Evaluation of Curcuma longa Aqueous Extract for Bactericidal Activity Toward Enterobacter ludwigii Isolated from Diseased Catla catla

Disease outbreaks are a significant concern for sustainable freshwater aquaculture, especially in unmanaged pond systems. In this study, we investigated a pathogen associated with mortality in Catla catla from an unmanaged village pond in Ladwa, India. The pond entirely depended on natural productivity, with no supplemental feeding. Affected fish showed signs of illness, including reduced appetite, lethargy, and severe gill hemorrhages. Water quality analysis indicated stressful conditions, with elevated pH (9.2 ± 0.2), low dissolved oxygen (4.9 ± 0.3 mg/L), and high total ammonia (0.5 ± 0.2 mg/L). A dominant Gram-negative bacterium was isolated from gill lesions and identified as Enterobacter ludwigii (strain HAUAAHM_EL1; GenBank PP989393) using both biochemical tests and 16S rRNA sequencing. Considering the need for sustainable disease management, the antibacterial potential of an aqueous extract of Curcuma longa (turmeric) was tested in vitro. Broth microdilution assays showed concentration-dependent inhibition of the pathogen, with a minimum inhibitory concentration (MIC) of 2.5 mg/mL and a minimum bactericidal concentration (MBC) of 10 mg/mL. An MBC/MIC ratio of 4 confirmed the extract's bactericidal effect. These results confirm that E. ludwigii can act as an opportunistic pathogen in unmanaged aquaculture ponds and suggest that turmeric extract, which is easily available and eco-friendly, may serve as a potential antibacterial agent. Further research is required to evaluate the in vivo effectiveness of the extract, standardize its phytochemical composition, and examine the antimicrobial resistance profile of the isolate before recommending its practical application in aquaculture systems.