Detection of AmpC-beta-lactamases producing isolates among carbapenem resistant P. aeruginosa isolated from burn patient

Background and Objectives: Pseudomonas aeruginosa is responsible for devastating nosocomial infections among severely burn patients. Class C of cephalosporinase (AmpC-beta-lactamases) is important cause of multiple beta-lactam resistance in P. aeruginosa. The aim of this study was to detect the AmpC-beta-lactamases producing isolates among carbapenem resistant P. aeruginosa isolated from burn patient. Material and Methods: a total of 100 isolates of carbapenem resistant P. aeruginosa isolates from different burn patients were investigated. Three phenotypic methods were selected for identification of the AmpC-beta-lactamases producing isolates. Results: Fifty four isolates were AmpC producer as detected by AmpC disk test. Seventeen isolates were identified as AmpC producer using combined disk method. Fifty two isolates showed a twofold or threefold dilution difference between the minimum inhibitory concentration of imipenem or ceftazidime and the minimum inhibitory concentration of imipenem or ceftazidime plus cloxacillin. One isolate was identified as AmpC producer using three methods. Three isolates produced AmpC as detected by both AmpC disk test and combined disk methods and 19 isolates were found as AmpC producer using both AmpC disk test and minimum inhibitory concentration methods. Six isolates were AmpC producer as shown by the MICs of both imipenem and ceftazidime. Conclusion: According to the results of this study, AmpC-beta-lactamase looks to be the main mechanism of resistance of Pseudomonas aeruginosa to cephalosporins and carbapenems in the study hospital.