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.