Novel intermicrobial molecular interaction: Pseudomonas aeruginosa Quinolone Signal (PQS) modulates Aspergillus fumigatus response to iron

Abstract

<i>Pseudomonas aeruginosa</i> (Pa) and <i>Aspergillus fumigatus</i> (Af), the commonest bacterium and fungus in compromised host airways, compete for iron (Fe). The Pseudomonas quinolone signal (PQS), a Pa quorum sensing molecule, also chelates Fe, and delivers Fe to the Pa cell membrane using Pa siderophores. In models of Af biofilm formation or preformed biofilms, PQS inhibited Af in a low Fe environment. AfΔ<i>sidA</i> (mutant unable to produce siderophores) biofilm was more sensitive to PQS inhibition than wild-type (WT), as was planktonic AfΔ<i>sidA</i> growth. PQS decreased WT Af growth on agar. All these inhibitory actions were reversed by Fe. The Pa siderophore pyoverdin, or Af siderophore inhibitor celastrol, act cooperatively with PQS in Af inhibition. These findings all indicate PQS inhibition is owing to Fe chelation. <i>Remarkably</i>, in high Fe environments<i>,</i> PQS <i>enhanced</i> Af biofilm at 1/100 to 1/2000 Fe concentration required for Fe alone to enhance. Planktonic Af growth, and on agar, Af conidiation, were also enhanced by PQS+Fe compared to Fe alone. In contrast, neither AfΔ<i>sidA</i> biofilm, nor planktonic AfΔ<i>sidA</i>, were enhanced by PQS-Fe compared to Fe. When Af siderophore ferricrocin (FC),+PQS, were added to AfΔ<i>sidA,</i> Af was then boosted more than by FC alone. Moreover, FC+PQS+Fe boosted AfΔ<i>sidA</i> more than Fe, FC, FC+Fe, PQS+FC or PQS+Fe. Thus PQS-Fe maximal stimulation requires Af siderophores. PQS inhibits Af via chelation under low Fe conditions. In a high Fe environment, PQS paradoxically stimulates Af efficiently, and this involves Af siderophores. PQS production by Pa could stimulate Af in cystic fibrosis airways, where Fe homeostasis is altered and Fe levels increase, supporting fungal growth.