Investigation of Colistin Susceptibility in Carbapenem-Resistant Gram-Negative Bacteria

Abstract

Objective: We evaluated colistin susceptibility in carbapenem-resistant isolates and compared the automated microbiology system (VITEK 2 Compact, bioMérieux, France) with the broth microdilution (BMD) reference method. Materials and Methods: A total of 174 carbapenem-resistant Gram-negative isolates collected from various clinical specimens during 2022–2023 were included in the study. Identification and antimicrobial susceptibility testing were performed using conventional methods and the VITEK 2 Compact automated system (bioMérieux, France). Colistin susceptibility was confirmed by the BMD reference method. Results: Among 90 Enterobacterales isolates tested with BMD, 39 (43.3%) were susceptible and 51 (56.7%) were resistant. In the non-fermentative group, 67 (79.8%) of 84 isolates were susceptible, and 17 (20.2%) were resistant. When compared with BMD, VITEK 2 demonstrated an essential agreement (EA) of 68.8%, a categorical agreement (CA) of 80%, a very major error (VME) rate of 7.8%, and a major error (ME) rate of 35.9% for Enterobacterales. For non-fermentative isolates, EA and CA were markedly lower (54.7% and 73.8%, respectively), while VME (47%) and ME (20.9%) rates exceeded acceptable ISO performance thresholds. Conclusion: Although automated systems such as VITEK 2 streamline laboratory workflow, their colistin susceptibility results show poor concordance with the BMD reference method, particularly for non-fermentative isolates. The high error rates observed in this study highlight the critical importance of confirming automated colistin results with BMD to avoid misclassification and inappropriate antimicrobial therapy.

Keywords

• Antimicrobial susceptibility testing
• automated system
• broth microdilution
• carbapenem resistance
• colistin

Ethical Statement

This study was conducted in accordance with the principles of the Declaration of Helsinki and relevant international guidelines. Ethical approval was obtained from the Bolu Abant İzzet Baysal University Clinical Research Ethics Committee (Date: 11.04.2023, decision no: 2023/106).

Karbapeneme Dirençli Gram-Negatif Bakterilerde Kolistin Duyarlılığının Araştırılması

Abstract

Amaç: Karbapenem dirençli izolatlarda kolistin duyarlılığını değerlendirdik ve VITEK 2 Compact Otomatik Mikrobiyoloji Sistemi'ni sıvı mikrodilüsyon (BMD) referans yöntemi ile karşılaştırdık. Materyal ve Metot: Çalışmaya 2022–2023 yılları arasında çeşitli klinik örneklerden izole edilen toplam 174 karbapeneme dirençli Gram-negatif bakteri dâhil edildi. İzolatların tanımlanması ve antibiyotik duyarlılık testleri konvansiyonel yöntemler ve VITEK 2 Compact sistemi kullanılarak gerçekleştirildi. Kolistin duyarlılığı, BMD referans yöntemiyle doğrulandı. Bulgular: BMD yöntemiyle test edilen 90 Enterobacterales izolatının 39’u (%43,3) kolistine duyarlı, 51’i (%56,7) dirençli bulundu. Non-fermentatif gruptaki 84 izolattan 67’si (%79,8) duyarlı, 17’si (%20,2) dirençliydi. VITEK 2 sonuçları BMD ile karşılaştırıldığında, Enterobacterales grubunda esas uyum (EA) %68,8, kategorik uyum (CA) %80, çok büyük hata (VME) %7,8 ve büyük hata (ME) %35,9 olarak saptandı. Non-fermentatif bakterilerde EA %54,7, CA %73,8 olup VME (%47) ve ME (%20,9) oranları ISO performans kriterlerinin belirgin şekilde üzerindeydi. Sonuç: Kolistin direncinin belirlenmesinde VITEK 2 gibi otomatize sistemler laboratuvar iş akışını kolaylaştırsa da referans yöntem olan BMD’ye kıyasla daha düşük uyum ve daha yüksek hata oranlarına sahiptir. Bu çalışmada saptanan uyumsuzluk ve hata oranlarının ISO standartlarının altında kalması, özellikle non-fermentatif türlerde kolistin duyarlılık sonuçlarının BMD ile doğrulanmasının klinik açıdan kritik önem taşıdığını göstermektedir.

Keywords

• Antibiyotik duyarlılık testi
• karbapenem direnci
• kolistin
• otomatize sistem
• sıvı mikrodilüsyon

Ethical Statement

Bu çalışma, Helsinki Bildirgesi ilkeleri ve ilgili uluslararası kılavuzlar doğrultusunda yürütülmüştür. Bolu Abant İzzet Baysal Üniversitesi Klinik Araştırmalar Etik Kurulu'ndan etik onay alınmıştır (Tarih: 11.04.2023, karar no: 2023/106).

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