Background and Aim: The emergence of antibiotic-resistant bacterial pathogens has been increasingly reported, which has resulted in a decreasing ability to treat bacterial infections. Therefore, this study investigated the presence of Aeromonas spp., including its antibiotic resistance in various fish samples, Oreochromis spp., Clarias gariepinus, and Pangasius hypophthalmus, obtained from Kelantan and Terengganu, Malaysia. Materials and Methods: In this study, 221 fish samples, of which 108 (Oreochromis spp., n=38; C. gariepinus, n=35; and P. hypophthalmus, n=35) were from Kelantan and 113 (Oreochromis spp., n=38; C. gariepinus, n=35; and P. hypophthalmus, n=40) were from Terengganu, were caught using cast nets. Then, samples from their kidneys were cultured on a Rimler Shott agar to isolate Aeromonas spp. Polymerase chain reaction (PCR) was used to confirm this isolation using specific gene primers for species identification. Subsequently, the isolates were tested for their sensitivity to 14 antibiotics using the Kirby–Bauer method, after which the PCR was conducted again to detect resistance genes: sul1, strA-strB, aadA, blaTEM, blaSHV, tetA-tetE, and tetM. Results: From the results, 61 isolates were identified as being from the genus Aeromonas using PCR, of which 28 were Aeromonas jandaei, 19 were Aeromonas veronii, seven were Aeromonas hydrophila, and seven were Aeromonas sobria. Moreover, 8, 12, and 8 of A. jandaei; 4, 3, and 12 of A. veronii; 6, 0, and 1 of A. hydrophila; and 3, 3, and 1 of A. sobria were obtained from Oreochromis spp., C. gariepinus, and P. hypophthalmus, respectively. In addition, the isolates showed the highest level of resistance to ampicillin (100%), followed by streptomycin (59.0%), each kanamycin and nalidixic acid (41.0%), neomycin (36.1%), tetracycline (19.7%), sulfamethoxazole (14.8%), and oxytetracycline (13.1%). Resistance to gentamicin and ciprofloxacin both had the same percentage (9.8%), whereas isolates showed the lowest resistance to norfloxacin (8.2%) and doxycycline (1.6%). Notably, all Aeromonas isolates were susceptible to chloramphenicol and nitrofurantoin. Results also revealed that the multiple antibiotic resistances index of the isolates ranged from 0.07 to 0.64, suggesting that the farmed fish in these areas were introduced to the logged antibiotics indiscriminately and constantly during their cultivation stages. Results also revealed that the sul1 gene was detected in 19.7% of the Aeromonas isolates, whereas the tetracycline resistance genes, tetA and tetE, were detected in 27.9% and 4.9% of the isolates, respectively. However, β-lactam resistance genes, blaTEM and blaSHV, were found in 44.3% and 13.1% of Aeromonas isolates, respectively, whereas strA-strB and aadA genes were found in 3.3% and 13.1% of the isolates, respectively. Conclusion: This study, therefore, calls for continuous surveillance of antibiotic-resistant Aeromonas spp. in cultured freshwater fish to aid disease management and better understand their implications to public health.
Aeromonas infections are becoming a serious risk issue in commercial aquaculture, and a wide range of fish and shellfish species has been documented as being vulnerable. Five isolates of Aeromonas hydrophila were identified from African catfish (Clarias gariepinus) cultured in Selangor, West Malaysia in this study. A conventional rapid identification approach (API 20E strip) was used for preliminary identification based on the biochemical properties of the isolated bacteria. Polymerase chain reaction (PCR) with the specific primer 16 rDNA, on the other hand, was used as an accurate and confirmed identification. A pathogenicity test via intramuscular (IM) injection was used to investigate the virulence of A. hydrophila. With a high degree of similarity (98%) to the NCBI or Genbank databases, the isolates were identified as A. hydrophila. The LD50 was calculated using pathogenicity test findings and was found to be 2.1 ´ 106.33 CFU mL−1, while 1 ´ 108 CFU mL−1 in the experimentally injected fish, resulted in 100% mortality. Several organs, including the kidney, liver, and spleen, showed histopathological abnormalities. Those changes mainly include increase in the presence of hemosiderin deposits, congested portal vessels, vacuolated hepatocytes, generalised loss of tubular cells, and oedematous degeneration in the infected organs.
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