Effect of N-Acetylcysteine in Combination with Antibiotics on the Biofilms of Three Cystic Fibrosis Pathogens of Emerging Importance

Abstract

Cystic fibrosis (CF) is a genetic disorder causing dysfunctional ion transport resulting in accumulation of viscous mucus that fosters chronic bacterial biofilm-associated infection in the airways. <i>Achromobacter xylosoxidans</i> and <i>Stenotrophomonas maltophilia</i> are increasingly prevalent CF pathogens and while <i>Burkholderia cencocepacia</i> is slowly decreasing; all are complicated by multidrug resistance that is enhanced by biofilm formation. This study investigates potential synergy between the antibiotics ciprofloxacin (0.5-128 µg/mL), colistin (0.5-128 µg/mL) and tobramycin (0.5-128 µg/mL) when combined with the neutral pH form of <i>N</i>-Acetylcysteine (NAC_neutral) (0.5-16.3 mg/mL) against 11 cystic fibrosis strains of <i>Burkholderia, Stenotrophomonas</i> and <i>Achromobacter</i> sp. in planktonic and biofilm cultures. We screened for potential synergism using checkerboard assays from which fraction inhibitory concentration indices (FICI) were calculated. Synergistic (FICI ≤ 0.5) and additive (0.5 > FICI ≥ 1) combinations were tested on irreversibly attached bacteria and 48 h mature biofilms via time-course and colony forming units (CFU/mL) assays. This study suggests that planktonic FICI analysis does not necessarily translate to reduction in bacterial loads in a biofilm model. Future directions include refining synergy testing and determining further mechanisms of action of NAC to understand how it may interact with antibiotics to better predict synergy.