The invention relates to a therapy of an infection with a drug resistant bacterium wherein a characteristic of multivalent phenol derivatives and/or extracts from "Tara" which enhance the activity of .beta.-lactam antibiotics on the drug resistant bacteria is utilized. Specifically, the invention relates to an enhancer of .beta.-lactam antibiotics comprising a multivalent phenol derivative or its pharmaceutically acceptable salt, a pharmaceutical composition comprising a .beta.-lactam antibiotic and a multivalent phenol derivative and/or extract from "Tara" for the therapy of an infection with a drug resistant bacterium, a disinfectant for the same, and a functional food comprising a multivalent phenol derivative and/or extract from "Tara".