Indole-Acetic Acid Impairs Pseudomonas aeruginosa Virulence and Alters Lung Infection in Mice

Patients in intensive care units, especially those immunocompromised, are prone to opportunistic infections, such as respiratory and urinary tract infections. Extended antibiotic use disrupts the production of microbiome-derived metabolites, including those involved in colonization resistance to Pseudomonas aeruginosa, which is known for its multidrug resistance. Hence, prior antibiotic treatment has been shown to increase susceptibility to P. aeruginosa infection, but the role of microbiota-derived metabolic cues in this context is still elusive. This study investigates how tryptophan metabolites from the indigenous microbiota affect P. aeruginosa virulence. In vitro tests on motility, biofilm production, and pigment quantification (pyocyanin and pyoverdine) were performed on P. aeruginosa strains (PAO1, PA103, PA14) and clinical isolates. Additionally, gene expression related to virulence was analyzed, and the effects of tryptophan metabolites on experimental lung infection in mice were evaluated. Indole, indoleacetic acid (IAA), and indoleacrylic acid (IA) reduced motility and pigment production. IAA and indole promoted biofilm formation, with indole having a stronger effect. Clinical isolates showed significant phenotypic diversity, and IAA was more effective at inhibiting virulence traits than indole or IA. Mice infected with bacteria grown in the presence of IAA had lower lethality and fewer polymorphonuclear leukocyte influx compared to the control group. This suggests that tryptophan metabolites, especially IAA, can modulate P. aeruginosa virulence and may help control infection progression.