Pseudomonas aeruginosa Can Inhibit Growth of Streptococcal Species via Siderophore Production

Abstract

Cystic fibrosis (CF) is a genetic disease that causes patients to accumulate thick, dehydrated mucus in the lung and develop chronic, polymicrobial infections due to reduced mucociliary clearance. These chronic polymicrobial infections and subsequent decline in lung function are significant factors in the morbidity and mortality of CF. <i>Pseudomonas aeruginosa</i> and <i>Streptococcus</i> spp. are among the most prevalent organisms in the CF lung; the presence of <i>P. aeruginosa</i> correlates with lung function decline, and the <i>Streptococcus milleri</i> group (SMG), a subgroup of the viridans streptococci, is associated with exacerbations in patients with CF. Here we characterized the interspecies interactions that occur between these two genera. We demonstrated that multiple <i>P. aeruginosa</i> laboratory strains and clinical CF isolates promote the growth of multiple SMG strains and oral streptococci in an <i>in vitro</i> coculture system. We investigated the mechanism by which <i>P. aeruginosa</i> enhances growth of streptococci by screening for mutants of <i>P. aeruginosa</i> PA14 that are unable to enhance <i>Streptococcus</i> growth, and we identified the <i>P. aeruginosa</i><i>pqsL</i>::Tn<i>M</i> mutant, which failed to promote growth of <i>Streptococcus constellatus</i> and <i>S. sanguinis</i> Characterization of the <i>P. aeruginosa</i> Δ<i>pqsL</i> mutant revealed that this strain cannot promote <i>Streptococcus</i> growth. Our genetic data and growth studies support a model whereby the <i>P. aeruginosa</i> Δ<i>pqsL</i> mutant overproduces siderophores and thus likely outcompetes <i>Streptococcus sanguinis</i> for limited iron. We propose a model whereby competition for iron represents one important means of interaction between <i>P. aeruginosa</i> and <i>Streptococcus</i> spp.<b>IMPORTANCE</b> Cystic fibrosis (CF) lung infections are increasingly recognized for their polymicrobial nature. These polymicrobial infections may alter the biology of the organisms involved in CF-related infections, leading to changes in growth, virulence, and/or antibiotic tolerance, and could thereby affect patient health and response to treatment. In this study, we demonstrate interactions between <i>P. aeruginosa</i> and streptococci using a coculture model and show that one interaction between these microbes is likely competition for iron. Thus, these data indicate that one CF pathogen may influence the growth of another, and they add to our limited knowledge of polymicrobial interactions in the CF airway.