Isolation, Characterisation and Virulence of Aeromonas spp. From Peruvian Rainbow Trout: First Description of A. popoffii-Associated Mortality

Aquaculture is one of the most important food sectors, providing nutrition and employment throughout the world. However, production is under threat due to the occurrence of pathogens. Aeromonas spp. are bacterial agents that affect the aquaculture industry, including rainbow trout (Oncorhynchus mykiss) farms, causing severe economic losses. The objective of this study was to characterise several isolates of Aeromonas spp. obtained from rainbow trout in Junín, Peru. Molecular analysis revealed the presence of four species: A. sobria (n = 12), A. salmonicida (n = 3), A. popoffii (n = 2), and A. media (n = 1). Biochemical characterisation revealed that several isolates tolerated 3% NaCl, with only A. salmonicida surviving in 4% NaCl. Virulence factor analysis demonstrated increased virulence at higher temperatures for A. sobria, while A. popoffii exhibited virulence in colder conditions. A. media displayed limited virulence properties, while A. salmonicida was consistently virulent across all temperatures. Aeromonas salmonicida isolates exhibited intense and prolonged motility—a behaviour rarely associated with this species—that persisted across a broad temperature range from 14°C to 37°C. The virulence genes aer, fla, ahyB, lip, act, ser, hly and altA were present in all A. salmonicida isolates, with both A. popoffii isolates lacking only the fla gene. Experimental infection resulted in mortality of up to 100% for A. salmonicida (7.2 × 10⁶ and 7.2 × 10⁷ CFU/fish), 40% for A. sobria (4 × 10⁷ CFU/fish), and 20% for A. popoffii (4 × 10⁷ CFU/fish). No direct relation was found between specific gene presence or combinations thereof and mortality. Our findings highlight the need to develop effective management strategies, including the development of a multivalent vaccine to enhance Peruvian trout aquaculture sustainability.