Aeromonas spp. are opportunistic pathogenic bacteria associated with a multitude of diseases in ornamental fish. In this study, virulence properties and antibiotic resistance patterns of 43 Aeromonas strains isolated from 46 zebrafish were investigated. The isolates were identified as Aeromonas veronii biovar veronii (n = 26), A. veronii biovar sobria (n = 3), Aeromonas hydrophila (n = 8), A. caviae (n = 3), Aeromonas enteropelogenes (n = 2) and Aeromonas dhakensis (n = 1) by gyrB gene sequencing. The sequence divergence within and between the species ranged from 0-5·80% and 4·90-8·00%. Each species formed a distinct group in a neighbour-joining phylogenetic tree. The lipase production, biofilm formation, DNase activity, gelatinase production, caseinase production and β-hemolysis were phenotypically observed in 34 (79·07%), 33 (74·74%), 30 (69·77%), 25 (58·14%), 22 (51·18%) and 21 (48·84%) isolates. The virulence genes were detected by polymerase chain reaction (PCR) in following frequencies- aer (86·05%), hlyA (83·72%), gcaT (83·72%), lip (72·09%), act (67·44%), fla (65·12%), ascV (58·14%), ast (55·81%), ser (41·86%), ahyB (39·53%) and alt (25·58%). Every isolate was resistant to at least four antibiotics in disk diffusion test. The multiple antibiotic resistance (MAR) index values ranged from 0·22-0·50 among the isolates. Our study suggests that zebrafish can be a potential reservoir of virulent and multi-drug resistant Aeromonas spp. SIGNIFICANCE AND IMPACT OF THE STUDY: Aeromonas spp. are Gram-negative and facultative anaerobic bacteria which are ubiquitous in aquatic environments. Virulence properties and antibiotic resistance of ornamental fish-borne Aeromonas spp. are poorly understood. The virulence factors as well as multiple antibiotic resistance profiles of zebrafish-borne Aeromonas spp. were characterized for the first time in Korea. Most of the isolates were positive for phenotypic virulence traits and harboured several virulence genes revealing the virulence potential of zebrafish-borne Aeromonas spp. Additionally, the high multiple antibiotic resistance (MAR) index values displayed by the isolates highlight the necessity of responsible use of antibiotics in the ornamental fish industry.

To our knowledge, this is the first study reporting Aeromonas spp. in scallop. This implies that not only the common varieties like oysters, but other bivalves can also harbour potentially pathogenic aeromonads which may have impacts on consumer health.

Aeromonas sp. are opportunistic pathogenic bacteria which are associated with various diseases in ornamental fish, aquaculture raised species and wild fisheries. In our study, antimicrobial resistance patterns, antimicrobial resistance genes and class 1 integron gene cassettes of 52 guppy‐borne Aeromonas sp. were examined. The isolates were identified as A. veronii (n = 34), A. dhakensis (n = 10), A. hydrophila (n = 3), A. caviae (n = 3) and A. enteropelogenes (n = 2) by gyrB gene sequencing. Every isolate was resistant to at least four antimicrobials in disc diffusion test. The resistance to amoxicillin, nalidixic acid and oxytetracycline was 100% among the tested isolates. 92·30, 76·92, 71·15, 51·92, 51·92 and 50·00% of the isolates were resistant to ampicillin, rifampicin, imipenem, cephalothin, tetracycline and trimethoprim respectively. The multiple antibiotic resistance index values ranged from 0·28 to 0·67. PCR amplification of antimicrobial resistance genes implied the occurrence of tetracycline resistance (tetA (65·39%), tetE (25·00%) and tetB (15·38%)), plasmid‐mediated quinolone resistance (qnrS (26·92%) and qnrB (17·31%)) and aminoglycoside resistance (aphaAI‐IAB (7·69%) and aac (6′)‐Ib (3·84%)) genes in the isolates. The IntI gene was positive for 36·54% of the isolates and four class 1 integron gene cassette profiles (aadA2, qacE2‐orfD, aadA2‐catB2 and dfrA12‐aadA2) were identified. These data suggest that ornamental guppy can be a reservoir of multidrug‐resistant Aeromonas sp. which comprise different antimicrobial resistance genes and class 1 integrons. Significance and Impact of the Study Antimicrobial resistance genes and integron gene cassettes of ornamental fish‐borne aeromonads are poorly studied. The antimicrobial resistance patterns, antimicrobial resistance genes and class 1 integron gene cassettes of Aeromonas sp. isolated from ornamental guppy were characterized for the first time in Korea. The incidence of different antimicrobial resistance genes and class 1 integron gene cassettes were observed in multidrug‐resistant Aeromonas isolates. This result suggests that better management practices are necessary to prevent and address the serious consequences of indiscriminate and inappropriate antimicrobial use, and the distribution of multidrug‐resistant bacteria.

Aeromonas spp. are Gram‐negative opportunistic bacteria which have been commonly associated with fish diseases. In this study, antibiogram, antimicrobial resistance genes and integrons of 43 zebrafish‐borne Aeromonas spp. were studied. The isolates were identified as six Aeromonas species (A. veronii biovar veronii (n = 26), A. veronii biovar sobria (n = 3), A. hydrophila (n = 8), A. caviae (n = 3), A. enteropelogenes (n = 2) and A. dhakensis (n = 1)). Antibiogram of the isolates indicated that most of them were resistant to amoxicillin (100·00%), nalidixic acid (100·00%), oxytetracycline (100·00%), ampicillin (93·02%), tetracycline (74·42%), rifampicin (67·44%) and imipenem (65·15%). Multiple antimicrobial resistance (MAR) index values ranged from 0·19–0·44 to 90·70% isolates showed multidrug resistance. PCR of antimicrobial resistance genes revealed that the tetracycline resistance gene (tetA) was the most predominant (67·44%) among the isolates. The qnrS (53·49%), tetB (30·23%), tetE (30·23%), qnrB (23·26%) and aac(6’)‐Ib‐cr (4·65%) genes were also detected. Class 1 integrase (IntI1) gene was found in 46·51% of the isolates. Two types of class 1 integron gene cassette profiles (qacG‐aadA6‐qacG and drfA1) were identified. The results showed that zebrafish‐borne aeromonads can harbour different types of antimicrobial resistance genes and class 1 integrons. Significance and Impact of the Study Aeromonas spp. are important pathogens found in diverse environments. Antimicrobial resistance genes and integrons of ornamental fish‐borne Aeromonas spp. are not well studied. The antibiogram, antimicrobial resistance genes and class 1 integrons of Aeromonas spp. isolated from zebrafish were characterized for the first time in Korea. The prevalence of tetracycline resistance genes, plasmid‐mediated quinolone resistance genes and class 1 integron gene cassettes were observed among the isolates. The qacG‐aadA6‐qacG gene cassette was identified for the first time in Aeromonas spp. The results suggest that the wise use of antimicrobials is necessary for the better management of the ornamental fish.

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