Use of Calgary and microfluidic BioFlux systems to test the activity of fosfomycin and tobramycin alone and in combination against cystic fibrosis Pseudomonas aeruginosa biofilms [PublishAheadOfPrint]

Pseudomonas aeruginosa is a major cause of morbidity and mortality in chronically infected cystic fibrosis patients. Novel in vitro biofilm models, which reliably predict therapeutic success of antimicrobial therapies, should be implemented. The activity of fosfomycin, tobramycin and fosfomycin-tobramycin combination was tested against 6 susceptible P. aeruginosa strains isolated from respiratory samples of cystic fibrosis patients by using two in vitro biofilm models: a closed system (Calgary device) and an open model based on microfluidics (BioFlux). All but one of the isolates formed biofilm. The fosfomycin and tobramycin minimal biofilm inhibitory concentrations (MBIC) were 1,024->1,024 μg/ml and 8-32 μg/ml, respectively. According to fractional inhibitory concentration analysis, the combination behaved synergistically in all the isolates except in the P. aeruginosa ATCC 27853 strain.

The dynamic formation of the biofilm was also studied with the BioFlux system and the MIC and MBIC of each antibiotic were tested. For the combination, the lowest tobramycin concentration that was synergistic with fosfomycin was used. The captured images were analyzed measuring the intensity of colored pixels, which is proportional to the biofilm biomass. A statistically significant difference was found when comparing the intensity of the inoculum with the intensity in the microchannel where the MBIC of tobramycin or fosfomycin or their combination was used (p<0.01) but not when applying the MIC (p>0.01).

Fosfomycin-tobramycin demonstrated to be synergistic against cystic fibrosis P. aeruginosa strains in biofilm models, both when testing with the Calgary and the microfluidic BioFlux systems. These results support the clinical use of this combination.

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