院内で最も多く検出される多剤耐性菌“MRSA”

MRSA : Most frequently isolated multidrug resistant pathogen

MRSAの検出は，オキサシリン(MPIPC)やセフォキシチン(CFX)の感受性，PBP2′の検出，mecA遺伝子の検出で行う．本菌はメチシリン(β-ラクタム系薬)耐性黄色ブドウ球菌の略だが，現実的には多剤(multi-drug)耐性黄色ブドウ球菌である．抗MRSA薬であるバンコマイシン(VCM)の最小発育阻止濃度(MIC)が4μg/ml以上の株は中程度耐性(I)と判定されるが，臨床効果は期待できない．さらに2μg/mlで感性(S)と判定される株の治療も難しい．したがって，MICを用いた薬物動態学/薬力学理論(PK/PD)による科学的根拠に基づいた抗菌薬の投与設計が必要となる．また，市中関連型MRSA(CA-MRSA)の中でも，白血球破壊毒素(PVL)産生株の存在を認識して対応すべきである．

Methicillin-resistant Staphylococcus aureus (MRSA) may be defined as the S. aureus strains, which are (i) resistant to oxacillin and cefoxitin, and which show (ii) the occurrence of penicillin binding protein (PBP2′) and mecA gene.　Though the original designation of MRSA implied methicillin-resistant, the bacterium is virtually multidrug resistant to structurally and functionally dissimilar chemotherapeutic agents.　The first choice antibiotics for the treatment of MRSA infections may be to use glycopeptide antibiotics such as vancomycin.　MRSA strains which show the minimum growth inhibitory concentration (MIC) of vancomycin 4-8 μg/ml were defined as intermediately vancomycin susceptible.　However, the vancomycin therapy for such MRSA infection often results in failure.　It is said that vancomycin treatment may be ineffective even in infection of MRSA with MIC of vancomycin 2 μg/ml.　Therefore, vancomycin therapy based on the conventional in vitro MIC determination should be reconsidered.　One of the solutions could be to determine MIC of vancomycin based on pharmacokinetics/pharmacodynamics (PK/PD) analyses and to design a suitable vancomycin dosage.　Another concern on MRSA infection would be the markedly increasing trend of community-acquired MRSA, which produces the life-threatening toxin, Panton-Valentine leukocidin.