____________________________________________________________________________________________ *Corresponding author: Email: stokajian@lau.edu.lb; British Microbiology Research Journal 2(4): 243-250, 2012 SCIENCEDOMAIN international www.sciencedomain.org Molecular Characterization, Multiple Drug Resistance, and Virulence Determinants of Pseudomonas aeruginosa Isolated from Lebanon Sima Tokajian 1* , Rola Timani 1 , Nahla Issa 1 and George Araj 2 1 Genomics and Proteomics Research lab, Lebanese American University, Byblos, Lebanon. 2 Department of Pathology and Laboratory Medicine, American University of Beirut, Medical Centre, Beirut, Lebanon. Authors’ contributions This work was carried out in collaboration between all authors. Author ST supervised, did all the coordination and analyses and wrote the manuscript, author RT did the PFGE and author NI did the toxin profiling and the antibiotic testing and author GA supervised and provided the clinical samples. All authors read and approved the final manuscript. Received 13 th September 2012 Accepted 19 th November 2012 Published 19 th December 2012 ABSTRACT Aims: Typing and characterization of 100 P. aeruginosa clinical isolates by pulse field gel electrophoresis (PFGE) to detect changes in the clonal composition of local strains and to correlate banding patterns with site of infection, drug resistance and Type III secretion system effectors. Methodology: A total of 100 P. aeruginosa isolates obtained from clinical specimens were used to study resistance profiles, PFGE banding patterns and virulence determinants. Results: Results from antimicrobial susceptibility testing yielded showed that 77 of the strains were multi drug resistant (MDR). Grouping isolates as non-susceptible when tested intermediate and resistant accordingly showed that resistance was 25% each for imipenem and piperacillin-tazobactam, while it was 29% for ceftazidime these drugs are among the ones most commonly used in treating infections caused by P. aeruginosa. Studying effectors released by the type III secretion system including exoU and exoS revealed that 48% of the isolates harbored exoS toxin gene and 46% the exoU, with 3% having both and 8% having none. When the different pulsotypes were compared on a dendogram, 45 Short Communication