Antibacterial Effects of Bicarbonate in Media Modified to Mimic Cystic Fibrosis Sputum

Abstract

Cystic fibrosis (CF) is a hereditary disease caused by mutations in the gene encoding an epithelial anion channel. In CF, Cl^- and HCO₃^- hyposecretion, together with mucin hypersecretion, leads to airway dehydration and production of viscous mucus. This habitat is ideal for colonization by pathogenic bacteria. We have recently demonstrated that HCO₃^- inhibits the growth and biofilm formation of <i>Pseudomonas aeruginosa</i> and <i>Staphylococcus aureus</i> when tested in laboratory culture media. Using the same bacteria our aim was to investigate the effects of HCO₃^- in artificial sputum medium (ASM), whose composition resembles CF mucus. Control ASM containing no NaHCO₃ was incubated in ambient air (pH 7.4 or 8.0). ASM containing NaHCO₃ (25 and 100 mM) was incubated in 5% CO₂ (pH 7.4 and 8.0, respectively). Viable <i>P. aeruginosa</i> and <i>S. aureus</i> cells were counted by colony-forming unit assay and flow cytometry after 6 h and 17 h of incubation. Biofilm formation was assessed after 48 h. The data show that HCO₃^- significantly decreased viable cell counts and biofilm formation in a concentration-dependent manner. These effects were due neither to extracellular alkalinization nor to altered osmolarity. These results show that HCO₃^- exerts direct antibacterial and antibiofilm effects on prevalent CF bacteria.