Aeromonas hydrophila is causing substantial economic losses in world aquaculture. This study determined the tolerance limit (LD50-96h) of A. hydrophila in Arapaima gigas, and also investigated the clinical signs after intradermal inoculation. Arapaima gigas fingerlings were inoculated intraperitoneally with 0 (control), 1.0×10(5), 1.0×10(6), 1.0×10(7), 1.0×10(9) and 1.0×10(10)CFU/mL of A. hydrophila for the determination of LD50-96h, which was 1.8×10(8)CFU/mL. In another trial with intradermal inoculation of 1.8×10(8)CFU/mL A. hydrophila, there was a 91.6% of mortality between 8 and 23h, and several clinical signs were found. As follows: depigmentation in the tegument, lesions in the tail and fins, loss of balance, reduction of respiratory movements, hemorrhagic foci, necrotic hemorrhages in the kidney, liver and swim bladder, splenomegaly, ascites in the abdominal cavity and hyperemia, enlargement of the gall bladder, among other clinical signs observed. The results showed that A. gigas has a relative tolerance to A. hydrophila when compared to other Neotropical fish species.

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.

The increasing prevalence of multi-drug resistant (MDR) Escherichia coli in distinct ecological niches, comprising water sources and food-producing animals, such as fish species, has been widely reported. In the present study, quinolone-resistant E. coli isolates from Arapirama gigas, a major fish species in the Brazilian Amazon rivers and fish farms, were characterized regarding their antimicrobial susceptibility, virulence, and genetic diversity. A total of forty (40) specimens of A. gigas, including 20 farmed and 20 wild fish, were included. Thirty-four quinolone-resistant E. coli isolates were phenotypically tested by broth microdilution, while resistance and virulence genes were detected by PCR. Molecular epidemiology and genetic relatedness were analyzed by MLST and PFGE typing. The majority of isolates were classified as MDR and detected harboring blaCTX-M, qnrA and qnrB genes. Enterotoxigenic E. coli pathotype (ETEC) isolates were presented in low prevalence among farmed animals. MLST and PFGE genotyping revealed a wide genetic background, including the detection of internationally spread clones. The obtained data point out A. gigas as a reservoir in Brazilian Amazon aquatic ecosystems and warns of the interference of AMR strains in wildlife and environmental matrices.

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