Siderophore-Mediated Competition Between Cystic Fibrosis Pathogens Increases Virulence Factor Production

The airways of people with cystic fibrosis (CF) are colonized by a polymicrobial community and coinfections with multiple pathogens may exacerbate disease. By analyzing 16S rRNA gene sequencing datasets, we determined that Achromobacter - Pseudomonas coinfection was associated with worse lung function than infection with either pathogen alone, or coinfections with Pseudomonas and other CF bacterial pathogens. We discovered Achromobacter spp. induced production of the siderophore pyoverdine by Pseudomonas aeruginosa strain PAO1 and other clinical isolates on synthetic CF sputum medium (SCFM), supplemented with physiologically relevant iron concentrations. We then tested 21 Achromobacter strains, encompassing seven species, and determined that 17 (81%) induced pyoverdine production. We sequenced the full genomes of these Achromobacter and found 18 (86%) encoded a biosynthetic gene cluster for the production of alcaligin. This compound is an iron-chelating siderophore that has previously been shown to be involved in colonization of the respiratory tract by Bordetella spp. Using mass spectrometry imaging, we confirmed that Achromobacter dolens strain RMS0405 produces alcaligin during coculture with PAO1. Further, we found that supplementing SCFM with increasing iron abolishes pyoverdine induction by PAO1 during coculture. Pyoverdine is known to contribute directly to virulence and regulate expression of other virulence factors. Using transcriptomics and proteomics, we determined that RMS0405 induces a low-iron stress response in PAO1 and stimulates the production of exotoxin A and other known virulence factors. Together, our results indicate that Achromobacter spp. and P. aeruginosa may exacerbate virulence in the airways of people with CF as a byproduct of competition for iron.