Abstract
Patients with cystic fibrosis (CF) experience lifelong respiratory infections, which are a significant cause of morbidity and death. These infections are polymicrobial in nature, and the predominant bacterial species undergo a predictable series of changes as patients age. Young patients have populations dominated by opportunists that are typically found within the microbiome of the human nasopharynx, such as nontypeable Haemophilus influenzae (NTHi); these are eventually supplanted, and the population within the CF lung is later dominated by pathogens such as Pseudomonas aeruginosa. In this study, we investigated how initial colonization with NTHi impacts colonization and persistence of P. aeruginosa in the respiratory tract. Analysis of polymicrobial biofilms in vitro by confocal microscopy revealed that NTHi promoted greater P. aeruginosa biofilm volume and diffusion. However, sequential respiratory infection of mice with NTHi followed by P. aeruginosa resulted in significantly lower levels of P. aeruginosa, compared to infection with P. aeruginosa alone. Coinfected mice also had reduced airway tissue damage and lower levels of inflammatory cytokines, compared with P. aeruginosa-infected mice. Similar results were observed after instillation of heat-inactivated NTHi bacteria or purified NTHi lipooligosaccharide endotoxin prior to P. aeruginosa introduction. Based on these results, we conclude that NTHi significantly reduces susceptibility to subsequent P. aeruginosa infection, most likely due to priming of host innate immunity rather than a direct competitive interaction between species. These findings have potential significance with regard to therapeutic management of early-life infections in patients with CF.