Evaluation of long-term co-administration of tobramycin and clarithromycin in a mature biofilm model of cystic fibrosis clinical isolates of Pseudomonas aeruginosa

Pseudomonas aeruginosa colonisation and chronic lung infection associated with biofilm formation is a major cause of morbidity and mortality in cystic fibrosis (CF) patients. There is thus an urgent need to develop alternative ways to treat biofilm-associated clinical infections. A kinetic study of twice-daily co-administration of the antibiotics tobramycin and clarithromycin was performed over 28 days on 12-day-old mature P. aeruginosa biofilms formed on microplate pegs for 23 clinical isolates of various phenotypes and genotypes to simulate the treatment of CF patients with inhaled tobramycin through aerosolisation (TOBI^®). Drug activity was assessed by enumeration of persistent bacteria before, during and after treatment. A mature (12-day-old) biofilm model was chosen because a previous study suggested that such a biofilm was a more realistic in vitro model than a 24-h-old biofilm. Synergistic activity of the drug combination was confirmed on biofilms of 9/23 P. aeruginosa isolates. Of these nine isolates, total destruction of the biofilm was observed for five of them. Combination treatment was superior or equivalent to treatment with tobramycin alone, as activity was observed on 47.8% of the isolates with the combination versus 26.1% with tobramycin alone. No correlation was observed between drug susceptibility profiles and the phenotype or genotype of the isolates.