Multicentre study of thein vitroactivity of ceftolozane/tazobactam and other commonly used antibiotics againstPseudomonas aeruginosaisolates from patients in the UK

Objectives To evaluate the in vitro activity of ceftolozane/tazobactam and other commonly used antipseudomonal antibiotics against geographically spread Pseudomonas aeruginosa isolates in the UK using disc susceptibility testing. Methods The in vitro activity of ceftolozane/tazobactam and nine other commonly used antipseudomonal antibiotics was evaluated. Isolates were collected between January 2015 and April 2018. Susceptibility results were interpreted using EUCAST 2018 criteria. Results Overall, 1326 clinical isolates from 14 centres in the UK were tested. The majority of the isolates were collected from non-cystic fibrosis (non-CF) patients ( n  =   1123, 85.0%). In addition, 199 cystic fibrosis (CF) isolates were collected from 10 centres. Overall susceptibility to ceftolozane/tazobactam was 89.3% ( n  =   1181), which included 128 CF and 1053 non-CF isolates. The other antibacterial agents with the highest susceptibility were tobramycin (92.4%, n  =   1221) and piperacillin/tazobactam (90.7%, n  =   1199). Susceptibility to all antibacterial agents was lower for CF isolates. Piperacillin/tazobactam was the most active of the antibacterial agents tested, followed by ceftolozane/tazobactam (70.4% and 64.3%, respectively), and <60% of CF isolates were susceptible to ceftazidime and the carbapenems. The reason for the higher rates of susceptibility to piperacillin/tazobactam and lower susceptibility to ceftazidime compared with other studies is unclear. Conclusions The data presented here support the need to investigate the place of ceftolozane/tazobactam as a treatment option in the management of pseudomonal infections, particularly in patients with CF. The results highlight the importance of routine testing of new antibacterial agents and of making the data available to clinicians to make appropriate and informed treatment choices.