Trends of antimicrobial resistance and combination susceptibility testing of cystic fibrosis multidrug-resistant Pseudomonas aeruginosa: A ten-year update

Ijeoma Okoliegbe* (Corresponding Author), Karolin Hijazi, Kim Cooper, Corinne Ironside, Ian Gould

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

9 Citations (Scopus)
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Background: Antimicrobial combination therapy is a time/resource- intensive procedure commonly employed in the treatment of cystic fibrosis (CF) pulmonary exacerbations caused by P. aeruginosa. Ten years ago the most promising antimicrobial combinations were proposed, but there has since been the introduction of new β-lactam+β-lactamase inhibitor antimicrobial combinations. The aims of this study were i) to compare in vitro activity of these new antimicrobials with other anti23 pseudomonals agents and suggest their most synergistic antimicrobial combinations. ii) to determine antimicrobial resistance rates and study inherent trends of antimicrobials over ten years.Methods: A total of 721 multidrug-resistant P. aeruginosa isolates from 183 patients were collated over the study period. Antimicrobial susceptibility and combination testing were carried out using the Etest method. The results were further assessed using the fractional inhibitory concentration index (FICI) and the susceptible breakpoint index (SBPI).Results: Resistance to almost all antimicrobial agents maintained a similar level during the studied period. Colistin (p<0.001) and tobramycin (p=0.001) were the only antimicrobials with significant increasing isolate susceptibility while an increasing resistance trend was observed for levofloxacin. The most active antimicrobials were colistin, ceftolozane/tazobactam, ceftazidime/avibactam, and gentamicin. All combinations with β-lactam+β-lactamase inhibitors produced some synergistic results. Ciprofloxacin+ceftolozane/tazobactam (40%) and amikacin+ceftazidime (36.7%) were the most synergistic combinations while colistin combinations gave the best median SPBI (50.11). Conclusions: This study suggests that effective fluoroquinolone stewardship should be employed for CF patients. It also presents in vitro data to support the efficacy of novel combinations for use in the treatment of chronic P. aeruginosa infections.
Original languageEnglish
Article numbere02483-20
Number of pages10
JournalAntimicrobial Agents and Chemotherapy
Issue number6
Early online date5 Apr 2021
Publication statusPublished - 18 May 2021

Bibliographical note

The authors would like to thank the laboratories and clinicians who use the Cystic Fibrosis Antibiotics Susceptibility testing service (CFASS) for their support in sending samples. CFASS is an adult patient testing facility funded by the National Services Division of the Common Services Agency. IMG serves as a consultant to and/ speaker to Pfizer and MSD. All other authors declare no competing interests.


  • Pseudomonas aeruginosa
  • Cystic Fibrosis
  • Antimicrobial susceptibility testing
  • Synergy testing
  • Etest


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