An in-vitro analysis of β-lactam combinations with aminoglycosides versus colistin among multi-drug resistant Pseudomonas aeruginosa

Abstract

We tested 94 multi-drug resistant Pseudomonas aeruginosa isolates by the checkerboard methods using only serum achievable concentration (SAC) wells. Among combinations when both agents were non-susceptible, a colistin-β-lactam combination significantly increased the likelihood of inhibiting P. aeruginosa growth in at least 1 SAC well compared to an aminoglycoside-β-lactam combination (15/19 (79.0%) vs 39/160 (24.4%); p= 0.0001).  When the beta-lactam was non-susceptible but the aminoglycoside or colistin agent was considered susceptible, almost all combinations were able to inhibit growth of at least 1 SAC well with no significant differences between combinations. However, in this subset use of an aminoglycoside combination significantly increased the likelihood of inhibiting P. aeruginosa growth in all SAC wells (59/128 (46.1%) vs 1/211 (0.5%); p= 0.0001). In conclusion, for a known β-lactam resistant P. aeruginosa a regimen with an aminoglycoside is preferred to colistin when both are considered susceptible. However, for pan-drug resistant P. aeruginosa a colistin combination may be preferred due to the probability of inhibiting growth of at least one SAC well.