Biochemical and Morphological Characterization of Bacteria Isolated from Freshwater in Sudan

The microbial quality of freshwater fish is a crucial indicator of both aquatic environmental health and food safety. The present study aimed to isolate and characterize bacteria from three common freshwater fish species, including Oreochromis niloticus, Clarias gariepinus, and Synodontis alberti, collected from three sites in Khartoum, Sudan. Six fish of each species were collected from each sampling location where the species were present. Total viable bacterial counts in fish gill and intestinal tissues obtained from Green Belt Sewage, Jebel Aulia fish landing, and Al Mourda fish market ranged from 1.2 × 10⁴ to 3.9 × 10⁵ CFU/g, with higher bacterial loads generally observed in intestinal tissues compared to gills. The highest bacterial load (3.9 × 10⁵ CFU/g) was recorded in the intestine of Clarias species collected from Green Belt Sewage. The current results demonstrated a predominance of Gram-negative, rod-shaped bacteria in gill and intestinal tissue samples across all sampling sites. All isolates were catalase-positive and capable of fermenting glucose, indicating facultative anaerobic metabolism. Oxidase activity of bacterial isolates differed by location; fish samples from Al Murda Fish Market had the highest number of oxidase-positive isolates, suggesting the potential presence of Aeromonas and Pseudomonas species. Urease activity was predominantly observed in isolates of fish samples collected from Al Murda and Jebel Aulia, suggesting a greater risk of fish spoilage or pathogenicity. Notably, isolates from the Green Belt Sewage fish samples comprised Gram-positive coccus, potentially identified as Staphylococcus spp., underscoring the likelihood of anthropogenic contamination. The present results indicated that fish obtained from all sampling locations generally exhibited similar microbial communities. However, the differences in enzyme activity across fish from all sites likely reflected variations in environmental factors, sanitation practices, and potential public health risks.