In vitro activity of double and triple antimicrobial combinations against carbapenem-resistant Pseudomonas aeruginosa biofilm

Objectives Pseudomonas aeruginosa is a common microorganism in chronic infections due to biofilm formation and antibiotic resistance. This study aimed to compare the synergistic effects of antibiotic combinations against carbapenem-resistant P. aeruginosa (CRPA) in planktonic and biofilm-embedded states. Methods Twelve CRPA bloodstream isolates from the Asian Bacterial Bank (2016–2018) were analysed. The minimum biofilm eradication concentrations (MBECs) were determined using the peg lid system, and antimicrobial interactions were assessed using biofilm checkerboard assays, testing three double combinations (colistin–rifampin, colistin–imipenem, and rifampin–ceftazidime/avibactam) and two triple combinations (colistin–rifampin–imipenem and colistin–rifampin–ceftazidime/avibactam). Results The MBEC values of all four antimicrobial agents (rifampin, colistin, imipenem, and ceftazidime/avibactam) were significantly higher than their corresponding minimum inhibitory concentration values (P < 0.001). Although single agents required markedly elevated concentrations to eradicate CRPA biofilms, approximately half of the double and triple antimicrobial combinations demonstrated synergistic activity. In the biofilm phase, synergism rates were comparable between triple combinations (colistin–rifampin–ceftazidime/avibactam, 50%; colistin–rifampin–imipenem, 66%) and double combinations (colistin–rifampin, 42%; rifampin–ceftazidime/avibactam, 42%; colistin–imipenem, 66%). The triple combinations showed lower FBEC indices (colistin–rifampin–imipenem: median 0.17; colistin–rifampin–ceftazidime/avibactam: 0.34) than the corresponding double combinations (colistin–rifampin: 0.53; colistin–imipenem: 0.20; rifampin–ceftazidime/avibactam: 0.78), although these differences were not statistically significant. Conclusions Our study provides experimental evidence that antimicrobial combination therapy may offer advantages over single agents for CRPA biofilm eradication, supporting further investigation into the role of such regimens in biofilm-associated infections.