Characterization of Pathogenic Pseudomonas alcaligenes Isolated from Koi Carp in China

Pseudomonas alcaligenes infection is rare in aquaculture. In this study, we provide the first report on the characterization of P. alcaligenes from koi (a variant of Common Carp Cyprinus carpio) in China. A gram-negative bacterium was isolated from the diseased koi and was named KCP-516. Morphological and biochemical tests as well as phylogenetic tree analyses derived from 16S ribosomal RNA, gyrase subunit A, and gyrase subunit B gene sequencing all strongly indicated that the isolate KCP-516 was P. alcaligenes. In liquid medium, the optimal growth conditions were 25°C, 2.5% NaCl, and pH 8. The pathogenicity of the isolate was demonstrated in koi, with 7.0 × 10⁴  CFU/g fish weight identified as the dose lethal to 50% of test fish. The results will provide a scientific reference for the diagnosis and treatment of P. alcaligenes infection.