Characterization of Saa, a novel autoagglutinating adhesin produced by locus of enterocyte effacement-negative shiga-toxigenic Escherichia coli strains that are virulent for humans

The capacity of Shiga toxigenic Escherichia coli (STEC) to adhere to the in testinal mucosa undoubtedly contributes to pathogenesis of human disease. T he majority of STEC strains isolated from severe cases produce attaching an d effacing lesions on the intestinal mucosa, a property mediated by the loc us of enterocyte effacement (LEE) pathogenicity island. This element is not essential for pathogenesis, as some cases of severe disease, including hem olytic uremic syndrome (HUS), are caused by LEE-negative STEC strains, but the mechanism whereby these adhere to the intestinal mucosa is not understo od. We have isolated a gene from the megaplasmid of a LEE-negative O113:H21 STEC strain (98NK2) responsible for an outbreak of HUS, which encodes an a uto-agglutinating adhesin designated Saa (STEC autoagglutinating adhesin). Introduction of saa cloned in pBC results in a 9.7-fold increase in adheren ce of E. coli JM109 to HEp-2 cells and a semilocalized adherence pattern. M utagenesis of saa in 98NK2, or curing the wild-type strain of its megaplasm id, resulted in a significant reduction in adherence. Homologues of saa wer e found in several unrelated LEE-negative STEC serotypes, including O48:H21 (strain 94CR) and O91:H21 (strain B2F1), which were also isolated from pat ients with HUS. Saa exhibits a low degree of similarity (25% amino acid [aa ] identity) with YadA of Yersinia enterocolitica and Eib, a recently descri bed phage-encoded immunoglobulin binding protein from E. coli. Saa produced by 98NK2 is 516 aa long and includes four copies of a 37-aa direct repeat sequence. Interestingly, Saa produced by other STEC strains ranges in size from 460 to 534 aa as a consequence of variation in the number of repeats a nd/or other insertions or deletions immediately proximal to the repeat doma in.