Differential gene expression by Pseudomonas aeruginosa during interaction with respiratory mucus.

Pseudomonas aeruginosa is a common respiratory tract pathogen that causes serious infections in patients with cystic fibrosis. A number of putative virulence factors have been characterized in several laboratories, and some have been implicated in human infections, based on criteria such as the phenotype of isolates from infected patients, an immune response to a particular antigenic factor, and the effect of a virulence factor on infectivity in an animal model. We have developed a series of genetic tools to study the selective regulation of expression of P. aeruginosa genes during interactions of the pathogen with host tissues. These tools are based on direct enrichment of bacteria, when a particular promoter is induced or repressed. We have found that interaction of bacteria with mucus from patients with cystic fibrosis results in marked induction of expression of several genes, including one that encodes a lipopolysaccharide biosynthetic enzyme, a gene for a protein responsible for uptake of the ferric pyochelin siderophore, and a new gene homologous with a class of iron-responsive repressors. The tools described here are useful for identification of induced or repressed genes in various animal models of infection or in controlled laboratory conditions that mimic natural infections of humans. Such genes might not be detectable when bacteria are cultured in laboratory conditions, and these tools are therefore useful for general probing of a bacterial genome for genes regulated during different stages of infection.