Antibiotic resistant Pseudomonas aeruginosa upregulate iron piracy genes during macrophage infection and evade killing

Abstract

Abstract Antibiotic resistant bacterial lung infections are the primary cause of morbidity and mortality in people with cystic fibrosis (CF). Recently, we showed that aztreonam resistant P. aeruginosa with a mutated nalD gene were more virulent than wild type (WT), even in the absence of antibiotic treatment. Yet, the mechanisms underlying the increased virulence were unknown. Because nalD− mutants increased antibiotic efflux pump expression, we hypothesized that their hypervirulence was dependent on the efflux pump. Surprisingly, deletion of the efflux pump did not affect nalD− virulence in an acute lung infection. To determine other potential mechanisms underlying the enhanced virulence, we performed RNA-seq on WT and nalD− grown in vitro and found that nalD− increased efflux pump expression relative to WT, as expected, but also differentially regulated hundreds of genes. Exopolysaccharide genes involved in biofilm formation were among the most highly upregulated genes. Consistent with this differential gene expression, the nalD− mutant exhibited increased biofilm formation, reduced C3 complement opsonization, and also replicated better than WT in the presence of macrophages. Subsequent RNA-seq analyses of infected macrophages revealed that nalD− upregulated iron siderophore systems relative to WT. We predict that this increased iron piracy facilitates subversion of nutritional immunity during infection. Because individuals with CF frequently experience anemia, antibiotic resistance mutations that increase iron piracy could promote bacterial growth during these periods of iron limitation.