Comparison of Automated Broth Microdilution System (Vitek-2) and Agar Dilution Method in the Detection of Fosfomycin Susceptibility in E. coli and K. pneumoniae Isolates Causing Urinary Tract Infection

Abstract

Introduction: Agar dilution is the reference method recommended by the European Committee on Antimicrobial Susceptibility Testing (EUCAST) for the detection of fosfomycin susceptibility in Gram-negative bacteria. However, this method is difficult to apply routinely in microbiology laboratories. For this reason, commercial automated systems have been developed to detect antibiotic susceptibility more easily and practically. In our study, we aimed to evaluate the utility of commercial automated broth microdilution system in routine studies to detect fosfomycin susceptibility by comparing it with the gold standard agar dilution method. Materials and Methods: A total of 100 isolates of E. coli and K. pneumoniae isolated as urinary tract infection agents from urine samples, which were sent to Haydarpaşa Numune Training and Research Hospital, Medical Microbiology Laboratory between 01.06.2019 and 31.12.2019 were included in our study. Isolates were identified with matrix assisted lazer desorption/ionization-time of flight mass spectrometry (MALDI-TOF MS) (BioMerieux, France), and antibiotic susceptibility tests were performed with an automated broth microdilution system (Vitek-2, BioMerieux, France). Fosfomycin susceptibility was studied with the reference agar dilution method and the results were evaluated according to the EUCAST V.10.0 recommendations. In the evaluation made to compare the two methods, categorical agreement (CA), essential agreement (EA), very major error (VME), and major error (ME) values were calculated and the results were evaluated according to ISO 20776-1 criteria (CA and EA ≥90%; ME and VME <3%). Results: When the data of the two methods were compared, CA and EA were found 100% for E. coli strains and 95.5% and 94.4% for K. pneumoniae strains, respectively. No ME or VME was detected for E. coli strains. While no VME was detected in K. pneumoniae strains, ME rate was 18%. Conclusion: Based on the high CA rates in our study, it was thought that the Vitek-2 system would be suitable for routine laboratory use. However, due to the high ME rates in K. pneumoniae strains, it was concluded that the results should not be reported unless confirmed with the reference method, especially in resistant isolates.

Keywords

• fosfomycin
• agar dilution
• automated broth microdilution method
• vitek-2
• gram-negative bacteria