What is the differential diagnosis for large Gram‑negative rods?

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太いグラム陰性桿菌の鑑別診断

太いグラム陰性桿菌を見た場合、まず臨床状況と検体部位を確認し、腸内細菌科（Enterobacteriaceae）、緑膿菌（Pseudomonas aeruginosa）、非発酵菌群、および嫌気性菌を鑑別する必要があります。

主要な鑑別診断

好気性・通性嫌気性菌

  • 腸内細菌科（Enterobacteriaceae） 1

    • 大腸菌（Escherichia coli）
    • クレブシエラ属（Klebsiella spp）
    • エンテロバクター属（Enterobacter spp）
    • これらは尿路感染症、敗血症、人工関節感染症で最も頻繁に検出される

  • 緑膿菌（Pseudomonas aeruginosa） 1

    • 人工関節感染症、尿路感染症の原因菌として重要
    • オキシダーゼ陽性で鑑別可能 2

  • その他の非発酵菌 3, 4, 5

    • Acinetobacter属
    • Alcaligenes属
    • Moraxella属

嫌気性菌

  • Prevotella属 6
    • 口腔内常在菌
    • Lemierre症候群の原因となりうる
    • 敗血症性血栓性静脈炎を引き起こす可能性

特殊な fastidious（発育困難な）グラム陰性桿菌 7, 8

  • Kingella kingae

    • 6-36ヶ月の小児における敗血症、骨髄炎、化膿性関節炎の重要な原因菌 8
    • 従来の培養では検出困難なことが多い

  • HACEK群

    • Aggregatibacter aphrophilus
    • Cardiobacterium hominis
    • Eikenella corrodens
    • Kingella spp

  • Pasteurella multocida

    • 動物咬傷に関連

  • Capnocytophaga属

    • 口腔内常在菌、免疫不全患者で問題となる

鑑別のための実践的アプローチ

初期評価で確認すべき項目

  1. 検体の種類と臨床状況

    • 血液培養：腸内細菌科、緑膿菌、嫌気性菌を考慮 1
    • 尿：腸内細菌科、緑膿菌が最多 1
    • 関節液：小児ではKingella kingaeを必ず考慮 8
    • 口腔・咽頭関連：Prevotella、Capnocytophaga、HACEK群

  2. 迅速鑑別検査 2

    • オキシダーゼ試験：緑膿菌（陽性）vs 腸内細菌科（陰性）
    • β-グルクロニダーゼ、β-ガラクトシダーゼ、インドール：大腸菌の推定同定（感度96-100%）
    • β-キシロシダーゼ：Klebsiella-Enterobacter群の推定（感度85-93%）

  3. 培養条件の最適化

    • 好気・嫌気培養の両方を実施 1
    • Fastidious菌が疑われる場合は血液寒天培地を使用
    • 人工関節感染症でCutibacterium acnes疑いの場合は14日間培養 1

同定困難例への対応

従来の生化学的同定で40%しか正確に同定できないfastidious菌については、16S rRNA遺伝子シーケンス解析が94%の同定率を示します 7。MALDI-TOF MSも有用です 8。

重要な注意点

  • グラム染色は尿中の白血球検出には不適切だが、尿路敗血症が疑われる場合の多数のグラム陰性桿菌検出には有用 1
  • 留置カテーテルからの培養は生物膜形成により定着菌を検出するため推奨されない 1
  • 混合感染の場合、臨床像と併せて抗菌薬選択を行う 8
  • 表現型同定で47%が誤同定または未同定となるため、重要症例では分子生物学的手法を考慮 7

References

3
Research

[Culture and differentiation of obligatory aerobic gram-negative rods from human material; a scheme for application in routine diagnosis (author's transl)].

Zentralblatt fur Bakteriologie, Parasitenkunde, Infektionskrankheiten und Hygiene. Erste Abteilung Originale. Reihe A: Medizinische Mikrobiologie und Parasitologie, 1976

5
Research

Differentiation of gram negative rods other than Enterobacteriaceae and Vibrionaceae by a micromethod for determination of carbon substrate assimilation.

Zentralblatt fur Bakteriologie, Mikrobiologie, und Hygiene. Series A, Medical microbiology, infectious diseases, virology, parasitology, 1984

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