Pseudomonas aeruginosa Alginate Overproduction Promotes Coexistence with Staphylococcus aureus in a Model of Cystic Fibrosis Respiratory Infection

Abstract

While complex intra- and interspecies microbial community dynamics are apparent during chronic infections and likely alter patient health outcomes, our understanding of these interactions is currently limited. For example, Pseudomonas aeruginosa and Staphylococcus aureus are often found to coinfect the lungs of patients with cystic fibrosis (CF), yet these organisms compete under laboratory conditions. Recent observations that coinfection correlates with decreased health outcomes necessitate we develop a greater understanding of these interbacterial interactions. In this study, we tested the hypothesis that P. aeruginosa and/or S. aureus adopts phenotypes that allow coexistence during infection. We compared competitive interactions of P. aeruginosa and S. aureus isolates from mono- or coinfected CF patients employing in vitro coculture models. P. aeruginosa isolates from monoinfected patients were more competitive toward S. aureus than P. aeruginosa isolates from coinfected patients. We also observed that the least competitive P. aeruginosa isolates possessed a mucoid phenotype. Mucoidy occurs upon constitutive activation of the sigma factor AlgT/U, which regulates synthesis of the polysaccharide alginate and dozens of other secreted factors, including some previously described to kill S. aureus. Here, we show that production of alginate in mucoid strains is sufficient to inhibit anti-S. aureus activity independent of activation of the AlgT regulon. Alginate reduces production of siderophores, 2-heptyl-4-hydroxyquinolone-N-oxide (HQNO), and rhamnolipids—each required for efficient killing of S. aureus. These studies demonstrate alginate overproduction may be an important factor driving P. aeruginosa coinfection with S. aureus.

Genotypic and phenotypic diversity ofStaphylococcus aureusfrom cystic fibrosis lung infections and their interactions withPseudomonas aeruginosa

“…Previously, it has been shown that nonmucoid PAO1 kills S. aureus lab isolate JE2, while mucoid PAO1 does not [13]. Therefore, we were curious whether this trend would be maintained with CF clinical isolates of S. aureus and consequently all of our 65 S. aureus isolates were assessed in a coculture assay with both nonmucoid and mucoid PAO1.…”

“…Therefore, we were curious whether this trend would be maintained with CF clinical isolates of S. aureus and consequently all of our 65 S. aureus isolates were assessed in a coculture assay with both nonmucoid and mucoid PAO1. While a number of different techniques have been used to look at how P. aeruginosa and S. aureus survive in coculture [13, 51-55], we have developed a simple, repeatable, well-controlled, quantitative assay for monitoring these interactions in vitro . This coculture assay allows for these bacteria to come in contact with one another on a solid surface as they might in a biofilm in the lung.…”

“…Research attempting to understand this health decline has mainly focused on the killing interactions between these two microbes, where it has been noted that P. aeruginosa readily kills S. aureus in vitro [9-12]. Furthermore, we have previously found that the mucoid phenotype of P. aeruginosa , which is associated with a chronic infection state, aids in coexistence with S. aureus [13], but this was performed on one reference isolate of S. aureus from wound infection, JE2.…”

Coexistence withPseudomonas aeruginosaaltersStaphylococcus aureustranscriptome, antibiotic resistance and internalization into epithelial cells

“…Two types of interactions between S. aureus and P. aeruginosa could be observed with CF patient isolates: the well-described competitive phenotype, where P. aeruginosa inhibits S. aureus growth, 16 and the newly described phenotype of coexistence, where P. aeruginosa is unable to outcompete S. aureus 19–21 . In order to quantify the importance of this last phenotype, we collected 50 pairs of S. aureus and P. aeruginosa from 36 co-infected CF patients.…”

Allelic polymorphism shapes community function in evolving Pseudomonas aeruginosa populations

“…Studies on P. aeruginosa populations isolated from individual lungs, have demonstrated divergent evolution, resulting in heterogeneous populations of P. aeruginosa within patients (16,17,19). This genetic adaptation and diversification is likely to impact on levels of pathogenicity and the efficacy of antibiotic treatment (16,20), and could potentially impact how other species of microbe colonize the CF lung (4,(21)(22)(23)(24)(25)(26)(27).…”