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Journals / Osmangazi Tıp Dergisi / 2026 / Cilt: 48 Sayı: 5

MALDI-TOF MS Pretreatment for Gram-Positive Bacteria in Routine Clinical Laboratories: Does Formic Acid Really Improve Identification?

Rutin Klinik Laboratuvarlarda Gram-Pozitif Bakteriler İçin MALDI-TOF MS Ön İşlemi: Formik Asit Tanımlamayı Gerçekten İyileştiriyor mu?

Abdurrahman Gülmez(Aydın Atatürk Devlet Hastanesi)0000-0003-3953-5267
Ayşe Nur Ceylan(Istanbul Basaksehir Cam and Sakura City Hospital, Medical Microbiology Laboratory, Istanbul, Turkey)0000-0002-0049-6873
Selda Kömeç(Istanbul Basaksehir Cam and Sakura City Hospital, Medical Microbiology Laboratory, Istanbul, Turkey)0000-0002-6726-0048
Beyza Öncel(Istanbul Basaksehir Cam and Sakura City Hospital, Medical Microbiology Laboratory, Istanbul, Turkey)0000-0002-2857-8635
Kamuran Şanlı(Istanbul Basaksehir Cam and Sakura City Hospital, Medical Microbiology Laboratory, Istanbul, Turkey)0000-0003-0814-5637
Pages
790–798
Publication Date
2026-08-26
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AI Summary & Key Takeaways

AI Insight
⚡ TL;DR(Too Long; Didn't Read)

Overall identification rates were significantly higher with FA pretreatment than with DT (99.3% vs. 79.7%).

📌 3 Key Takeaways

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Objective & Hypothesis:Accurate identification of Gram-positive bacteria by matrix-assisted laser desorption ionization–time of flight mass spectrometry (MALDI-TOF MS) may be limited by the complex structure of their cell walls.
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Key Findings & Data:This study evaluated the impact of on-plate formic acid (FA) pretreatment on MALDI-TOF MS identification performance in a routine clinical microbiology laboratory.
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Conclusion & Impact:Overall identification rates were significantly higher with FA pretreatment than with DT (99.3% vs. 79.7%).
🔬Design:Araştırma Makalesi
📍Scope:Comprehensive Sample
⚙️Variables:Matriks yardımlı lazer desorpsiyon/iyonizasyon (MALDI) · Gram pozitif bakteriler · mikrobiyoloji

Abstract

Accurate identification of Gram-positive bacteria by matrix-assisted laser desorption ionization–time of flight mass spectrometry (MALDI-TOF MS) may be limited by the complex structure of their cell walls. This study evaluated the impact of on-plate formic acid (FA) pretreatment on MALDI-TOF MS identification performance in a routine clinical microbiology laboratory. A total of 148 non-repetitive Gram-positive clinical isolates were analyzed using two preparation methods: direct transfer (DT) and on-plate FA pretreatment. Identification scores were compared with an emphasis on high-confidence species-level identification (log score ≥ 2.0). Statistical analyses were performed using the Wilcoxon signed-rank and McNemar tests. Overall identification rates were significantly higher with FA pretreatment than with DT (99.3% vs. 79.7%). The proportion of isolates achieving species-level identification increased from 27.1% with DT to 76.2% with FA pretreatment. Mean MALDI-TOF MS log scores for all isolates were significantly higher with FA (2.104 ± 0.186) than with DT (1.861 ± 0.265; p < 0.001). Notably, FA pretreatment also resulted in a significant increase in mean log scores among isolates that had already achieved species-level identification using DT (p = 0.001). The most pronounced improvement was observed among Staphylococcus spp ., particularly coagulase-negative staphylococci, with complete concordance between DT and FA-assisted species-level identifications. In conclusion, on-plate FA pretreatment significantly enhances identification accuracy and diagnostic confidence for Gram-positive bacteria in routine MALDI-TOF MS workflows. Building on these findings, we propose a pragmatic, score-based, and species-aware diagnostic algorithm in which FA pretreatment is reflexively applied to Gram-positive isolates, yielding a DT log score < 2.0, with primary FA application recommended for staphylococci (particularly coagulase-negative species) and for less commonly encountered Gram-positive organisms.

Özet

Gram-pozitif bakterilerin matriks yardımlı lazer desorpsiyon/iyonizasyon–uçuş zamanı kütle spektrometrisi (MALDI-TOF MS) ile doğru tanımlanması, hücre duvarlarının karmaşık yapısı nedeniyle bazı durumlarda sınırlı olabilmektedir. Bu çalışmada, rutin bir klinik mikrobiyoloji laboratuvarında MALDI-TOF MS ile bakteri tanımlama performansı üzerine plak üzerinde uygulanan formik asit (FA) ön işleminin etkisi değerlendirilmiştir. Çalışmaya, benzersiz toplam 148 Gram-pozitif klinik izolat dahil edilmiştir. İzolatlar iki farklı hazırlama yöntemi kullanılarak analiz edilmiştir: doğrudan transfer (DT) yöntemi ve plak üzerinde FA ön işlemi. Tanımlama skorları karşılaştırılırken, özellikle yüksek güvenilirlikte tür düzeyinde tanımlama (log skor ≥ 2,0) kriteri esas alınmıştır. İstatistiksel analizlerde Wilcoxon işaretli sıralar testi ve McNemar testi kullanılmıştır. Genel tanımlama oranı, FA ön işlemi uygulanan örneklerde DT yöntemine kıyasla anlamlı derecede daha yüksek bulunmuştur (%99,3 ve %79,7). Tür düzeyinde tanımlama elde edilen izolat oranı DT yöntemi ile %27,1 iken, FA ön işlemi ile bu oran %76,2’ye yükselmiştir. Tüm izolatlar için ortalama MALDI-TOF MS log skorları, FA ön işlemi ile DT yöntemine kıyasla anlamlı derecede daha yüksek saptanmıştır (2,104 ± 0,186 ve 1,861 ± 0,265; p < 0,001). Ayrıca, DT yöntemi ile halihazırda tür düzeyinde tanımlanmış izolatlarda dahi FA ön işlemi sonrasında ortalama log skorlarında anlamlı bir artış gözlenmiştir (p = 0,001). En belirgin iyileşme, özellikle koagülaz-negatif stafilokoklar olmak üzere Staphylococcus türlerinde izlenmiş; DT ve FA destekli tanımlamalar arasında tür düzeyinde tam uyum saptanmıştır. Sonuç olarak, plak üzerinde uygulanan formik asit ön işlemi, rutin MALDI-TOF MS uygulamalarında Gram-pozitif bakterilerin tanımlanma doğruluğunu ve tanısal güvenilirliği anlamlı ölçüde artırmaktadır. Bu bulgulardan yola çıkarak, FA ön işleminin DT log skoru < 2,0 olan Gram-pozitif izolatlara refleks olarak uygulandığı; stafilokoklar (özellikle koagülaz-negatif türler) ve nadir karşılaşılan Gram-pozitif organizmalar için ise birincil olarak önerildiği, skor temelli ve tür bilinçli pragmatik bir tanı algoritması önermekteyiz.

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Cite This Article

Gülmez, A., Ceylan, A. N., Kömeç, S., Öncel, B., Şanlı, K. (2026). MALDI-TOF MS Pretreatment for Gram-Positive Bacteria in Routine Clinical Laboratories: Does Formic Acid Really Improve Identification?. Osmangazi Tıp Dergisi, Cilt: 48 Sayı: 5, 790–798. https://doi.org/10.20515/otd.1848228

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