Ofloxacin (OFL) and sulfamethoxazole (SMX) are common co-occurring antibiotic contaminants in aquatic environments, yet their long-term combined toxicity to freshwater fish remains poorly elucidated. In this study, adult mosquitofish (Gambusia affinis) were used as a model to investigate histopathological alterations, oxidative stress responses, gene expression, and gut microbiota changes after 30 days of exposure to environmentally relevant concentrations of OFL and SMX (0 ng/L, 50 ng/L, 1 μg/L, and 20 μg/L), either individually or in combination. The results showed that both single and combined exposures induced liver and intestinal damage. Oxidative stress responses exhibited clear tissue specificity, with activation of antioxidant defenses in the liver, whereas the intestine was mainly characterized by decreased SOD and GST activities, as well as reduced MDA content. Changes in gene expression were relatively limited, with significant alterations observed only in hepatic sod2 and hsp90 and intestinal hsp70 in certain treatment groups. Gut microbiota analysis showed that OFL exerted a stronger disruptive effect than SMX, as reflected by increased alpha diversity, reduced abundance of core genera, and functional remodeling, whereas combined exposure triggered weaker microbial community restructuring relative to single exposures. Overall, OFL and SMX induced tissue-specific toxicity in mosquitofish by causing tissue injury, oxidative stress imbalance, and gut microbiota dysbiosis, with OFL showing the stronger overall effect.
Ding et al. (2026) studied this question.