Pseudomonas aeruginosaCan 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. Pseudomonas aeruginosa and Streptococcus spp. are among the most prevalent organisms in the CF lung; the presence of P. aeruginosa correlates with lung function decline and the Streptococcus milleri group (SMG), a subgroup of the viridans streptococci, is associated with exacerbations in patients with CF. Here we characterize the interspecies interactions that occur between these two genera. We demonstrated that multiple P. aeruginosa laboratory strains and clinical CF isolates promote the growth of multiple SMG strains and oral streptococci in an in vitro coculture system. We investigated the mechanism by which P. aeruginosa enhances growth of streptococci by screening for mutants of P. aeruginosa PA14 unable to enhance Streptococcus growth, and we identified the P. aeruginosa pqsL::TnM mutant, which failed to promote growth of S. constellatus and S. sanguinis. Characterization of the P. aeruginosa ΔpqsL mutant revealed that this strain cannot promote Streptococcus growth. Our genetic data and growth studies support a model whereby the P. aeruginosa ΔpqsL mutant overproduces siderophores, and thus likely outcompetes Streptococcus sanguinis for limited iron. We propose a model whereby competition for iron represents one important means of interaction between P. aeruginosa and Streptococcus spp.ImportanceCystic 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 P. aeruginosa 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 add to our limited knowledge of polymicrobial interactions in the CF airway.

Availability of Zinc Impacts Interactions BetweenStreptococcus sanguinisandPseudomonas aeruginosain Co-culture

“…We report that co-cultivation with P. aeruginosa results in enhanced growth of S. sanguinis SK36 on either plastic or CF-derived airway cells, while P. aeruginosa growth is relatively unaffected during coculture. These data are consistent with previous reports from our group and others that coculture of streptococci with P. aeruginosa promotes the growth of streptococci, but with no obvious benefit to P. aeruginosa growth (8–10). Interestingly, in a study examining the interactions between P. aeruginosa and oral streptococci, including S. sanguinis (12, 63), P. aeruginosa growth was inhibited when streptococci were grown as a pioneer colonizer; streptococci can produce hydrogen peroxide (H 2 O 2 ) to react with excess nitrite in the medium to generate reactive nitrogenous intermediates (RNI) for the inhibition of P. aeruginosa growth (12, 13, 63).…”

“…We have previously shown that P. aeruginosa can enhance S. sanguinis SK36 growth either as planktonic or biofilm cells in coculture compared to that of S. sanguinis SK36 monoculture, while the P. aeruginosa population was not significantly impacted by the presence of S. sanguinis (8, 10), a finding confirmed here Fig. S1A.…”

“…CFTR dysfunction affects several body systems, and progressive lung disease due to chronic and recurrent microbial infections is the leading cause of morbidity and mortality in individuals with CF (4, 5). It has been shown that CF airway infections are polymicrobial (6, 7), and the composition and interspecies interactions within the polymicrobial communities can have profound and diverse consequences, including on bacterial growth (8–10), as well as disease progression and therapeutic outcomes (6, 11).…”

“…An example of such microbial interactions includes the CF-associated streptococcal species, the presence of which may influence the growth and/or virulence of other CF pathogens, including the important pathogen Pseudomonas aeruginosa (12, 13). In turn, P. aeruginosa can impact the growth and/or persistence of streptococci (8–10, 14), with the net impact of these interactions resulting in exacerbation (4, 15–17) or less server loss of lung function (11, 16, 18–21). Therefore, understanding how these pathogens interact with each other and their multicellular host to impact disease progression, as well as how these interactions are modified by the CF airway environment, is of high significance.…”

“…We recently showed that P. aeruginosa can enhance the growth of multiple oral Streptococcus spp. in coculture conditions (8, 10), but the molecular mechanism underlying such interactions is still poorly understood. In the present work, we used a comprehensive Streptococcus sanguinis SK36 mutant library to determine the genetic requirements for S. sanguinis SK36 to benefit from interaction with P. aeruginosa .…”