Abstract: OBJECTIVE To explore the mechanisms of berberine (BBR) in inhibiting the capsular biosynthesis of hypervirulent Klebsiella pneumoniae (hvKp) and enhancing the immune clearance of the host. METHODS The impact of BBR on proliferation of the hvKp was evaluated through growth curves, the output of capsular polysaccharide (CPS) was determined by sulfuric acid - phenol method, the viscosity was detected by low-speed centrifugation method, and the capsular ultrastructure was observed with the use of transmission electron microscope (TEM). The transcriptional levels of capsular synthesis gene cluster (cps) and biofilm-related genes were detected by means of quantitative real-time PCR (qRT-PCR); the effect of BBR on regulation of hvKp virulence was assessed via seroresistance test and THP-1 macrophage phagocytosis assay, and the biofilm formation ability was observed by crystal violet staining and laser scanning confocal microscope. RESULTS BBR did not affect the final growth of hvKp but could concentration-dependently reduce the output of CPS and culture viscosity (P＜0.05). It was proved by the TEM that the capsules appeared with structural damages such as breakage and sparseness after treatment with BBR. The transcriptional levels of key genes of cps gene cluster (manC, galF, rmpA, rmpA2, magA and rmpD) declined by 50.40% to 90.00% after the treatment with 20 μg/ml of BBR for 6 hours(P＜0.05). Further assays indicated that the pretreatment with BBR made the survival rate of hvKp in serum of 50% of the healthy people decrease from 48.12% to 23.91%(P＜0.05), with the bacterial load in macrophages increasing from 2 570 CFU/ml to 3 810 CFU/ml(P＜0.05), the biofilm formation volume decreasing by 48.40%(P＜0.05); both the thickness of biofilm and coverage rate declined(P＜0.05). CONCLUSIONS BBR can inhibit the transcription of cps gene cluster, weaken the capsular synthesis of hvKp, reduce the seroresistance, enhance the macrophage phagocytosis, and inhibit the biofilm formation, which may provide experimental bases for assisted treatment of hvKp infection with BBR.