Abstract
Agglutination of Serratia marcescens in human urine is known to be caused by interaction of urinary mucin and the bacterial pili. This phenomenon, urinary mucin agglutination (UMA), is thought to be an expression of nonimmune-host-defense mechanism against bacterial infection of urinary tract. The piliated bacteria are trapped by urinary mucin and excreted at urination as a large clump of bacteria and mucin. In this study, the mechanism of the agglutination was examined with a piliated clinical isolate of S. marcescens (US5) and partially purified human urinary mucin. The following results were obtained:
i) Urinary mucin was purified by centrifugation and washing with phosphate buffered saline. Partially purified mucin showed a single peptide band in SDS-PAGE and morphologically a homogeneous mass of fine fibrous molecules in electron microscopy.
ii) A single fiber of the mucin molecule was approximately 14nm in diameter and consisted of filaments with double helix. Each filament made a turn at every 28nm.
iii) Either by heating at 55C for 30min or by treating with proteinase and hyaluronidase, the mucin lost its agglutinability.
iv) Agglutination was inhibited by addition of D-mannose or some of its derivatives to the reaction mixture. This suggests that the interaction of pili with mucin was mediated by the mannose-residue in the mucin molecule probably located at the bridging structure between the axial fiber and the helical fiber.
v) Such strains that have the pili eliciting mannose-resistant hemagglutination were not agglutinated in urine. Furthermore, the type 1 pili of E. coli eliciting mannose-sensitive agglutination also showed no agglutination human urine. This suggests that some factor other than mannose-specific lectin activity of pili may participate in the agglutination reaction. The size of pili seems to be an important factor for the agglutination. Such pili that have diameters larger than the pitch size of helix of the mucin molecule may not agglutinated by mucin.
