Association of colony variation in Serratia marcescens with the differential expression of protease and type 1 fimbriae

Several clinical isolates of Serratia marcescens were found to dissociate on peptone glycerol agar into colonies with red and pink or white and gray phenotypes that differ in the expression of proteolytic activity and mannose-sensitive type of hemagglutination. Colonies of red and white type were proteolytically active but did not express hemagglutination, whereas pink and gray colonies were protease-deficient but agglutinated guinea pig erythrocytes. Site-directed mutagenesis of a red laboratory strain S. marcescens SM6 resulted in selection of protease negative derivative prt::G7 which expressed the pink phenotype with hemagglutinating activity. It is suggested that a DNA-regulatory element may be involved in this type of colony variation.