Antimicrobial activity of fosfomycin and tobramycin in combination against cystic fibrosis pathogens under aerobic and anaerobic conditions

BackgroundThere is a need for new antibiotics or combination of antibiotics that possess activity against increasingly resistant cystic fibrosis (CF) respiratory pathogens such as Pseudomonas aeruginosa and MRSA.MethodsThe antimicrobial activity of a novel 4:1 (w/w) fosfomycin:tobramycin (F:T) combination against CF respiratory pathogens under both aerobic and anaerobic conditions was determined by MIC, time-kill and biofilm studies, and compared with activity of fosfomycin and tobramycin, individually.ResultsF:T and fosfomycin had excellent activity against P. aeruginosa and were more active than tobramycin against P. aeruginosa under anaerobic conditions with lower MIC50, MIC90 and geometric mean values. F:T (p < 0.001) and fosfomycin (p < 0.001) MICs for P. aeruginosa were significantly lower under anaerobic conditions with tobramycin MICs significantly higher (p < 0.001). F:T and fosfomycin also had high activity against MRSA with both being more active than tobramycin. In time–kill studies, F:T was rapidly bactericidal against all 15 P. aeruginosa and 3/5 MRSA isolates tested. F:T also demonstrated bactericidal activity against P. aeruginosa grown in biofilm under both aerobic and anaerobic conditions.ConclusionsF:T has promising in vitro antimicrobial activity against MRSA and P. aeruginosa with greater activity under anaerobic conditions similar to those found in the CF lung.