Synergistic effects of colistin-rifampin-based triple antimicrobial combination therapy against Carbapenem-resistant Pseudomonas aeruginosa: a time-kill assay

Background: Our research aimed to investigate the potential of in vitro triple antimicrobial synergism against carbapenem-resistant Pseudomonas aeruginosa (CRPA) as a strategy to overcome antimicrobial resistance. Methods: We used 12 CRPA blood isolates stocked in the Asian Bacterial Bank between 2016 and 2018. All isolates were tested by multi-locus sequencing and carbapenemase multiplex PCR. To assess the antimicrobial interactions, we performed time-kill assays using double or triple combination regimens. These regimens included CST and/or rifampin combined with IPM, MEM, or CZA. The assay was conducted at 1× and 0.5× MICs. Results: Among the 12 CRPA isolates, nine produced metallo-beta-lactamases (6 IMP-6, 2 VIM-2 and 1 NDM-1). In the time-kill assay, the median viable bacterial count for CST-rifampin was the lowest among double combinations after 24h incubation (2.25 log cfu/mL at 1× MIC and 3.71 log cfu/mL at 0.5× MIC). In contrast, all triple combinations achieved 0 log cfu/mL at both 1× MIC and 0.5× MIC. Compared with CST-rifampin (synergism: 25% at 1× MIC, 42% at 0.5× MIC; bactericidal: 50% at 1× MIC, 42% at 0.5× MIC), all triple combinations showed greater synergism and bactericidal activity at both 1× MIC (50%-75% for synergism, 75%-83% for bactericidal activity) and 0.5× MIC (58%-75% for both). Conclusions: Our findings suggest that CST-rifampin-based triple antimicrobial combinations exhibit greater synergy and bactericidal activity in eradicating CRPA compared with double antimicrobial combinations.