HYPER-INVASIVE MUTANTS DEFINE A NOVEL PHO-REGULATED INVASION PATHWAY IN ESCHERICHIA-COLI

We have isolated two transposon insertion mutations of the pst-phoU op eron which result in the constitutive expression of the phoA gene prod uct, alkaline phosphatase. The two mutations also render Escherichia c oli invasive towards cultured HEp-2 cells and define a novel Pho-regul ated invasion pathway. The presence of the large 'invasion' plasmid de rived 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. Transductio n 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 ba cteria grown in standard laboratory rich media, may become functional in the host when invasive bacteria encounter nutrient starvation and/o r other related stress conditions.