Objectives: This study aimed to evaluate the efficacy of inhaled cationic antibiotics, including tobramycin (TOB) and colistin (CST), using an in vitro alginate bead model that simulates Pseudomonas aeruginosa lung biofilms.
Methods: Bioluminescent P. aeruginosa were encapsulated within alginate beads and dispersed in either Mueller-Hinton broth (MHB) or artificial sputum medium (ASM). The impact of bead size and culture medium on TOB and CST efficacy was assessed by monitoring bioluminescence kinetics, followed by colony-forming unit (CFU/mL) measurements. Antibiotic efficacy was quantified using a Hill inhibitory model to analyze variations in CFU/mL in response to TOB and CST concentrations.
Results: The TOB EC50 was found to be 8-fold higher when P. aeruginosa was encapsulated in larger beads (1200 μm) compared to smaller beads (60 μm). TOB efficacy further decreased twofold when larger beads were dispersed in ASM. In contrast, CST demonstrated superior efficacy, being four times more potent than TOB, with corresponding EC50 values of 20.5 ± 2.8 times MIC and 78.4 ± 10.2 times MIC, respectively. No change in MICs was observed for either antibiotic, even after exposing bacteria to 200 times the MIC.
Conclusions: This P. aeruginosa biofilm model highlights how alginate and mucus modulated the efficacy of TOB and CST, and suggested the superior efficacy of CST in eradicating pulmonary biofilms.