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Identification of the mucin-binding adhesin of Pseudomonas cepacia isolated from patients with cystic fibrosis

In previous experiments, we have shown that isolates of Pseudomonas cepacia from sputa of patients with cystic fibrosis (CF), particularly those with severe lung infection, exhibited specific binding to purified respiratory or intestinal mucins (U. Sajjan, M. Corey, M. Karmali, and J. Forstner, J. Clin. Invest. 89:648-656, 1992). The present report describes the identification of the adhesin as a protein located on fimbriae of mucin-binding P. cepacia. From a total of 53 isolates available (from 22 patients with CF), we used three mucin-binding and three non-mucin-binding isolates for our experiments. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of crude P. cepacia homogenates was performed, the separated proteins were blotted onto nitrocellulose and overlaid with purified mucin, and mucin-binding components were detected with an antimucin antibody and then a second-antibody-alkaline phosphatase conjugate system. Only mucin-binding isolates exhibited a positively stained band at an Mr of 22,000. The 22-kDa protein was purified, and a polyclonal antibody specific for it was developed in rabbits. By electron microscopy and immunogold labelling, both the antibody and mucin (separately) were localized to pili present over the entire surface of the bacterial cells. Non-mucin-binding isolates did not have (or had very few) pili and did not stain with either mucin or the antibody to the 22-kDa protein. The purified 22-kDa protein and its antibody were each able to inhibit piliated P. cepacia binding to mucin. The amino acid composition of the 22-kDa protein was dissimilar to those of the major pilin proteins of Escherichia coli (type 1 pilus) and P. aeruginosa (PAK and PAO1 strains). Both the pili of P. aeruginosa PAK and PAO1 and antibodies to these pili failed to inhibit P. cepacia binding to mucin. Thus, P. cepacia adhesion to mucin is mediated by a pilin-associated 22-kDa protein which differs from epithelial-cell-binding pilin proteins of P. aeruginosa. We postulate that the 22-kDa adhesin may play a role in the virulence of P. cepacia lung infections of patients with CF.