. were recovered from diseased freshwater fishes from 13 farms in Thailand. All isolates were identified using biochemical characteristics, matrix‐assisted laser desorption ionization‐time of flight mass spectrometry (MALDI‐TOF MS), polymerase chain reaction assays, and the gyrB gene sequence analysis. The result of MALDI‐TOF MS showed 100% (86 isolates) accuracy at genus‐level identification, and 88.4% (76 isolates) accuracy at species‐level identification. Six species of Aeromonas were confirmed through nucleotide sequencing and phylogenetic analysis of the gyrB gene Aeromonas veronii (72.1%), Aeromonas jandaei (11.6%), Aeromonas schubertii (9.3%), Aeromonas diversa (3.5%), Aeromonas hydrophila (2.3%), and Aeromonas punctata (1.2%). Antimicrobial susceptibility tests for all isolates revealed resistance against amoxicillin (99%), ampicillin (98%), oxolinic acid (81.4%), oxytetracycline (77%), trimethoprim‐sulfamethoxazole (24%), and enrofloxacin (21%). The multiple antibiotic resistance (MAR) index varied between 0.14 and 0.86, with MAR values more than 0.2 in 99% of isolates. Furthermore, four diverse multidrug‐resistant (MDR) patterns were found among Aeromonas isolates. Our finding show that A. veronii is the most abundant species in Thai cultured freshwater fish with the highest MDR patterns.

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This study describes the etiological agent of Vibriosis along with its distribution and antimicrobial resistance profiles among farmed Asian sea bass (Lates calcarifer) in Thailand. The study isolated 283 Vibrionaceae from 15 Asian sea bass farms located around the provinces of the Andaman Sea and Gulf of Thailand coasts to uncover the distribution and antimicrobial resistance profiles. Bacterial identification based on a combination of the biochemical characteristics, Matrix‐assisted laser desorption/ionization‐time of flight (MALDI‐TOF) mass spectrometry (MS) analysis, and the species‐specific PCR demonstrated the predominant Vibrionaceae were Vibrio harveyi (n = 56), Photobacterium damselae (n = 35), and V. vulnificus (n = 31), respectively. According to a laboratory challenge experiment, among the six isolates, only V. harveyi was found to cause clinical signs of muscle necrosis and scale loss in Asian sea bass. Antibiotics resistance test results exhibited high resistance to antibiotics such as metronidazole (100%), streptomycin (97%), clindamycin (96%), colistin sulphate (70%) and amoxicillin (59%). Remarkably, 100% of Vibrionaceae isolates are susceptible to florfenicol. The 28 of 29 resistance profiles were multidrug resistances (MDR), with V. vulnificus having the highest MAR value (0.66). The findings of this study advise that a surveillance program, as well as preventive and control measures, be developed for Vibrionaceae to reduce production loss, pathogen proliferation, and antibiotic abuse, whereas AMR data indicate substantial health problems for aquatic animals and humans.