Investigation of Tigecycline Susceptibility of Multidrug-Resistant Acinetobacter Baumannii Isolates by Disc Diffusion, Agar Gradient and Broth Microdilution Tests

Abstract

The use of tigecycline is becoming increasingly important because of the high levels of antibiotic resistance in Acinetobacter baumannii (A. baumannii) isolates. In this prospective study, multidrug-resistant A. baumannii isolates were obtained from various tissue and fluid samples of patients admitted to or treated at various departments and tested in Laboratory of Microbiology, Duzce University Medical Faculty between January 2013 and December 2015. Tigecycline resistance in multidrug-resistant A. baumannii isolates were analyzed using the disc diffusion test (DDT), agar gradient test (AGT), and gold standard test [broth microdilution test (BMT)]. A. baumannii isolates resistant to multiple drugs were included in the study (N=94). Using the BMT method, 89 (95%), 4 (4%) and 1 (1%) A. baumannii isolates were determined as tigecycline susceptible, intermediate and resistant isolates, respectively. Using the Food and Drug Administration criteria, the rates of major error (ME), minor error (mE) and categorical agreement (CA) for DDT were 26%, 67% and 9%, respectively. In contrast, for AGT, the rates of ME, mE and CA were 0%, 4%, 95%, respectively. Tigecycline resistance as assessed by BMT showed no increase between 2013 and 2015. Accordingly, isolates found to be resistant or intermediate by DDT should be confirmed by BMT. Due to the ease of application, AGT is a safe method of detecting susceptibility.

Keywords

• Acinetobacter baumannii
• Agar gradient test
• Broth microdilution test
• Disc diffusion test
• Multidrug-resistance
• Tigecycline