The aim of the thesis work was to analyse the in vitro effectiveness of ceftolozane/tazobactam against 87 carbapenem or ceftazidime resistant Pseudomonas aeruginosa strains. Among these strains there were 22 extremely drug resistant (XDR) strains which were susceptible to only colistin. From the results obtained, 84 strains were susceptible to the novel drug. This shows that the new drug is a highly effective drug in the treatment of complicated infections and ventilator-associated pneumonia caused by Pseudomonas aeruginosa.