A lung pathogen grows stronger in iron-rich environments, but at the cost of its own virulence, revealing a hidden trade-off behind chronic infection.
Study: Iron dictates the growth, biofilm formation, and virulence of Pseudomonas aeruginosa in pulmonary infections. Image credit: Christoph Burgstedt/Shutterstock.com
By modeling different iron environments relevant to persistent pulmonary infection, a recent study in Frontiers in Microbiology found that pre-exposure of Pseudomonas aeruginosa to iron-rich laboratory conditions promotes bacterial growth and biofilm formation in experimental lung infection models, while contradictingly reducing virulence and host tissue damage.
The findings highlight a trade-off in which iron availability supports pulmonary persistence but dampens the expression of key virulence factors, helping explain why chronic P. aeruginosa lung infections can be difficult to eradicate yet vary widely in disease severity.
Growth and virulence of P. aeruginosa
In hospitals and healthcare facilities, P. aeruginosa emerges as a versatile opportunistic pathogen that causes infections across multiple body systems, including wounds, the urinary tract, and the respiratory system. Of all these infections, persistent lung infections prove particularly difficult to treat, primarily because of the bacterium’s remarkable ability to form biofilms.
Bacterial cells in biofilms are surrounded by a self-produced protective matrix that adheres to surfaces. As a model organism for biofilm research, P. aeruginosa demonstrates how these structures enable bacteria to withstand hostile environments and resist treatment, making chronic pulmonary infections especially stubborn.
P. aeruginosa deploys a sophisticated range of virulence factors to establish and maintain infection. This includes siderophores for iron acquisition, motility structures such as flagella, toxic compounds such as pyocyanin and exotoxin A, tissue-degrading enzymes including elastase and various proteases, and hemolysins, phospholipase C, and rhamnolipids. Each factor serves a distinct purpose in overwhelming host defenses.
Limited investigation of iron’s role in persistent pulmonary infection models
Iron plays a central role in P. aeruginosa biofilm development and virulence. Though the body isolates iron in proteins, P. aeruginosa produces siderophores, molecules that bind iron to support growth and survival. Local environmental conditions, particularly iron availability, can dramatically alter bacterial growth and virulence, potentially explaining why chronic infections are difficult to treat.
The lung environment is unique in that the body restricts iron overall, yet pulmonary disease states may expose bacteria to fluctuating iron levels that influence pathogenicity. While iron regulation has been widely studied, relatively few investigations have combined in vitro and in vivo models to examine how iron exposure shapes P. aeruginosa behavior specifically in persistent lung infection contexts, or how these changes translate to host pathology.
Assessing how different iron concentrations affect P. aeruginosa
The current study investigates how varying iron concentrations affect P. aeruginosa growth, biofilm formation, and virulence using laboratory models that simulate iron-replete and iron-restricted conditions relevant to pulmonary infection. Clinical P. aeruginosa isolates from persistent lung infections, and the reference strain PAO1, were tested under varying iron conditions. Iron-deficient conditions were created by supplementing tryptic soy broth (TSB) with 0 to 500 µM dipyridyl (DP), an iron chelator. PAO1 growth slowed progressively with increasing DP, with minimal growth at 500 µM. TSB with 400 µM DP was selected as the optimal iron-deficient condition.
To confirm growth inhibition was iron-specific, FeCl₃ was used. P. aeruginosa was cultured under three distinct iron conditions. The iron-replete condition used TSB alone. The iron-deficient condition used TSB with 400 µM DP. The partially iron-restored condition used TSB with 400 µM DP plus 5 µM FeCl₃, which partially reverses chelation without fully restoring iron to baseline TSB levels.
Environmental iron facilitates pulmonary persistence while decreasing virulence
All P. aeruginosa strains demonstrated optimal growth in iron-replete environments, with significantly greater growth than in either the iron-restricted or partially iron-restored conditions. All strains exhibited significantly enhanced biofilm formation in the iron-replete environment compared to both the partially iron-restored and iron-restricted conditions.
The study noted that all P. aeruginosa strains exhibited significantly reduced production of virulence determinants, including protease, pyocyanin, exotoxin A, phospholipase C, alkaline protease, elastase, siderophore, and hemolysin, in the iron-replete medium compared to both the partially iron-restored and iron-restricted conditions. Production of these virulence determinants was also significantly lower under the partially iron-restricted condition than under iron restriction.
Galleria mellonella larvae infected with P. aeruginosa pre-cultured in iron-replete medium demonstrated significantly higher survival rates than those infected with bacteria grown in iron-restricted conditions. Under iron-replete conditions, these bacterial strains also exhibited significantly greater adhesion to pulmonary epithelial cells.
Consistent with the in vitro observations, mice infected with P. aeruginosa pre-cultured in iron-replete medium exhibited significantly higher pulmonary bacterial loads than those infected with bacteria pre-cultured in iron-restricted medium. Despite this increased bacterial burden, mice infected with iron-replete cultured bacteria demonstrated significantly lower levels of pulmonary PCT, IL-6, and IL-10.
Histological analysis revealed striking differences. Mice infected with P. aeruginosa cultured in iron-replete medium showed minimal granulocyte infiltration of alveolar walls and mildly irregular bronchiolar epithelium. In contrast, those infected with bacteria grown under iron restriction exhibited extensive pathology, including alveolar narrowing and collapse, significant immune cell infiltration, bronchiolar irregularities, perivascular edema, occasional hemorrhage, and interstitial vascular congestion.
Rethinking iron-targeted strategies for chronic lung infections
Environmental iron availability during bacterial growth paradoxically affects P. aeruginosa in pulmonary infection models. It promotes bacterial growth and biofilm formation while simultaneously reducing virulence and host tissue damage. This decoupling of bacterial burden from disease severity may help explain why P. aeruginosa establishes persistent lung infections that are treatment-resistant yet exhibit variable clinical outcomes.
The findings suggest that iron-restricted environments may push P. aeruginosa toward a more aggressive, virulent state, whereas iron-rich conditions enable robust growth with diminished pathogenicity. The authors caution that therapeutic strategies aimed solely at iron deprivation could unintentionally exacerbate lung inflammation and tissue damage, highlighting the need for approaches that balance bacterial clearance with modulation of iron-responsive virulence pathways, as well as for future studies that directly probe how manipulating iron availability in the lung environment influences long-term infection outcomes.
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