Insight into characteristics and pathogenicity of five rapidly growing non-tuberculous Mycobacterium species isolated from the Siamese fighting fish, Betta splendens

Abstract

Siamese fighting fish (Betta splendens) play an important role in the global aquarium trade, but their susceptibility to mycobacteriosis could challenge the long-term sustainability of the industry. Thus, this study aimed to characterize rapidly growing non-tuberculous mycobacteria (RGM) isolated from the fish and to investigate their pathogenicity. Five RGM species were investigated, namely Mycobacterium chelonae, M. cosmeticum, M. farcinogenes, M. mucogenicum, and M. senegalense. The isolates were phenotypically and biochemically characterized and assessed for susceptibility to 18 antibiotics and five disinfectants. The pathogenicity of the isolated bacteria was evaluated through experimental infection of the fish by intraperitoneal injection. The results of phenotypic and biochemical typing allowed the identification of some of the isolates and provided preliminary information to distinguish between the bacteria. All isolates were resistant to at least four antibiotics, with multiple antibiotic resistance indexes ranging from 0.22 to 0.61, with M. chelonae having the highest value. Four disinfectants (i.e., ethanol, formalin, chlorine, and povidone‑iodine) showed strong antibacterial activity, whereas potassium permanganate proved less effective. An LD50 value within 7 days showed that M. chelonae had the highest virulence at 7.12 × 105 CFU/fish, followed by M. mucogenicum at 4.25 × 106 CFU/fish and a range of 1.01–1.65 × 107 CFU/fish for the other isolates. Depending on the dose and the isolate administered, some fish displayed acute disease symptoms, while others developed a chronic condition. The acute disease was characterized by short median survival, severe peritonitis, and tissue necrosis. Fish with a chronic infection survived the 42-day trial but were emaciated and had systemic granulomas within their viscera. The findings of this study have not only improved our understanding of the nature of these RGM species but also promoted the development of control strategies to mitigate the negative impact of mycobacteriosis on the Siamese fighting fish industry.