The prevalence, putative virulence genes and antibiotic resistance profiles of Aeromonas spp. isolated from cultured freshwater fishes in peninsular Malaysia

Aeromonas species are ubiquitous in diverse aquatic habitats. High stocking density and improper farm practices can increase the susceptibility of cultured freshwater fishes to motile Aeromonas septicemia (MAS) outbreak. However, polymorphic phenotypes and genotypes can lead to misidentification of Aeromonas isolates from diseased fish. This study characterized a large collection of Aeromonas spp. that were collected from nine important fish farms in Peninsular Malaysia, and to compare these isolates based on their virulence and antibiotic resistance profiles. A total of 124 isolates were obtained from fish with clinical symptoms of MAS that grew on Aeromonas selective agar, were Gram-negative and were oxidase-positive, indicating these as presumptive Aeromonas spp. Based on a phylogenetic analysis of these isolates using gyrB sequences, along with phenotypic analysis based on a series of biochemical tests, five species of Aeromonas were identified among these isolates. Among these isolates, Aeromonas dhakensis was obtained most frequently (43%), followed by A. veronii (22%), A. hydrophila (20%), A. caviae (8%) and A. jandaei (7%). Moreover, 50% of the isolates were found to harbour at least half of the virulence genes tested such as aer, ahp, alt, hly, lip, fla, ela and/or act, while 2% of the isolates were found to harbour none. The Aeromonas isolates displayed resistance to amikacin (5%), ampicillin (100%), cefotaxime (7%), amoxicillin (65%), gentamicin (2%), chloramphenicol (2%) and tetracycline (15%), but not for levofloxacin (0%). The highest multiple antibiotic resistance (MAR) index of 0.63 was found in 1% of the isolates, while the mean MAR index was determined at 0.25. This study reported the prevalence of Aeromonas in cultured freshwater fishes in Peninsular Malaysia, where A. dhakensis was observed as the predominant species. However, the presence of Aeromonas with multiple virulence genes and antibiotic resistance ability should alarm the freshwater fish industry in this country.

Automated summary

The study characterized a large collection of Aeromonas species from fish farms in Peninsular Malaysia, identifying A. dhakensis as the predominant species. Half of the isolates carried at least half of the virulence genes tested and displayed resistance to multiple antibiotics, highlighting the need for vigilance in the freshwater fish industry.