AbstractObjectivesCystic fibrosis (CF) patients are often colonized withPseudomonas aeruginosa. During treatment,P. aeruginosacan develop subpopulations exhibiting variablein vitroantimicrobial (ABX) susceptibility patterns. Heteroresistance (HR) may underlie reported discrepancies betweenin vitrosusceptibility results and clinical responses to various ABXs. Here, we sought to examine the presence and nature ofP. aeruginosapolyclonal HR (PHR) and monoclonal HR (MHR) to ceftolozane/tazobactam in isolates originating from CF pulmonary exacerbations.MethodsThis was a single-centre, non-controlled study. Two hundred and forty-sixP. aeruginosaisolates from 26 adult CF patients were included. PHR was defined as the presence of different ceftolozane/tazobactam minimum inhibitory concentration (MIC) values amongP. aeruginosaisolates originating from a single patient specimen. Population analysis profiles (PAPs) were performed to assess the presence of MHR, defined as ≥4-fold change in the ceftolozane/tazobactam MIC from a singleP. aeruginosacolony.ResultsSixteen of 26 patient specimens (62%) contained PHRP. aeruginosapopulations. Of these 16 patients, 6 (23%) had specimens in which PHRP. aeruginosaisolates exhibited ceftolozane/tazobactam MICs with categorical differences (i.e. susceptible versus resistant) compared to results reported as part of routine care. One isolate, PSA 1311, demonstrated MHR. Canonical ceftolozane/tazobactam resistance genes were not found in the MHR isolates (MHR PSA 1311 or PHR PSA 6130).ConclusionsCeftolozane/tazobactam PHR exists amongP. aeruginosaisolates in this work, and approximately a quarter of these populations contained isolates with ceftolozane/tazobactam susceptibiilty interpretations different from what was reported clinically, supporting concerns surrounding the utility of traditional susceptibility testing methodology in the setting of CF specimens. Genome sequencing of isolates with acquired MHR to ceftolozane/tazobactam revealed variants of unknown significance. Future work will be centred on determining the significance of these mutations to better understand these data in clinical context.