Localization of Burkholderia cepacia Complex Bacteria in Cystic Fibrosis Lungs and Interactions with Pseudomonas aeruginosa in Hypoxic Mucus

The localization of Burkholderia cepacia complex (Bcc) bacteria in the cystic fibrosis (CF) lung, alone or during co-infection with Pseudomonas (P.) aeruginosa, is poorly understood. We performed immunohistochemistry for Bcc and P. aeruginosa on 21 co-/or singly-infected CF lungs obtained at transplantation or autopsy. Parallel in vitro experiments examined growth of two Bcc species, Burkholderia cenocepacia and Burkholderia multivorans, in environments similar to those occupied by P. aeruginosa in the CF lung. Bcc was predominantly identified in the CF lung as single cells or small clusters within phagocytes and mucus, but not as "biofilm-like structures". In contrast, P. aeruginosa was identified in biofilm-like masses, but densities appeared to be reduced during co-infection with Bcc. Based on chemical analyses of CF and non-CF respiratory secretions, a test medium was defined to study Bcc growth and interactions with P. aeruginosa in an environment mimicking the CF lung. When test medium was supplemented under anaerobic conditions with alternative electron acceptors, B. cenocepacia and B. multivorans used fermentation rather than anaerobic respiration to gain energy, consistent with the identification of fermentation products by HPLC. Both Bcc species also expressed mucinases that produced carbon sources from mucins for growth. In the presence of P. aeruginosa in vitro, both Bcc species grew anaerobically, but not aerobically. We propose that Bcc: 1) invades a P. aeruginosa infected CF lung when the airway lumen is anaerobic; 2) inhibits P. aeruginosa biofilm-like growth; and 3) expands the host bacterial niche from mucus to also include macrophages.