Determination of polymyxin B sensitivity in clinical insulations of carbapenem resistant escherichia coli and klebsiella pneumoniae by broth microdilution method and comparison of this method with disk diffusion test

Background-Aim: Increasing resistance rates in Gram-negative bacteria limit the treatment options, making it necessary to use polymyxins in clinical use and to study in vitro susceptibility tests. In this study, it was aimed to determine polymyxin B susceptibility in carbapenem resistant Klebsiella pneumoniae (K. pneumoniae) and Escherichia coli (E. coli) isolates by using a commercial broth bmicrodilution (BMD) based test. Materials-Method: The study included 128 carbapenem resistant isolates (85 K. pneumoniae, 43 E. coli) isolated from various patient samples sent from intensive care units (ICU) and clinics to Dicle University Hospitals Medical Microbiology Laboratory between February 10, 2021 and June 10, 2022. Polymyxin B susceptibility of the isolates was studied with the HiMIC™ Plate Kit (HiMedia, India), a commercial BMD based kit, according to the manufacturer's recommendations. In addition, Kirby Bauer disc diffusion test was performed with 300 µg polymyxin B (Bioanalyse, Turkey) discs and the compatibility of this test with SMD was investigated. Results: Out of a total of 43 E. coli isolates, 5 (11,63%) were resistant and 38 (88,37%) were "intermediate". Polymyxin B resistance was found to be 15% and 11,1% in the clinics and ICU patients, respectively. Of the 85 K. pneumoniae isolates, 18 (21,18%) were resistant, and 67 (78,82%) were "intermediate". Polymyxin B resistance was found in 14,3% (2/14) of the clinics patients and in 30,2% (16/53) of the ICU patients. Polymyxin B resistance observed in isolates of patients hospitalized in the ICU was found to be higher than isolates of patients hospitalized in the clinic (p<0,05). No agreement was found between the disc diffusion test and SMD. Conclusion: High polymyxin B resistance may be associated with the increased use of colistin in our hospital in recent years. This study provides important data on polymyxin B resistance in our hospital and city. Disc diffusion test results should not be relied upon in detecting polymyxin resistance. Key Words: Polymyxin B, Klebsiella pneumoniae, Escherichia coli, antibiotic resistance, broth microdilution, disk diffusion test