Pathogenic Providencia rettgeri AR2HAU from sick Heteropneustes fossilis (Singhi catfish)

Over the past decade, aquaculture has become the fastest-growing food production sector worldwide. Among fish diseases, bacterial infections are the most common, affecting both internal and external organs. During the present study, 13 out of 28 bacterial isolates were selected for detailed screening. Based on hemolytic activity, the isolate AR2 ( Providencia rettgeri ) was identified and selected for further experimental investigation. This bacterium was found to be pathogenic to Heteropneustes fossilis (Singhi catfish). In agar diffusion assays, Withania somnifera showed the highest antibacterial effect among tested herbal extracts, while Eisenia fetida extract was the most effective among earthworm samples. Providencia rettgeri is a Gram-negative, opportunistic bacterium known to cause infections under stress or unfavorable conditions. The present study reports the isolation of pathogenic P. rettgeri strain AR2HAU from diseased H. fossilis . Experimental infection of healthy fish with this isolate confirmed its pathogenicity, producing visible symptoms such as skin darkening, fluid accumulation in the body cavity, and redness around the mouth and anal regions. Histopathological analysis of infected intestinal tissue revealed mild infiltration of mononuclear cells—mainly lymphocytes—within the mucosal layer, along with noticeable goblet cell hyperplasia.