In　the　last　decades　the　worldwide　rise　in　antibiotic　resistance　has　intensified　the　development　of　new　antimicrobial　agents.　Photodynamic　antimicrobial　chemotherapy　(PACT)　has　been　used　successfully　to　inactivate　bacteria.　We　herein　report　a　new　zinc　phthalocyanine　based　photosensitizer　conjugated　with　polylysine　moiety　(ZnPc-PL).　This　photosensitizer　significantly　inactivated　Porphyromonas　gingivalis,　the　primary　pathogenic　bacteria　responsible　for　periodontitis.　No　obvious　phototoxicity　was　found　to　either　mammalian　bone　marrow　stromal　cells　(BMSC)　or　human　periodontal　ligament　cells　(HPDLC),　indicating　the　high　selectivity　of　ZnPc-PL　toward　bacteria.　Furthermore,　we　established　an　experimental　periodontitis　model　on　beagle　dogs　to　test　the　antimicrobial　efficacy　in　vivo.　The　amount　of　gingival　crevicular　fluid　(GCF)　and　the　activity　of　crevicular　fluid　aspartate　aminotransferase　(AST)　were　monitored　and　were　found　to　reduce　significantly　in　the　ZnPc-PL　treated　group　compared　to　the　controls　(laser　only　and　no　treatment).　In　addition,　PACT　with　ZnPc-PL　caused　a　reduction　in　the　bacterial　burden　by　100-fold　compared　to　controls.　Taken　together,　these　findings　suggest　ZnPc-PL　is　a　promising　antimicrobial　photosensitizer　for　the　treatment　of　periodontal　diseases.　©　2011　World　Scientific　Publishing　Company.