Aeromonas is a rod-shaped Gram-negative bacterial genus that has been frequently reported to cause diseases in farmed fish, resulting in mortality. This has been related to their ability to produce virulence factors driven by genes encoded within their genome that confer them advantages in the pathogenic process. Being necessary to explore the potential of Aeromonas spp to cause mortality in native species of importance in aquaculture in Peru, such as the fish paco Piaractus brachipomus, the present study evaluates the virulence genes of Aeromonas spp isolated from Amazonian fish and lethality in P. brachipomus.
Bacteria (n=17) isolated from diseased Peruvian Amazonian fish were previously biochemically and molecularly identified by API 20E, and the analysis of 16S rRNA and gybB genes, respectively. From DNA extractions of the isolates, the presence of eight virulence genes: aerolysin (aer), cytotonic enterotoxin (altA), cytotoxic enterotoxin (act), hemolysin (hly), serine protease (ser), lipase (lip), flagellin (fla) and elastase (ahyB), were evaluated. PCR amplified products were analyzed by electrophoresis and evaluated for their presence (+), or absence (-) in agarose gels,. For experimental infection, P. brachypomus (9 ± 1 cm), acclimatized and kept in tanks under controlled conditions were infected intraperitoneally with 0.1 µL of bacterial inoculum in 0.85% saline solution, for 2 treatments, equivalent to 106, 107 and 108 CFU/fish. The control group was injected with 0.1 µL of saline. The experiments were carried out at 28 °C, for 20 days.
It was observed that fla and ahyB genes were present in all the isolates evaluated, the lip gene was present in all bacteria except in one A. caviae. A. dhakensis and four A. hydrophila had the highest number of virulence genes and caused high mortality (> 90%) in P. brachypomus from the lowest doses (106 CFU/fish), compared to other bacterial species in this study. A. veronii (with 5 virulence genes +) and one A. caviae isolate (4 genes +) produced mortality (60 and 30%, respectively) from 107 CFU/fish. A. jandaei (3 genes +) and A. caviae (2 genes +) isolates caused mortality only at the 108 CFU/fish dose. The results showed that bacteria harboring more virulence genes were associated with high mortality. An exception was two A. hydrophila isolates that produced high mortality (> 90%) and harbored two virulence genes. Other components involved in A. hydrophila virulence will help to explain this observation.
Given the importance of native fish aquaculture in Peru, this study highlights the ability of species of the genus Aeromonas to harbor several virulence genes, especially A. hydrophila and A. dhakensis, and to cause high mortality in paco. It also highlights the pathogenic potential of A. caviae, A. veronii and A. jandaei, bacterial species that could constitute a threat in the future for Peruvian native Amazonian fish.
Acknowledgment: This work was funded by the National Council of Science and Technology - CONCYTEC, Contract 129-2020 FONDECYT, and PpR - 2022, Meta 03 Desarrollo Tecnológico, IMARPE, Ministerio de la Producción.