Interdependence between iron acquisition and biofilm formation in Pseudomonas aeruginosa

Abstract

Bacterial biofilms remain a persistent threat to human healthcare due to their role in the development of antimicrobial resistance. To combat multi-drug resistant pathogens, it is crucial to enhance our understanding of not only the regulation of biofilm formation, but also its contribution to bacterial virulence. Iron acquisition lies at the crux of these two subjects. In this review, we discuss the role of iron acquisition in biofilm formation and how hosts impede this mechanism to defend against pathogens. We also discuss recent findings that suggest that biofilm formation can also have the reciprocal effect, influencing siderophore production and iron sequestration.

Keywords: Pseudomonas aeruginosa; biofilm; exopolysaccharides; iron acquisition; nutritional immunity; siderophore.

Fig. 1.. Interfering with bacterial iron acquisition inhibits biofilm formation.

Under iron-replete conditions, free iron can be either directly transported into the bacterium by active transport systems or indirectly transported via iron scavenging molecules (e.g., pyoverdine, pyochelin, PQS, etc.). Under these conditions bacteria retain the ability to form mature biofilms. In the presence of iron chelators (such as EDTA, EDDHA, or heterologous siderophores that P. aeruginosa cannot utilize, like deferoxamine), however, iron availability is restricted and biofilm formation is compromised. This indicates that iron uptake is necessary for biofilm formation. Furthermore, certain metals like gallium can compete against iron for bacterial iron-scavenging molecules, preventing iron uptake and inhibiting biofilm formation.

Fig. 2.. Hosts and pathogens compete to sequester iron from the environment.

Host cells secrete iron-sequestering proteins such as transferrin and lactoferrin to minimize free extracellular iron. Some bacterial pathogens secrete siderophores to compete against these proteins and scavenge the trace amounts of free iron. Certain siderophores, such as enterobactin (from E. coli and S. typhimurium) and pyoverdine (from P. aeruginosa), can directly remove ferric iron from iron-bound transferrin and lactoferrin, increasing pathogen iron uptake and promoting biofilm formation. To interfere with siderophore activity, certain host cells secrete lipocalin 2 (also known as NGAL) to recognize and bind to siderophores, preventing their function. Production of siderophores that can evade lipocalin 2 has also been linked to pathogenicity.

Fig. 3.. P. aeruginosa biofilm can store excess iron.

In P. aeruginosa biofilms, secreted exopolysaccharides and Pf4 bacteriophages can bind iron, sequestering it in the extracellular matrix. Alginate and Pf4 can sequester ferric iron, Pel exopolysaccharide can sequester ferrous iron, and Psl exopolysaccharide can sequester both. In the case of Psl, P. aeruginosa can utilize iron-bound Psl for growth and bio-film formation under conditions of iron restriction. Furthermore, aggregated cells (in both planktonic and sessile states) exhibit increased pyoverdine production, suggesting another regulatory mechanism for iron uptake.