EXPRESSION OF CAPSULAR POLYSACCHARIDE DETERMINES SERUM RESISTANCE IN ESCHERICHIA-COLI K92

The amount of capsular polysaccharide expression has been shown to be the major determinant of serum resistance in Escherichia coli K1. E. c oli K92, like K1, is a polymer of sialic acid molecules. It differs fr om K1 by containing both a (2.8) and a (2.9) linkages. Four strains of E. coli K92 were tested for serum resistance. Three strains were seru m-resistant (50% normal human serum), one strain was moderately serum- sensitive. The serum-resistant strains expressed significantly more ca psular polysaccharide than did the serum-sensitive strain. For each of the serum-resistant strains, six mutants were isolated by selection f or resistance against infection with a K92-specific bacteriophage. All of the mutants expressed less capsular polysaccharide than the respec tive wild-type strains. All mutants were more sensitive to serum killi ng than the wild-type strains. In all groups, the mutants with lowest expression of capsular polysaccharide were highly serum-sensitive. Cha nges of outer membrane proteins or lipopolysaccharide patterns that we re present in some mutants did not correlate with serum resistance pro perties of the mutants. Furthermore, it was investigated whether the p resence of active serum had an influence on capsule expression. In the serum-sensitive strain, the presence of serum induced a significant a nd concentration-dependent increase of capsule expression. Serum had n o effect on capsule expression by the serum-resistant strains. We conc lude from the data that the expression of K92 capsular polysaccharide determines serum resistance in the strains examined.