Objective: Acinetobacter baumannii is an aerobic, gram negative and glucose non-fermenting opportunistic pathogen that is frequently associated with outbreaks mostly in intensive care units. One of the main reasons for the growing interest in A. baumannii in recent years is its tendency to produce outbreaks and its ability to acquire and to accumulate resistance determinants that lead to the emergence of multidrug resistant (MDR) strains. The survival of this organism in hospital environment for long periods, its genetic flexibility and high adaptability has resulted in rapid and global spread of MDR A. baumannii strains resistant to many antimicrobial classes, including carbapenems, especially in the last few years. Carbapenems are important agents of the beta-lactam group that are used in the treatment of Acinetobacter baumannii infections. Carbapenem resistance in Acinetobacter species has been increasingly reported throughout the world and has become the indicator of emerging antimicrobial resistance. Carbapenem resistant A. baumannii is considered as an important health problem today because of limited number of treatment options. The most common mechanisms of carbapenem resistance in Acinetobacter species involves OXA-type enzymes. A. baumannii can produce four different families of enzymes including OXA-23, OXA-24, OXA-51 and OXA-58 and derivatives of all these 4 enzymes that can hydrolyze carbapenems. In this study, we aimed to investigate OXA-type Class D beta-lactamases on carbapenem-resistant- A. baumannii strains isolated from various clinics of our hospital.

Material and Methods: One hundred clinical isolates of carbapenemresistant A. baumannii isolated from various clinical specimens were included in the study. Isolates were identified and their antibiotic susceptibility tests were performed by VITEK 2 system (bioMérieux, USA). Minimal Inhibitory Concentration (MIC) values for imipenem and meropenem were determined by broth microdilution method according to the recommendations of CLSI (Clinical and Laboratory Standards Institute). The presence of OXA carbapenemases namely bla_OXA-23, bla_OXA-24, bla_OXA-51, bla_OXA-58 gene regions in isolates was investigated by PCR.

Results: All isolates were found to be resistant to piperacillin, cefepime and ceftazidime according to the antimicrobial susceptibility test results. All A. baumannii clinical isolates in the study appeared to be susceptible to colistin. All isolates were found to be resistant to imipenem and meropenem with broth microdilution method. A. baumannii specific bla_OXA-51 was found to be positive in all isolates. While bla_OXA-23 was found to be positive in 93% of the isolates, bla_OXA-24 and bla_OXA-58 gene regions weren't detected in isolates.

Conclusion: The high level of resistance to beta lactam antibiotics and the high prevalence of OXA type carbapenemases are remarkable. As a result of the study,it was concluded that carbapenem resistance may occur as a result of the overproduction of OXA-51 type natural oxacillinases and the presence of OXA-23 gene. Additionally, further studies are needed to investigate other resistance mechanisms.