Barbs as Sentinels of Antimicrobial Resistance

Mass fish mortalities in Lake Victoria have caused major economic losses, with Barbus altianalis now listed on the IUCN Red List. This study assessed water quality, barb species diversity, parasites, microbiomes, and antimicrobial susceptibility testing (AST) in River Nyando. Kidney swabs were processed for microbiome isolation and 16S rRNA sequencing. Antimicrobial susceptibility testing (AST) was performed using the Kirby–Bauer test. Four antibiotics, tetracycline (TE, 30 µg), ciprofloxacin (CIP, 5 µg), trimethoprim/sulfamethoxazole (SXT, 25 µg), and erythromycin (E, 15 µg) were tested, with inhibition zones validated against CLSI M100 (2025) and EUCAST (2025) ranges. Water quality analysis revealed elevated nutrient levels, with nitrates (0.11±0.19 mg/L), nitrites (0.02±0.06 mg/L), and soluble reactive phosphorus (0.07±0.27 mg/L), exceeding optimal thresholds of <0.1, <0.01, and <0.02 mg/L, respectively. Out of 89 barb specimens nine species across three genera were identified, including Labeobarbus altianalis (23.3%), Labeo victorianus (13.3%), and Barbus apolograma (20.0%). Parasite analysis identified 18 species from eight genera, with copepods (Lernaea and Ergasilus) most prevalent, while microbiome sequencing revealed 14 pathogenic bacterial genera. AST showed reduced susceptibility in Aeromonas hydrophila (TE 19.0 mm, CIP 28.6 mm, SXT 23.8 mm) and Pseudomonas aeruginosa (TE 14.0–16.2 mm, CIP 16.8–17.0 mm, SXT 14.6–16.0 mm), while Vibrio anguillarum (TE 26.8 mm, CIP 29.4 mm, SXT 28.2 mm) and Yersinia ruckeri (TE 30.0–33.6 mm, CIP 38.0–38.2 mm, SXT 36.4–38.6 mm) remained highly susceptible. Elevated nutrient concentrations coincided with resistant pathogens, suggesting eutrophication may exacerbate fish health risks. Although wild barbs cannot be treated directly, AST provides vital surveillance data. As ecological sentinels, they reveal how antimicrobial resistance circulates across aquatic ecosystems. These findings highlight the combined impact of nutrient pollution, parasitic infections, and multidrug resistance, providing baseline evidence for fisheries management, conservation strategies, pollution control, and public health preparedness in Lake Victoria.