Potential opportunistic role of Kosakonia sacchari in common carp (Cyprinus carpio) from unmanaged village ponds in Hisar

Abstract

Intensification of freshwater aquaculture increases the risk of opportunistic bacterial infections, particularly in small, unmanaged village ponds where water quality often deviates from optimal ranges. This study reports the isolation and characterization of Kosakonia sacchari from diseased common carp (Cyprinus carpio) in an unmanaged pond in Hisar, India the first such record from this region. Clinically affected fish exhibited signs of systemic infection, including lethargy, pallid gills, haemorrhagic skin lesions, and intestinal congestion. Pond water analysis indicated stressful conditions (temperature ≈ 33 °C; pH 8.5; dissolved oxygen 4.1 mg L⁻¹; total ammonia 0.7 mg L⁻¹). The intestinal isolate was a Gram-negative rod producing a biochemical profile (catalase ⁺, oxidase ⁻, MR ⁻/VP ⁺, lysine decarboxylase ⁺, citrate ⁺, indole ⁻, urease ⁻) characteristic of the genus Kosakonia. Physiological assays showed mesophilic growth (optimum 27-32 °C), preference for neutral to mildly alkaline media (pH 7.0-9.0), and moderate halotolerance (growth up to 3% NaCl). Partial 16S rRNA gene sequencing (GenBank accession no. PP990623) showed 100% similarity to K. sacchari reference sequences, and phylogenetic analysis clustered the isolate within the K. sacchari clade with strong bootstrap support (>83%). Collectively, these findings confirm the occurrence of K. sacchari in diseased carp from Indian freshwater systems and suggest a potential opportunistic role under environmental stress. Further experimental infection trials and whole-genome sequencing are recommended to establish pathogenicity and elucidate virulence or adaptation determinants.