The influence of human respiratory epithelia on Pseudomonas aeruginosa gene expression

Abstract

The opportunistic pathogen Pseudomonas aeruginosa can cause acute or chronic infections in humans. Little is known about the initial adaptation of P. aeruginosa to host tissues and the factors that determine whether a P. aeruginosa-epithelial cell interaction will manifest as an acute or a chronic infection. To gain insights into the initial phases of P. aeruginosa infections and to identify P. aeruginosa genes regulated in response to respiratory epithelia, we exposed P. aeruginosa to cultured primary differentiated human airway epithelia. We used a P. aeruginosa strain that causes acute damage to the epithelia and a mutant with defects in Type III secretion and in rhamnolipid synthesis. The mutant did not cause rapid damage to epithelia as did the wildtype. We compared the transcriptomes of the P. aeruginosa wildtype and the mutant to each other and to P. aeruginosa grown under other conditions, and we discovered overlapping sets of differentially expressed genes in the wildtype and mutant exposed to epithelia. A recent study reported that exposure of P. aeruginosa to epithelia is characterized by a repression of the bacterial iron-responsive genes. These findings were suggestive of ample iron availability during infection. In contrast, we found that P. aeruginosa shows an iron-starvation response upon exposure to epithelial cells. This observation highlights the importance of the iron starvation response in both acute and chronic infections and suggests opportunities for therapy.

Figure 1. Measurements of transepithelial electrical resistance (R_t) of airway epithelia grown in millicells and infected with the parent PAO1 or the mutant PAO-SC11

The inoculum size is ~2 × 10⁷ CFU/millicell. R_t values are reported as averages of 5 millicells at each time point expressed as a percentage of the starting values. The range was +/−15%.

Figure 2. Absolute expression profiles of genes during growth of P. aeruginosa in TSB and on epithelia

Wildtype P. aeruginosa PAO1 in TSB (A), the mutant PAO-SC11 in TSB (B), wildtype P. aeruginosa PAO1 on epithelia (C) and, the mutant PAO-SC11 on epithelia (D). The profile was generated by combining lists of genes assigned a present call in at least one of the two replicates for each growth condition. Depicted are the averaged expression values normalized to the median. Genes are displayed in the order of the hierarchical clustering of their expression profiles as determined using the Spearman corelation in GENESPRING.

Figure 3. Venn diagram showing overlaps between genes regulated in the parent PAO1 and the mutant PAO-SC11 upon exposure to epithelia

Blue, genes regulated in wildtype PAO1 exposed to epithelia versus grown in TSB. Yellow, genes regulated in the mutant PAO-SC11 exposed to epithelia versus grown in TSB. Red, genes regulated in a comparison of wildtype PAO1 exposed to epithelia and the mutant PAO-SC11 exposed to epithelia.