Retrospective identification and pathogenicity of Aeromonas dhakensis in freshwater fish

Abstract

Motile Aeromonas species, which are pathogenic to freshwater fish, constitute a complex group characterized by closely related phenotypic traits. This phenotypic similarity has historically contributed to frequent misidentification, most notably the incorrect classification of diverse isolates as Aeromonas hydrophila. In this study, we retrospectively identified five isolates, previously presumed to be A. hydrophila, which were collected during disease outbreaks in tilapia (Oreochromis sp.) and striped catfish (Pangasianodon hypophthalmus) dating back to the period 2010–2017. A polyphasic approach, combining phenotypic characterization, sequencing of four housekeeping genes (16S rRNA, rpoB, rpoD, and gyrB), and multilocus sequence analysis (MLSA), revealed that four of the isolates were in fact Aeromonas dhakensis, while only one was confirmed as A. hydrophila. Representatives of both species were selected to assess pathogenicity in juvenile Nile tilapia through intraperitoneal injection at three different bacterial doses (10⁵, 10⁶, and 10⁷ CFU/fish), followed by a 14-day observation period. The results showed dose-dependent mortality, ranging from 10 to 100% for A. dhakensis and 30% to 90% for A. hydrophila. Infected fish exhibited similar clinical signs and histopathological changes regardless of the bacterial species. These findings suggest that A. dhakensis is an overlooked pathogenic species capable of causing disease in tilapia and potentially other freshwater fish, emphasizing the need for continued surveillance and the development of effective control measures.