Purification and characterization of Serratia marcescens US5 pili

The pili of Serratia marcescens US5 isolated from a patient with urinary tract infection were purified and characterized. During the aeration culture, the pili were detached from the bacteria and were precipitated by the addition of ammonium sulfate. The purification of the pili was carried out by ion-exchange chromatography and gel filtration on Sepharose 4B. In electron microscopy, the purified pilus showed a filament of 3 nm in diameter and 0.3 micron in average length. The molecular weight of the protein subunit of the purified pili was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Two protein bands appeared. One has a molecular size of 19,000 daltons, and the other has a molecular size of 39,000 daltons. The isoelectric point was 3.7. The content of hydrophobic amino acids in purified pili subunits was 42% of the total amino acid content. Further purification of pili by isopycnic centrifugation failed to remove the large protein band. No identical protein bands to pili proteins were detected in the electrophoresis pattern of the outer membrane proteins extracted from S. marcescens US5 in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. These two proteins might be a dimer of a small molecule. A survey of clinically isolated strains of S. marcescens revealed that more than 60% of the strains had this type of pili. These results suggest that these pili are widely distributed among strains of S. marcescens.