In vitro activity of ceftolozane–tazobactam and ceftazidime–avibactam against clinical isolates of meropenem-non-susceptible Pseudomonas aeruginosa: A two-centre study

Objectives This study aimed to compare the activity of ceftazidime–avibactam (C/A), ceftolozane–tazobactam (C/T) and three anti-pseudomonal β-lactams (piperacillin–tazobactam, ceftazidime and cefepime) against a collection of meropenem-non-susceptible Pseudomonas aeruginosa ( P. aeruginosa ) clinical isolates recovered from two centres in Turkey. Methods A total of 102 unique patient isolates of meropenem-non-susceptible P. aeruginosa were included in the study. MICs of antimicrobials were determined by the gradient diffusion method. Results Overall susceptibility rates for C/A and C/T were 83.3% and 82.4%, respectively. Both C/A and C/T had better activity than any one of the three anti-pseudomonal β-lactams. According to the MIC 50 values, C/T was the most potent agent against isolates. Although the susceptibility rates of isolates to C/T and C/A were similar, C/T (MIC 50 , 1 μg/mL) was four-fold more potent than C/A (MIC 50 , 4 μg/mL). The MIC 50 values of C/A and C/T for the isolates that were non-susceptible to three β-lactams were significantly higher than those for isolates that were non-susceptible to zero, one or two β-lactams. Also, the C/A MIC 50 value for the isolates that were non-susceptible to two β-lactams was higher than that for isolates which were non-susceptible to one β-lactam. Conclusions C/A and C/T showed good activity against meropenem-non-susceptible P. aeruginosa isolates. However, resistance to these agents was not uncommon among these isolates. The overall β-lactam susceptibility profile of isolates seems to have an effect on the probability of susceptibility to C/A and C/T. Antimicrobial susceptibility testing should be performed for C/A and C/T if these agents are considered for treatment of infections caused by meropenem-non-susceptible P. aeruginosa .