Antimicrobial susceptibility patterns of multidrug-resistant Pseudomonas aeruginosa and Acinetobacter baumannii against carbapenems, colistin, and tigecycline.

Objective: To examine susceptibility of Pseudomonas aeruginosa (P. aeruginosa) and Acinetobacter baumannii (A. baumannii ) against carbapenems along with colistin and tigecycline as alternative therapeutic options.

Methods: A total of 117 strains of multidrug-resistant (MDR) non-fermenting Gram negative bacteria isolated from non-duplicate samples were collected consecutively. We included one sample from each patient (84 isolates of A. baumannii and 33 isolates of P. aeruginosa isolated from patients seen at King Khalid University Hospital, Riyadh, Saudi Arabia, from June to December 2010). Isolates were identified by the MicroScan WalkAway 96 Plus system. The minimum inhibitory concentrations (MICs) were determined by E-test following the Clinical and Laboratory Standards Institute breakpoint recommendations.

Results: Most A. baumannii strains were resistant to imipenem (90.5%), meropenem (90.5%), and doripenem (77.4%). Whereas, a higher percentage of P. aeruginosa was resistant to imipenem (90.9%), and meropenem (81.8%), only 39.4% were resistant to doripenem. Colistin had excellent activity against both A. baumannii (100%) and P. aeruginosa (93.9%), while 89.3% of A. baumannii strains were susceptible to tigecycline.

Conclusion: Among the carbapenems, doripenem was found to be the most potent antimicrobial agent against P. aeruginosa, whereas colistin proved to be an effective alternative antimicrobial agent for treatment of A. baumannii or P. aeruginosa. Tigecycline remains the best therapeutic option for MDR A. baumannii.