CONGO-RED BINDING BY PORPHYROMONAS-GINGIVALIS IS MEDIATED BY A 66-KDAOUTER-MEMBRANE PROTEIN

Congo red was bound from solution by strains of Porphyromonas gingival is including W50, HG189, HC184 NCTC 11834 Bg 381, WPH35, the slower br own pigmenting colonial variant W50/BR1, and the avirulent mutant W50/ BE1, and by Porphyromonas endodontalis HC370 and Porphyromonas asaccha rolytica B537. SDS-PACE of whole cells of all species examined display ed a 66 kDa Congo-red-binding component which was also detected in the outer membranes of P. gingivalis W50 grown in the chemostat under bot h haemin limitation and haemin excess, and which corresponded to a Coo massie-blue-stained band of the same mobility. Pretreatment of haemin- excess batch-grown cells of P. gingivalis W50 with polymyxin B, which binds to lipid A, did not inhibit binding, whilst binding was enhanced in the presence of 2 M ammonium sulphate, suggesting the involvement of non-specific hydrophobic interactions. Binding was also reduced by pretreatment with trypsin and papain, and by 8-anilino-1-naphthalenesu lphonic acid, which binds to hydrophobic amino acids. The 66 kDa bindi ng component was sensitive to proteinase K digestion, and loss of Cong o red staining of this band correlated with the quantitative reduction in Congo red binding by whole cells. These data, and our previous wor k, show that Congo red and iron protoporphyrin IX (haemin) are bound t o different outer-membrane components, and that Congo red binding may be of little value as a marker to detect virulent strains of P. gingiv alis or those expressing haemin-binding proteins.