Hyper‐invasive mutants define a novel Pho‐regulated invasion pathway in Escherichia coli

We have isolated two transposon insertion mutations of the pst–phoU operon which result in the constitutive expression of the phoA gene product, alkaline phosphatase. The two mutations also render Escherichia coli invasive towards cultured HEp‐2 cells and define a novel Pho‐regulated invasion pathway. The presence of the large‘invasion’plasmid derived from an entero‐invasive E. coli (EIEC) clinical isolate in these mutants leads to enhanced invasiveness toward cultured HEp‐2 cells, a phenomenon referred to as the‘hyper‐invasive’phenotype. Transduction of a pst–phoU insertion mutation into clinical isolates of EIEC and Shigella flexneri results in constitutive PhoA expression and coupled hyper‐invasiveness in the former but not the latter. We speculate that the Pho‐regulated invasion pathway described here, while silent in bacteria grown in standard laboratory rich media, may become functional in the host when invasive bacteria encounter nutrient starvation and/or other related stress conditions.