Berberine is the primary alkaloid found in Barberry. Research appearing in the Annals of Tropical Medicine and Parasitology (1991 Aug;85(4):417-25) looked at what exposure to berberine sulfate did to various microorganisms. In vitro (in a Petri dish with nutrients), berberine inhibited the growth, and caused damaging cellular changes in Entamoeba histolytica, Giardia lamblia and Trichomonas vaginalis. It also morphological changes in parasites. The authors concluded, “These observations demonstrate that, in vitro, this drug (berberine) was effective against E. histolytica, G. lamblia and T. vaginalis.”

Berberine can be effective against Streptococcus. Other research, appearing in Antimicrobial Agents and Chemotherapy (1988;32:1370-1374) found that berberine sulfate is bacteriostatic for streptococci and that low doses of berberine blocked the adherence of streptococci to host cells.