Effects of sub-lethal antimicrobial photodynamic therapy mediated by haematoporphyrin monomethyl ether on polymyxin-resistant Escherichia coli clinical isolate.

Abstract Background and aim It is generally believed that bacteria can not develop resistance to antimicrobial photodynamic therapy (aPDT). This work employed a polymyxin-resistant Escherichia coli clinical isolate (E15017) to study whether it could become resistant to aPDT mediated by haematoporphyrin monomethyl ether (HMME) via consecutive photodynamic treatments at sub-lethal condition. Methods The sub-lethal and lethal photodynamic treatment conditions for E15017 were determined by colony forming units (CFU) assay. Bacterial cells of E15017 were treated with 20 cycles of repeated sub-lethal HMME-mediated aPDT, and subsequently subjected to aPDT at lethal condition. The antibiotic susceptibility, zeta-potential and membrane integrity of sub-lethal aPDT treated E15017 cells were also investigated. Results After 20 cycles of repeated HMME-mediated aPDT treatments at sub-lethal condition, E15017 cells didn't become more resistant to aPDT. Sub-lethal HMME-mediated aPDT decreased the MIC values of E15017 to ceftazidime and polymyxin E by 4 and 2-fold, respectively, and increased the electronegativity of bacterial surface and affected the bacterial membrane integrity. Conclusions The results obtained in this study confirmed that antibiotic-resistant bacteria could not develop resistance to aPDT, and HMME-mediated aPDT is an attractive potential treatment for MDR E. coli caused infections.