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Virulence and antimicrobial resistance potential of Aeromonas spp. associated with shellfish

2021; Oxford University Press; Volume: 73; Issue: 2 Linguagem: Inglês

10.1111/lam.13489

1472-765X

Liyana Arachchilage Dinithi Sandunika De Silva, M.V.K.S. Wickramanayake, Gang‐Joon Heo,

Antimicrobial Peptides and Activities

Letters in Applied MicrobiologyVolume 73, Issue 2 p. 176-186 Review ArticleFree Access Virulence and antimicrobial resistance potential of Aeromonas spp. associated with shellfish L.A.D.S. De Silva, Veterinary Medical Center and College of Veterinary Medicine, Chungbuk National University, Cheongju, KoreaSearch for more papers by this authorM.V.K.S. Wickramanayake, Veterinary Medical Center and College of Veterinary Medicine, Chungbuk National University, Cheongju, KoreaSearch for more papers by this authorG.-J. Heo, Corresponding Author gjheo@cbu.ac.kr orcid.org/0000-0003-4602-4086 Veterinary Medical Center and College of Veterinary Medicine, Chungbuk National University, Cheongju, Korea Correspondence Gang-Joon Heo, Laboratory of Aquatic Animal Medicine, Veterinary Medical Center and College of Veterinary Medicine, Chungbuk National University, Chungdae-ro 1, Seowon-gu, Cheongju, Chungbuk 28644, Republic of Korea. E-mail: gjheo@cbu.ac.krSearch for more papers by this author L.A.D.S. De Silva, Veterinary Medical Center and College of Veterinary Medicine, Chungbuk National University, Cheongju, KoreaSearch for more papers by this authorM.V.K.S. Wickramanayake, Veterinary Medical Center and College of Veterinary Medicine, Chungbuk National University, Cheongju, KoreaSearch for more papers by this authorG.-J. Heo, Corresponding Author gjheo@cbu.ac.kr orcid.org/0000-0003-4602-4086 Veterinary Medical Center and College of Veterinary Medicine, Chungbuk National University, Cheongju, Korea Correspondence Gang-Joon Heo, Laboratory of Aquatic Animal Medicine, Veterinary Medical Center and College of Veterinary Medicine, Chungbuk National University, Chungdae-ro 1, Seowon-gu, Cheongju, Chungbuk 28644, Republic of Korea. E-mail: gjheo@cbu.ac.krSearch for more papers by this author First published: 23 April 2021 https://doi.org/10.1111/lam.13489Citations: 1AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinked InRedditWechat Abstract Aeromonas spp. are associated with seafood-related outbreaks worldwide. In seafood industry, shellfish play a major role in global seafood production. With this emerging trend of shellfish consumption, shellfish-related bacterial infections are being reported frequently. Aeromonas spp. are natural contaminants found in shellfish. Although 36 species have been identified, some species including Aeromonas hydrophila, Aeromonas caviae and Aeromonas veronii biotype sobria have dragged major attention as foodborne pathogenic bacteria. The ability to elaborate a variety of virulence factors of Aeromonas spp. contributes to the pathogenic activities. Also, emerging antimicrobial resistance in Aeromonas spp. has become a huge challenge in seafood industry. Furthermore, multidrug resistance increases the risk of consumer health. Studies have supplied pieces of evidence about the emerging health risk of Aeromonas spp. isolated from seafood. Therefore, the present review was intended to highlight the prevalence, virulence and antimicrobial resistance of Aeromonas spp. isolated from various types of shellfish. Introduction According to Food Agriculture Organization (FAO), world aquaculture production constitutes 114·5 million tonnes which included 82·1 million tonnes of fish production. Total aquaculture fish production consisted of 54·3–47 million tonnes from inland aquaculture and 7·3 million tonnes from marine and coastal aquaculture, including 17·7 million tonnes of mollusks, bivalves and 9·4 million tonnes of crustaceans (FAO 2020). Seafood is an organism living in the sea that can be used as food for humans. Seafood consists of various kinds of fish and shellfish varieties (Thakur et al. 2019). Shellfish plays an important role in global seafood production (Venugopal and Gopakumar 2017). Shellfish-related illnesses have been reported all over the world which shows an increasing trend with time (Potasman et al. 2002). Aeromonas has been identified as pathogenic bacteria which can be a cause for seafood-related outbreaks (WHO 2008). Aeromonas is a Gram-negative, non-spore-forming rod-shaped and facultative anaerobic bacteria easily found in aquatic environments (WHO 2011). It belongs to family Aeromonadaceae which is abundant in aquatic environments. It causes various human infections, such as gastroenteritis, soft tissue infection, septicemia, hepatobiliary tract infections and occasionally pleuropulmonary infections, indwelling device-related infections, meningitis, peritonitis and haemolytic uremic syndrome (Chuang et al. 2011). The genus Aeromonas constitutes psychrophiles and mesophiles which causes diseases in warm and cold-blooded animals (Lakshmanaperumalsamy et al. 2015). The mesophile group has optimal growth at 35–37°C and is involved in a variety of human infections. Psychrophile group can be easily grown in 22–25°C and can result in nonmotile diseases in fish (Janda and Abbott 2010). Molluscan shellfish have been identified as a vector for marine biotoxins and infectious agents over the ages. Raw and partially under-cooked shellfish can easily be subjected to these kinds of infections. A high number of pathogenic bacteria in the gut of clams, mussels and oysters has been identified (Hoel et al. 2019). Improper packaging, shipment and stocking shellfish in contaminated containers increase the chance of contamination (Colakoglu et al. 2006). Virus, bacteria and toxin-producing dinoflagellates are abundant in shellfish, especially in filter-feeding organisms. Due to the increased concentration of pathogenic organisms in filter-feeders, filter-feeding shellfish are often implicated as sources of food-borne infections over the non-filter feeders (Rippey 1994). Pathogenic micro-organisms such as Vibrio, Aeromonas and Plesiomonas are natural contaminants that can be easily found in shellfish. Due to favourable conditions in the shellfish and their environment, bacteria can easily colonize shellfish and it increases the risk of infection (Colakoglu et al. 2006). Aeromonas outbreaks associated with shellfish have been identified over the years in various countries. Aeromonas spp. act as specific spoilage organisms (Dalgaard 2014). A study conducted on oysters cultured around Louisiana reported an Aeromonas foodborne outbreak and Aeromonas hydrophila was implicated as the root cause (Abeyta et al. 1986). A study conducted in France reported Aeromonas salmonicida as a spoilage micro-organism in shrimp aquaculture systems (Penaeus vannamei) (Macé et al. 2014). In Switzerland, an outbreak associated with A. hydrophila was identified in patients who consumed contaminated shrimps (Altwegg et al. 1991). Likewise, Aeromonas spp. related foodborne outbreaks are reported frequently. Hence, the object of this study is to summarize the potential pathogenic activities of Aeromonas spp. isolated from molluscan shellfish. This will help to increase consumer awareness about foodborne Aeromonas outbreaks. Shellfish as a seafood Seafood is considered one of the most nutritious foods. Shellfish are invertebrate animals, possessing two different body types (soft and segmented bodies). They are divided into two varieties, namely mollusks and crustaceans. Mollusks include mussels, oysters, clams, scallops, squid, cuttlefish, octopus abalone, sea snail, cockle and whelks. On the other hand, shrimp, lobster, crayfish, crab and krill are examples of crustaceans (Venugopal and Gopakumar 2017). Shellfish is a rich source of protein, iron, zinc, copper and vitamin B-12. It is composed of low-fat food, especially unsaturated fats, and rich in omega-3 fatty acids. It can be defined as a good food source for a healthy diet (Dong 2010). Scallops, cuttlefish, octopus, hard clams, abalones, top shells, pearl oyster, ark shells, cockles, sea urchins, sea cucumbers, hen clams, baby clams, jellyfish, shrimps, prawns, crabs and other types of shellfish are popularly consumed (Brief et al. 2018). These seafood are eaten in cooked, precooked, fresh, chilled and frozen forms (Brief et al. 2019). Taxonomy of Aeromonas spp. Aeromonas strain was first identified in 1891. It was classified under genus Vibrionaceae in 1965 by the International Committee of Systematic Bacteriology. In 1986, it was categorized under a new family, namely Aeromonadacae based on the results of 16S rRNA and 5S rRNA gene sequence analysis and DNA hybridization studies (Fernández-Bravo and Figueras 2020). The strains which were classified under the family of Aeromonadacae have optimal growth at 22–28°C. It was recorded as facultatively anaerobic Gram-negative bacteria which can mobilize with the help of polar flagellation (Tomás, 2012). Under the family of Aeromonadacae, a total of 36 species were identified as follows Aeromonas allosaccharophila, A. dhakensis, A. jandaei, A. salmonicida, A. aquatica, A. diversa, A. media, A. sanarellii, A. aquatilis, A. encheleia, A. molluscorum, A. schubertii, A. australiensis, A. enterica, A. lacus, A. simiae, A. bestiarum, A. eucrenophila, A. lusitana, A. sobria, A. bivalvium, A. finlandiensis, A. piscicola, A. taiwanensis, A. cavernicola, A. fluvialis, A. popoffii, A. tecta, A. caviae, A. hydrophila, A. rivipollensis, A. trota, A. crassostreae, A. intestinalis, A. rivuli and A. veronii (Fernández-Bravo and Figueras 2020). In the mid-1970s, aeromonads were classified into two groups, including mesophilic and psychrophilic groups, depending on their growth characteristics and biochemical constitutes. Later, it was divided into three different groups, including A. hydrophila, A. caviae and A. sobria based on their different biochemical features (Janda and Abbott 2010). Bacterial population of Aeromonas spp. in fish and shellfish Aeromonas spp. can be widely distributed in both finfish and shellfish. Table 1 shows some examples of the prevalence of Aeromonas spp. within fish and shellfish varieties. It seems both fish varieties have different kinds of Aeromonas spp. Mostly A. hydrophila, A. salmonicida and A. caviae has been reported frequently in fish studies. Table 1. Prevalence of Aeromonas spp. among finfish and shellfish Aeromonas spp. found in finfish Aeromonas spp. found in shellfish Source Bacterial species Reference Source Bacterial species Reference Pintado fish (pseudoplatystoma spp.) A. caviae, A. hydrophila, A. sobria Rall et al. (1998) Crab (Portunus pelagicus), shrimp (Metapenaeus stebbingi) A. hydrophila, A. sobria, A.caviae Farag (2006) Common carp (Cyprinus carpio), goldfish (Carassius auratus), tilapia (Oreochromis niloticus), ayu (Plecoglossus altivelis), channel catfish (Ictalurus punctatus) A. caviae, A. hydrophila, A. jandaei, A. sobria, A. veroni Sugita et al. (1994) Oysters (Crassostrea rhizophorea) A. caviae, A. eucrenophila, A. media, A. sobria, A. trota, A. veronii Evangelista-barreto et al. (2006) Common carp (Cyprinus carpio), crucian carp (Carassius carassius), gray mullet (Mugil cephalus) A. caviae, A. hydrophila, A. jandaei, A. sobria, A. veroni Sugita et al. (1995) Crab, shrimp, mussel, scallop, lobster A. caviae, A. hydrophila, A. veronii biovar sobria, A. encheleia Chang et al. (2008) Baltic herring (Clupeo harengus mambrus), rain- bow trout (Salmo guirdneri), vendace (Coregonus ulbulu), zander (Luciopercu spp.), perch (Perca juviutilis), bream (Abramis brama), roach (Rut&s rutilus), ide (Leu- ciscus idis), flounder (Platichthys Jeesus), salmon (Salmo salaris) A. hydrophila, A. cuviue, A. sobria Hänninen et al. (1997) Squid, prawn, mussel A.punctata, A.caviae, A.enteropelogenes, A.hydrophila, A.aquarorium Joseph et al. (2013) Common carp (Cyprinus carpio), anchovy (Engraulis encrasicholus), true sardine (Sardina pilchardus) A. caviae, A. hydrophila, A. veroni bv. sobria Yucel et al. (2004) Shrimp (Metapenaeus monoceros) A. hydrophila, A. sobria, A. veronii Fadel and El-Lamie (2019) Tilapia (Tilapia nilotica), catfish (Clarias batrachus) A. caviae, A. sobria, A. hydrophila Ashiru et al. (2011) Wedge-shells (Donax trunculus), mussels (Mytilus sp.), cockles (Cardium sp.), wedge-shells (Donax trunculus) and razor-shells (Ensis sp.) A. molluscorum Miñana-Galbis et al. (2004) Nile tilapia (Oreochromis niloticus), African catfish (Clarias gariepinus) A. hydrophila, A. sobria Wamala et al. (2018) Cockles (Cardium sp.) and razor shells (Ensis sp.) A. bivalvium Miñana-Galbis et al. (2007) According to the studies done on shellfish A. caviae, A. trota, A. sobria and A. eucrenophila, A. salmonicida, A. media, A. caviae, A. allosaccharophila, A. veronii and A. hydrophila species were identified as major species (Evangelista-Barreto et al. 2006; Latif-Eugenín et al. 2016). Fernández-Bravo and Figueras (2020) reported that diversity of Aeromonas spp. isolated from shellfish is similar to the diversity in fish varieties. Also, the ability of A. hydrophila to survive at −72°C in oysters for 18 months indicates that these organisms can withstand stress conditions (Abeyta et al. 1986). A high proportion of Aeromonas can be seen in microflora of shrimps (Leroi and Joffraud 2011). Table 1 shows the distribution of Aeromonas spp. among finfish and shellfish. In case of shellfish, the filter-feeding process shows a relationship with the bacterial population, as it can clear the water. At the same time, it can increase the bacterial population in shellfish. Some studies reported that it could perform either reduction or accumulation of pathogens (Burge et al. 2016). According to the findings of Ma et al. (2017), filter-feeding process of oysters leads to the accumulation of A. salmonicida in the gut. This proved that filter-feeding organisms have the capability of accumulating micro-organisms at high concentrations. However, studies show that the bacterial population of fish can depend on the surrounding aqueous medium, a significant amount of microflora present in skin, mucus, gills and gastrointestinal tract of the fish. Aeromonas can be detected as Gram-negative psychrotolerant bacteria around 0–25°C temperatures. On the other hand, the bacteria flora of shrimp-like shellfish depends on the geographic location and environment, temperature and salinity of the water-like parameters. Aeromonas spp. related infections There are several Aeromonas spp. (A. hydrophila, A. caviae and A. veronii biotype sobria) which are identified as human pathogens. They can be a root cause of gastrointestinal and extraintestinal infectious diseases in humans, such as cellulitis, wound infections, septicaemia, urinary tract infections and hepatobiliary infection (Vila et al. 2003). Consumption of raw shellfish has been identified as a major reason for bacterial infections in human. Also, immunocompromised patients can be considered as a high-risk group for shellfish-related bacterial infections in previous studies (Wittman and Flick 1995). Another risk group of people is patients who have diabetes mellitus, low levels of gastric acid and those who take medicines to reduce stomach acidity (Wittman and Flick 1995). In 2001 A. hydrophila, A. caviae and A. veronii biotype sobria were identified as potential sources for acute diarrhoea among children in Iran (Soltan Dallal and MoezArdalan 2004). Aeromonas veronii biotype sobria and A. caviae were found to be etiological agents of traveler’s diarrhoea (Vila et al. 2003). Aeromonas spp. has been identified as a foodborne pathogen especially in shellfish with several cases (Aberoum and Jooyandeh 2010). Aeromonas hydrophila Aeromonas hydrophila can cause aeromonad septicaemia (Xu et al. 2012). Aeromonas hydrophila is mesophilic and psychrotrophic. Psychotropic A. hydrophila strains are the most abundant type of pathogens (Lampila and State 2012). Aeromonas hydrophila is the most frequently reported Aeromonas spp. in foodborne Aeromonas outbreaks and abundantly found in fish, amphibians, reptiles, snails, cows and humans (Hazen et al. 1978). Diarrhoea was the most reported disease caused by A. hydrophila (Awaad et al. 2011). Aeromonas hydrophila was the main reason for the outbreak that occurred among the people who consumed oysters in Louisiana (Abeyta et al. 1986). Also, it has been identified as a pathogenic bacterial species which causes diseases in fish and crabs farms in China (Nielsen et al. 2001). Aeromonas caviae Aeromonas caviae is a strain that can be easily found in fish, marine water, beef, chicken and human faecal samples (Hannineni and Siitonen 1995). It has been frequently reported as a pathogen for intestinal infections in humans. Also, this species can cause infections in skin and soft tissues that are exposed to contaminated aquatic environments. Infections can be severe in immunocompromised patients (Evangelista-Barreto et al. 2006). Aeromonas caviae is the most frequently reported pathogenic bacteria in the traveler’s diarrhoea (Vila et al. 2003). Aeromonas veronii biotype sobria Aeromonas veronii biotype sobria strain has frequently been reported in human infections and clinical isolates (Janda 1991). Also, this mesophilic bacteria can be found in fish, amphibians and reptiles (Gauthier et al. 2017). It was also reported as pathogenic bacteria in traveller’s diarrhoea (Vila et al. 2003). Furthermore, it was reported as pathogenic bacteria in Korean fish farms that may cause secondary infections on ulcers and, may cause haemorrhagic erosion (Yu et al. 2015). Virulence factors of Aeromonas spp Virulence factors in bacteria are important in the invasion, replication and evading the host’s immune system. These factors play key roles in pathogenesis leading to damage host tissues, affect the host defense system, and even kill the organism (Vilches et al. 2004). Aeromonas strains have a wide range of virulence factors including adherence properties, lipopolysaccharides, siderophores, toxins, colonization, extracellular secretions, iron acquisition and quorum sensing mechanisms. Most of these virulence properties were reported in finfish (Guerra et al. 2007; Beaz-Hidalgo and Figueras 2013), but studies related to shellfish are limited. Several studies have proved the prevalence of virulence-related genes in Aeromonas spp. isolated from seafood. In Germany, cytotoxin producing aer and hlyA genes were screened in seafood-borne bacteria. Both genes were reported in A. hydrophila and A. sobria isolates (Ullmann et al. 2005). Also, A.hydrophila and A. caviae strains were positive for several virulence genes like hly, act, ast and aer (Tawab et al. 2017). Furthermore, shellfish that harboured A. salmonicida and A. media strains were positive for the hlyA, aerA, act and alt genes (Lee et al. 2020). Similarly, A. hydrophila, A. veronii, A. caviae, A. enteropelogenes and A. media strains isolated from shrimps were positive for the act, alt, ast, aerA, lip, ahyB, ser, fla, gcat and ascV genes that produce toxins and extracellular enzymes (De Silva et al. 2018a). Similarly, Aeromonas spp. were isolated from a well-known shellfish variety called yesso scallop (Patinopecten yessoensis). In that study act, alt, ast, aerA, lip, ahyB, ser, hlyA, fla, gcat and ascV genes were reported in A.hydrophila, A. salmonicida, A. media, A. caviae, A. veroni and A. enteropelogenes isolates (De Silva et al. 2018c). Also, act, alt, ast, aerA, lip, ahyB, ser, fla, gcat, ascV and hlyA genes have been reported in A. hydrophila, A. veroni, A. media, A. salmonicida, A. allosaccharophila, A. besriarum, A. culicicola, A. enteropelogenes, A. rivipollensis and A. meida strains isolated from cockles (Tegillarca granosa) (Dahanayake et al. 2020). Hard shell mussels (Mytilus coruscus) have been screened for Aeromonas related virulence markers and A. salmonicida, A. veroni, A. enteropelogenes, A. caviae, A. allosaccarophila and A. bivalvium strains were isolated and fla, aer, hlyA, ahyB, gcaT, ser, lip, alt, ast and act genes were detected in high frequencies (Hossain et al. 2020). Extracellular enzymes in shellfish The secretion of the extracellular enzymes is a significant feature in the process of pathogenicity of Aeromonas spp. Proteases, lipases, amylases, chitinases, nucleases and gelatinases are the most abundant extracellular enzymes (Tomás, 2012). These various types of enzymes are related to significant distribution and adaptability following environmental changes in Aeromonas spp. (Pemberton et al. 1997). Similar studies with virulence markers have evidenced the occurrence of extracellular enzymes in Aeromonas isolates. Aeromonas hydrophila, A. veronii, A. caviae and A. enteropelogenes strains isolated from Pacific white leg shrimps (Litopenaeus vannamei) exhibited high prevalence of DNase, protease, gelatinase and lipase traits over each sample (De Silva et al. 2018a). Similarly, A. hydrophila isolated from shrimp samples in India demonstrated high percentages of haemolysis, slime and protease activities. In addition, this study revealed extracellular product in both fish and shellfish (Illanchezian et al. 2010). Another study conducted by Yano et al. (2015) reported a prevalence of extracellular activities in A. veronii, A. aquariorum, A. caviae, A. jandaei and A. schubertii isolates from shrimp samples. Lipase, DNase and gelatinase activities were reported in high percentages. Yesso scallop (P. yessoensis) borne A. hydrophila, A. salmonicida, A. meida, A. caviae, A. veroni and A. enteropelogenes were reported as extracellular enzyme-producing strains that possess DNase, gelatinase, caseinase, β-hemolysis and lipase activities. Most of the activities were reported in more than 50% of the isolates (De Silva et al. 2018c). Hossain et al. (2020) also reported a high prevalence of DNase, gelatinase, caseinase, β-hemolysis and lipase activities in A. salmonicida, A. veroni, A. enteropelogenes, A. caviae, A.allosaccarophila and A. bivalvium strains isolated from hard shell mussels (Mytilus coruscus). Biofilm formation Biofilm formation is a major virulence factor in pathogenic bacteria and composed mainly of proteins, polysaccharides and DNA (Singh et al. 2017). This structure helps the antimicrobial resistance among Aeromonas spp. (Dias et al. 2018). Chenia and Duma (2017) reported that biofilm formation could be affected by available nutrients rather than temperature changes. Biofilm formation was reported in A. salmonicida recovered from finfish in United Kingdom (Desbois et al. 2020). Aeromonas veroni isolates were recovered from shrimp in China (Liu et al. 2020) and A. salmonicida, A. veroni, A. enteropelogenes, A. caviae and A.allosaccarophila were isolated from hard shell mussels (Hossain et al. 2020). Cytotoxicity Aeromonas spp. can produce wide range of exotoxins. There are two types of enterotoxines which are identified as cytotoxic and cytotonic. Cytotoxicity has been reported in 98·07 and 100% of A. hydrophila isolated from fish and shrimp samples, respectively (Illanchezian et al. 2010). However, cytotoxin production was more frequent in A. hydrophila isolated from oysters (93·3%), followed by shrimp (91·7%) and fish (87·5%) (Tsai and Chen 1996). Cytotoxicity enterotoxins have been identified in Aeromonas spp. previously isolated from fish (A. hydrophila, A. salmonicida, A. sobria, A. veronii, A. bestiarum and A. eucrenophila) and A. bivalvium and A. molluscorum were isolated from shellfish (Jung-Schroers et al. 2019). Usage of antimicrobial agents in aquaculture The aquaculture industry faces the challenge of the emergence and spread of diseases. Various compounds have been used to control these diseases amongst which antimicrobial agents are the most popular (Ibrahim et al. 2020). Antimicrobials are used at various stages in the aquaculture industry by using different methods such as feed medication, bioencapsulation, immersion baths, dipping and flushing (Smith et al. 2008). The study done by Lulijwa et al. (2020) reported that a total of 67 antimicrobial compounds had been used all over the world as prophylactic or therapeutics to defeat microbial infections in aquaculture. Tetracyclines, sulphonamides and quinolones are the main groups used as antimicrobial compounds and other compounds such as florfenicol and amoxicillin are also used. Of these, oxytetracycline, sulphadiazine and florfenicol have been identified as the most frequently used antimicrobials. Even though antimicrobials are important to reduce the spread of bacterial diseases, the continuous release of antimicrobials to the environment leads to the rise of antimicrobial resistance among fish and shellfish (Navarrete et al. 2008). In Thailand, around 600 metric tons were applied in shrimp farms and resulted in antimicrobial resistance (Defoirdt et al. 2011). Also, these antimicrobial agents are stable and non-degradable and remain in fish and shellfish which are later consumed by humans (Santos and Ramos 2016). Other agents such as florfenicol and amoxicillin are also used. These antimicrobials are used as prophylactic and therapeutic agents (Rodgers and Furones 2009). Antimicrobial contamination in the environment is one of the major reasons for the prevalence of antimicrobial resistance in microbes. Over usage of antimicrobials in aquaculture, hospital waste, industrial waste, agricultural waste and livestock animal waste were reported as possible ways for the accumulation of antimicrobial agents in water environments (Kraemer et al. 2019; Felis et al. 2020). Antimicrobial resistance properties of Aeromonas spp. in shellfish Over the past decades, the seafood industry has been affected by various bacterial diseases. This resulted in increasing the usage of antimicrobial agents which led to the development of antimicrobial resistance in the aquaculture industry. (Stratev and Odeyemi 2016). A previous study reported that Aeromonas pathogens are resistant to sulfonamide, trimethoprim, tetracycline, tetracycline, sulfonamide, trimethoprim and chloramphenicol (Patil et al. 2016). Also, antimicrobial resistance properties in Aeromonas spp. isolated from finfish and shellfish were reported in a previous study. Aeromonas hydrophila isolated from finfish and prawns were resistant to multiple antimicrobials including bacitracin, erythromycin, gentamicin, kanamycin, methicillin, nalidixic acid, neomycin, novobiocin, polymyxin-B, rifampicin, streptomycin, tetracycline, trimethoprim and vancomycin (Thayumanavan et al. 2003). Multidrug-resistant Aeromonas spp. isolated from Pacific white leg shrimps (Litopenaeus vannamei) were resistant to ampicillin, clindamycin, nalidixic, tetracycline, cephalothin, erythromycin and trimethoprim-sulfamethoxazole (De Silva et al. 2018a). However, A. hydrophila, A. enteropelogenes, A. veroni, A. salmonicida and A. sobria from pacific abalone have been reported as ampicillin, cephalothin, rifampicin, oxytetracycline, colistine sulphate, nalidixic acid and piperacillin resistant strains by Wickramanayake et al. (2019). Dahanayake et al. (2019) reported the presence of A. hydrophila, A. salmonicida, A. media, A. veroni, A. allosaccarophila and A. caviae in marketed Manila clam (Ruditapes philippinarum). Also, the isolated Aeromonas spp. isolates were reported as ampicillin, cephalothin, rifampicin, oxytetracycline, colistin sulphate, nalidixic acid and piperacillin resistant strains and most were multidrug-resistant. In contrast, yesso scallop (P. yessoensis) isolated Aeromonas isolates were reported as ampicillin, colistin, vancomycin, cephalothin, pipercillin, clindamycin, erythromycin, nalidixic acid, imipenen, meropenem, trimethoprim-sulfamethoxazole and rifampicin resistant (De Silva et al. 2018c). However, Aeromonas spp. isolated from marketed yesso scallop (P. yessoensis) were also screened for antimicrobial resistance genes by PCR. Antimicrobial resistance genes were more prevalent in A. veronii (aac(6ʹ)-Ib, tetE, qnrS and IntI1), A. salmonicida (IntI1, blaCTX, aac(3ʹ)-Ib, aac(6ʹ)-Ib, qnrS), A. hydrophila (IntI1, blaTEM, qnrS, aac(6’)-Ib and strA-strB) (De Silva et al. 2020). In another study, A. hydrophila, A. veronii, A. media, A. salmonicida, A. allosaccharophila, A. bestiarum, A. culicicola, A. enteropelogenes and A. rivipollensis isolated from marketed cockles (T. granosa) showed varied resistance to ampicillin, imipenem, rifampicin, cephalothin, piperacillin and colistin sulphate. Also, the antimicrobial resistance was confirmed by the presence of several antimicrobial resistance genes including blaSHV, blaCTX, tetE, aac(6ʹ)-Ib, strAstrB, qnrS and IntI1 (Dahanayake et al. 2020). Another study conducted by Hossain et al. (2020) on Aeromonas spp. isolated from hard-shelled mussel has confirmed cephalothin, nalidixic acid, oxytetracycline, rifampicin, trimethoprim/sulfamethoxazole resistance. Antimicrobial resistance genes are the markers for antimicrobial resistance in pathogenic bacteria. β-Lactam resistance genes are one of the most frequently identified antimicrobial resistance genes. They encode β-lactamase enzymes which hydrolyse the peptide bonds of the antimicrobials rendering them inactive, preventing bacterial peptidoglycan damage from the activity of antimicrobials (Tayler et al. 2010). The presence of tetracycline resistance genes is related to three different mechanisms that cause resistance called efflux pump, ribosomal protection and enzymic inactivation (Balassiano et al. 2007). Plasmid-mediated quinolone–resistant (PMQR) genes are the most important antimicrobial resistance gene groups. Particularly they