Synergistic degradation of catechin and minocycline hydrochloride in alkaline solution

Minocycline hydrochloride (MCH) is a broad-spectrum antibiotic commonly found in aquatic environments that can adversely affect aquatic ecosystems. Catechin, known for its low chemical stability, can undergo oxidative degradation under alkaline conditions. In this study, we used catechin basification to develop a method for degrading MCH and determined the effects of MCH degradation on bacterial growth. Approximately 38.0 and 65.4% of catechin was removed, with dimeric A- and B-type proanthocyanidins being formed, owing to the reduction of catechin via electron transfer after incubation at pH 10 for 2 and 24 h, respectively. MCH is stable in alkaline conditions but unstable when subjected to catechin in alkaline solutions. The percentages of degraded MCH and catechin were 57.1 and 65.4%, respectively, when mixed and incubated at pH 8 for 24 h. When MCH was incubated at pH 10 in the presence of catechin for 2 and 24 h, 62.3% and 87.4% of MCH and 56.2% and 98.3% of catechin were degraded, respectively, owing to the electron transfer induced by catechin basification, suggesting that the synergistic degradation of catechin and MCH is enhanced in alkaline solutions. The changes in the structure of MCH in the MCH-catechin system were confirmed by an ion signal at m/z 472, namely, the molecular formula C23H27N3O8, and a mass of 473.5 g/mol. The effects of MCH and the degraded MCH on bacterial growth curves were examined. The treatment of MCH upon mixing with catechin, followed by 2h incubation at pH 8, led to bacterial growth rates of 0.037 and 0.057 h−1, respectively. After treatment with MCH-catechin, the bacterial growth curve steepened, indicating that MCH degradation reduces its antimicrobial activity under alkaline conditions. The results of this study highlight the potential for developing a simple and safe method to degrade MCH using catechin in alkaline conditions.