Experimental co‐infection by Aeromonas hydrophila and Aeromonas jandaei in pirarucu Arapaima gigas (Pisces: Arapaimidae)

In fish, bacteriosis has been widely linked with Aeromonas species, but co-infection by these bacteria has been little addressed. The aims of this study were to report on an outbreak of disease in pirarucu Arapaima gigas caused by Aeromonas and to investigate experimental co-infection and characterize resistance profile, virulence factors and phenotypic and molecular differentiation. Fish samples with clinical signs of bacteriosis were collected and used to study experimental co-infection. The bacterial isolates were characterized phenotypically as Aeromonas hydrophila and Aeromonas jandaei. Virulence genes aerA, gcat, lip, dnase and hlyA were detected using the polymerase chain reaction, while the alt, act and ser genes were not found. Resistance to imipenem and ceftriaxone was observed; however, all isolates were susceptible to most of the antibiotics assayed. Phenotypic tests to determine the presence of metallo-β-lactamases showed positivity only for A. jandaei strains. Assays for the resistance genes kpc, ndm, imp, oxa-48 and vim showed negative results. The co-infection and pathogenicity of A. hydrophila in association with A. jandaei in A. gigas, established in accordance with Koch's postulate, provided experimental support for the existence of synergism between these bacteria. This has several implications relating to occurrences of this co-infection and determinants of virulence.