複数菌種、複数菌株の混在が自動細菌検査装置での菌種同定および

複数菌種、複数菌株の混在が自動細菌検査装置での菌種同定および薬剤感受性
試験の結果に与える影響. 臨床病理 56 (4) :283～289..
比嘉美也子・木佐貫京子・山根誠久・仲宗根勇
Influence of the Mixed Inoculum on the Species-Identification
andAntimicrobial Susceptibility Test Results by the Automated
Microbial Systems
Miyako HIGA, Kyoko KISANUKI, Nobuhisa YAMANE, NAKASONE
【Abstract】
The influence of the mixed inoculum on the species-identification and
antimicrobial susceptibility test results was evaluated for the three
automated microbial systems, WalkAway-40 (Dade MicroScan, West
Sacramento, CA, USA), VITEK 2 Compact (bioMerieux, Marcy l’Etoile,
France) and RAISUS (Nissui Pharmaceutical, Tokyo). In the evaluation,
two different species or two different strains were mixed in serial
ratios and adjusted to the inoculum cell suspension for the respective
systems, and then tested for the species-identification and
antimicrobial susceptibility. For the species-identification, all the
three automated systems experienced incorrect identifications others
from the species inoculated with higher likelihoods (>90%),
e.g. Enterobacter cloacae plusKlebsiella pneumoniae resulted in K.
ornithinolytica or E. aerogenes with 93% to 97% likelihoods at the
mixing ratio 9 to 1. Whereas, the mixings extended-spectrum βlactamase (ESBL)-producing and non-producingEscherichia coli,
methicillin-resistant (MRSA) and methicillin-susceptibleStaphylococcus
aureus, and vancomycin-resistant (VR) and vancomycin-susceptible
(VS) Enterococcus faecalis, always resulted in correct detection of a
small portion of resistant cells. However, minimum ratios of resistant
cells for the correct detection varied by the systems, that is, RAISUS
required 70% of MRSA, VITEK 2 Compact was 8%, and the WalkAway-40 was
1%. Also, when the cell suspension of VS E. faecalisspiked
with Proteus mirabilis was tested, the WalkAway-40 reported as being
very rare biotype, but both VITEK 2 Compact and RAISUS reported as the
test inoculum being VR E. faecalis.
With these results, it can be concluded that: First, incorrect
species-identifications others from the inoculated species easily
occur when the inoculum contains different species even at the ratio
visibly indiscernible on the primary isolation agar plate. Secondly,
the automated microbial systems always intend to detect antimicrobial
resistant cells in the inoculum rather than to detect major
susceptible cells to prevent us from reporting very major error
interpretation.
Key Words: Mixed culture (混合菌液), species-identification (菌種同
定), incorrect identification (誤同定), antimicrobial susceptibility
test (薬剤感受性試験), very major error (非常に大きな誤り)
【要旨】
菌種同定および薬剤感受性試験を目的に調製された菌浮遊液に、異なる菌種、
菌株が混在した時、それぞれの試験結果に与える影響を自動細菌検査装置 3 機
種で人為的に再現し、評価した。グラム陽性球菌および腸内細菌科の異なる 2
菌種を組み合わせた時の菌種同定への影響では、菌浮遊液を構成するふたつの
菌種とはまったく異なる菌種 (A + B = C) に、しかも高い相対同定確
率 (>90%)で同定される組み合わせがすべての機種で観察された。このような
誤同定は、混和した異なる菌種の細菌細胞の比率が 0.5～1%という極少ない量
の添加・混入で観察された。また、薬剤感受性試験では、薬剤耐性が問題とな
る ESBL 産生 E. coli、MRSA、VRE の検討で、いずれの自動機器も、添加・混入
された耐性菌を極低い混和比率の段階で正しく検出、判定し、very major
error の判定を回避していることが確認された。
以上の成績から、臨床細菌検査においても、菌種同定に際しての純培養は極め
て重要な手順であり、また広く臨床応用されている自動細菌検査装置での薬剤
感受性試験は耐性菌検出に重きを置いていることを正しく認識する必要があ
る。

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