Strain-specific iron-dependent virulence in Escherichia coli

For reasons unknown, certain Escherichia coli strains become highly virulen t when injected with hemoglobin or other soluble iron sources. Two clinical isolates (virulent and nonvirulent) showed equivalent hemoglobin-mediated growth acceleration in vitro. However, when injected intraperitoneally into mice without hemoglobin, the virulent strain was cleared more slowly (t(1/ 2), >4 h vs. <30 min). The virulent E. coli strain had a polysialic acid-co ntaining capsule, whereas the nonvirulent strain did not. Virulent E. coli grown at 20<degrees>C (which blocks polysialylation) were cleared as rapidl y as nonvirulent organisms. In another virulent E. coli strain having abund ant outer membrane polysialic acid, targeted deletion of the polysialyltran sferase accelerated host clearance and blocked iron-dependent virulence. Th e iron-dependent virulence of certain E. coli strains may represent the com bined effect of slow in vivo clearance-associated, in this case, with outer membrane polysialylation coupled with accelerated growth permitted by iron compounds.