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The plasminogen activator Pla of Yersinia pestisbelongs to the omptin family of enterobacterial surface proteases and is responsible for the highly efficient invasion of the plague bacterium from the subcutaneous infection site into the circulation. Y. pestishas been reported to invade human epithelial cells. Here, we investigated the role of Pla in bacterial invasion into human endothelial cells. Expression of Pla in recombinant Escherichia coliXL1(pMRK1) enhanced bacterial invasion into ECV304 cells. The invasiveness was not affected by substitution mutation at the residues S99 or D206 that are needed for the proteolytic activity of Pla. Pla-expressing bacteria adhered to the extracellular matrix of ECV304 cells. Only weak adhesion and poor invasion were seen with the recombinant E. coliXL1(pMRK2), which expresses the omptin homolog from E. coli. The results identify Pla as an invasion protein of Y. pestisand show that the invasive function does not involve the proteolytic activity of Pla.

Immobilization of plasminogen via its lysine‐binding sites is regarded as a prerequisite for its activation and function in fibrinolysis and pericellular proteolysis. In the present study, the interaction of plasminogen with fimbriae found on Escherichia colistrains causing invasive human infections was studied. Plasminogen displayed concentration‐dependent and saturable binding to immobilized type 1 fimbriae and, several fold lower binding to P and S fimbriae. The binding to fimbriae was effectively inhibited by ε‐aminocaproic acid indicating that it was mediated by the lysine‐binding sites of plasminogen. Binding studies with mutated fimbriae and inhibition tests indicated that the interaction was not dependent on the lectin subunit of the fimbriae. These results indicate the existence of a novel type of host‐microbe interaction which may be important in the invasion by bacteria of host tissues.

A strain of the phytopathogenic bacterial species, Erwinia rhapontici, was found to cause hemagglutination of human erythrocytes that was specifically inhibited by β-galactosides. Of the monosaccharides tested, N-acetyl galactosamine and galactose efficiently inhibited the hemagglutination. The most potent inhibitor identified was Galβ1—4GlcNAc that was 30–100-fold more potent than Galβ1—3GlcNac or Galβ1—3GalNAc. Fetuin had no effect on the hemagglutination whereas asialofetuin was inhibitory. No blood group specificity was found for the hemagglutinin. These findings indicate that the E. rhaponticistrain possesses a novel bacterial cell-binding activity with specificity for terminal N-acetyllactosamine residues.

Binding of P fimbriae of uropathogenic Escherichia colito purified human plasma fibronectin and human placental type IV collagen was studied. In an enzyme immunoassay, purified P fimbriae bound strongly to immobilized intact fibronectin and to the aminoterminal 30-kDa fragment and the 120–140-kDa carboxyterminal fragments of fibronectin. Binding to the gelatin-binding 40-kDa fragment of fibronectin was considerably weaker. No binding to immobilized type IV collagen was seen. The interaction between P fimbriae and immobilized fibronectin was not inhibited by α-D-Gal-(1-4)-β-D-Gal-1-O-Me, a receptor analog of P fimbriae. Moreover, a mutated P fimbria lacking the lectin activity behaved similarly in the adherence assays. Recombinant strains expressing the corresponding cloned fimbriae genes bound to immobilized fibronectin, but no binding to soluble 125I-labelled fibronectin was found. The results suggest that P fimbriae interact with immobilized fibronectin and that the binding mechanism does not involve the lectin activity of the fimbriae.

Escherichia colistrains isolated from patients with different levels of urinary tract infection and from healthy persons were tested for their ability to haemagglutinate endo-β-galactosidase-treated human erythrocytes. Among the 104 strains studied one revealed a strong agglutination reaction with the enzyme-treated erythrocytes. From the monosaccharides tested N-acetyl-D-glucosamine inhibited agglutination most effectively. Orosomucoid and asialo-orosomucoid had no effect on the haemagglutination whereas β-galactosidase treated asialo-orosomucoid was inhibitory. These findings indicate that the E. colistrain studied contains a novel cell-binding activity with specificity for terminal N-acetyl-D-glucosamine residues.