Abstract
Abstract—Hematoporphyrin derivative (HpD) is widely used in photoradiation therapy of tumors and other diseases, and has been shown to affect the viability of gram positive bacteria. This investigation assessed the efficiency of binding of HpD toBacillus subtilisandStreptococcus faecaliswhen HpD‐treated organisms were exposed to red light. Kinetic studies indicated that the amount of HpD bound increased with increasing external concentration of HpD until saturation of binding sites was reached.S. faecalishad a higher affinity for HpD and was more susceptible to photoinactivation thanB. subtilis.The data from this study suggest that differences in susceptibility of microorganisms to photoinactivation are directly related to the affinity of each strain for HpD.