EPIDEMIOLOGICAL STUDY OF NOSOCOMIAL MRSA INFECTION BY PLASMID DNA AND ANTIBIOTIC SUSCEPTIBILITY ANALYSES

为进行医院感染的流行病学调查，对1991年从旭川医科大学附属医院住院患者分离的87株耐甲氧西林金黄色葡萄球菌(MRSA)进行了质粒谱和药敏调查。质粒谱分为7个谱型(A-G谱)，21株无质粒。A谱47株(54%)，B谱10株，其余为C~G谱，共检测9种抗生素。只有米诺环素(Mino)的药敏模式明显分为敏感(MIC为0.5或1µg/ml)和耐药(MIC为4或8µg/ml)两种模式。47株对米诺敏感，40株耐药。因此，采用质粒谱和敏感度相结合的方法进行流行病学调查。从其中一个外科病房分离的菌株以A质粒型和米诺敏感为主，而在另一个外科病房、神经外科病房和泌尿外科分离的菌株以A质粒型和米诺耐药为主。对1991年后分离的部分MRSA菌株的质粒谱进行了进一步检测，发现B谱型成为优势菌株，而不是A谱型。我们还对远离旭川的鹿儿岛大学医院分离的菌株进行了调查，它们的质粒谱与朝日川株不同。我们还开始用脉冲场凝胶电泳(PFGE)检测MRSA菌株的基因组谱。对23株具有全部A质粒图谱的菌株进行了PFGE分析，将其分成10个图谱，因此，PFGE比质粒图谱更具区分性，甚至可以结合药敏模式作为流行病学工具。该方法将对400多株储存的MRSA菌株进行调查，以进一步进行医院内MRSA感染的流行病学调查。

Plasmid profile and antibiotic susceptibility of 87 strains of methicillin-resistant staphylococcus aureus (MRSA) isolated from inpatients in Asahikawa Medical College Hospital in 1991 were investigated for epidemiologic survey of nosocomial infection. Plasmid profile was classified into 7 profiles (profile A to G) and 21 strains had no plasmids. Forty-seven strains (54%) belonged to profile A,10 to profile B and the rest to profile C to G.Nine antibiotics were tested. Only the susceptibility pattern of minocycline (MINO) was clearly divided into two patterns, sensitive (minimal inhibitory concentration, MIC is 0.5 or 1 mug/ml) or resistant (MIC is 4 or 8 mug/ml). Forty-seven strains were MINO sensitive and 40 were resistant. Thus plasmid profile combined with MINO susceptibility was used for epidemiologic investigation. Most strains isolated from patients in one of the two surgery wards had plasmid profile A and MINO sensitive, whereas in another surgery ward, neurosurgery ward and urology word, profile A and MINO resistant strains were predominant. Profile C strains were identified just in otolaryngology ward.We further examined plasmid profiles of some of the strains isolated after 1991 and found that profile B became dominant rather than profile A.The strains isolated in Kagoshima University Hospital, which is located far from Asahikawa, were also investigated and they had the different plasmid profiles from those of Asahikawa strains.We also started to examine genomic profile of MRSA strains by pulsed field gel electrophoresis (PFGE). Twenty-three strains which had all plasmid profile A were classified into 10 profiles by PFGE.Thus PFGE is more discriminant than plasmid profiling, even combined antibiotic susceptibility pattern as epidemiological tool. More than 400 MRSA strains stocked will be investigated by this method for further epidemiological investigation of nosocomial MRSA infection.