Pathogenic Aeromonas spp in Amazonian fish: Virulence genes and susceptibility in Piaractus brachypomus, the main native aquaculture species in Peru

The aim of this study was to isolate and identify Aeromonas species, characterize their virulence genes factors, and to evaluate their lethality in Piaractus brachypomus. Presumptive Aeromonas (n = 34) were selected from 44 bacteria retrieved from diseased Amazonian fish. Nineteen isolates were chosen based on fish species, group, the location where were isolated, and colony morphology. Isolates were biochemically and molecularly identified as Aeromonas hydrophila, A. dhakensis, A. veronii, A. jandaei, and A. caviae using API 20E, API 20NE and the analysis of 16 S rRNA and gyrB genes, respectively. Eight virulences genes (aer, altA, act, hly, ser, lip, fla ahyB) were found in one A. dhakensis and two A. hydrophila isolates. A. veronii, A. jandaei, and another two A. hydrophila isolates harbored four, two and three virulence genes, respectively. All A. caviae were fla +, ahyB + and lip +, except two isolates that also presented ser and altA. P. brachypomus (9 ± 1 cm), infected intraperitoneally (4 ×10⁶, 4 ×10⁷, and 4 ×10⁸ CFU/fish), showed susceptibility to all Aeromonas species, each harboring a different virulence genes profile. Both A. hydrophila and A. dhakensis caused > 80% mortality at the lowest evaluated concentration (∼10⁶ CFU/fish). A. veronii and A. jandaei isolates produced 20% and 70% mortality, respectively, using ∼10⁷ CFU/fish, but with lower intensity compared to A. hydrophila and A. dhakensis. At the highest concentration (∼10⁸ CFU/fish), A. veronii caused 70% mortality, while A. caviae isolates caused mortality ranging from 50% to 80%. Remarkably, A. jandei resulted in 100% mortality at this level, similar to A. hydrophila and A. dhakensis. This report confirms the variety of Aeromonas species infecting farmed native Amazonian fish. The study also suggests that virulence genes evaluated in Aeromonas species may not always be an indicator of virulence. Moreover, P. brachypomus, an economically important aquaculture fish species could be susceptible not only to the high mentioned A. hydrophila but also to other less observed species such as A. dhakensis, A. veronii, A. jandaei, and A. caviae, bacteria that should be taken into account in health surveillance for the implementation of aquaculture disease prevention strategies.